erl_driver(3erl) C Library Functions erl_driver(3erl)
NAME
erl_driver - API functions for an Erlang driver.
DESCRIPTION
An Erlang driver is a library containing a set of native driver call-
back functions that the Erlang Virtual Machine calls when certain
events occur. There can be multiple instances of a driver, each in-
stance is associated with an Erlang port.
Warning:
Use this functionality with extreme care.
A driver callback is executed as a direct extension of the native code
of the VM. Execution is not made in a safe environment. The VM cannot
provide the same services as provided when executing Erlang code, such
as pre-emptive scheduling or memory protection. If the driver callback
function does not behave well, the whole VM will misbehave.
* A driver callback that crash will crash the whole VM.
* An erroneously implemented driver callback can cause a VM internal
state inconsistency, which can cause a crash of the VM, or miscel-
laneous misbehaviors of the VM at any point after the call to the
driver callback.
* A driver callback doing lengthy work before returning degrades re-
sponsiveness of the VM and can cause miscellaneous strange behav-
iors. Such strange behaviors include, but are not limited to, ex-
treme memory usage and bad load balancing between schedulers.
Strange behaviors that can occur because of lengthy work can also
vary between Erlang/OTP releases.
As from ERTS 5.5.3 the driver interface has been extended (see extended
marker). The extended interface introduces version management, the pos-
sibility to pass capability flags (see driver_flags) to the runtime
system at driver initialization, and some new driver API functions.
Note:
As from ERTS 5.9 old drivers must be recompiled and use the extended
interface. They must also be adjusted to the 64-bit capable driver in-
terface.
The driver calls back to the emulator, using the API functions declared
in erl_driver.h. They are used for outputting data from the driver, us-
ing timers, and so on.
Each driver instance is associated with a port. Every port has a port
owner process. Communication with the port is normally done through the
port owner process. Most of the functions take the port handle as an
argument. This identifies the driver instance. Notice that this port
handle must be stored by the driver, it is not given when the driver is
called from the emulator (see driver_entry).
Some of the functions take a parameter of type ErlDrvBinary, a driver
binary. It is to be both allocated and freed by the caller. Using a bi-
nary directly avoids one extra copying of data.
Many of the output functions have a "header buffer", with hbuf and hlen
parameters. This buffer is sent as a list before the binary (or list,
depending on port mode) that is sent. This is convenient when matching
on messages received from the port. (Although in the latest Erlang ver-
sions there is the binary syntax, which enables you to match on the be-
ginning of a binary.)
In the runtime system with SMP support, drivers are locked either on
driver level or port level (driver instance level). By default driver
level locking will be used, that is, only one emulator thread will exe-
cute code in the driver at a time. If port level locking is used, mul-
tiple emulator threads can execute code in the driver at the same time.
Only one thread at a time will call driver callbacks corresponding to
the same port, though. To enable port level locking, set the
ERL_DRV_FLAG_USE_PORT_LOCKING driver flag in the driver_entry used by
the driver. When port level locking is used, the driver writer is re-
sponsible for synchronizing all accesses to data shared by the ports
(driver instances).
Most drivers written before the runtime system with SMP support existed
can run in the runtime system with SMP support, without being rewrit-
ten, if driver level locking is used.
Note:
It is assumed that drivers do not access other drivers. If drivers ac-
cess each other, they must provide their own mechanism for thread-safe
synchronization. Such "inter-driver communication" is strongly discour-
aged.
Previously, in the runtime system without SMP support, specific driver
callbacks were always called from the same thread. This is not the case
in the runtime system with SMP support. Regardless of locking scheme
used, calls to driver callbacks can be made from different threads. For
example, two consecutive calls to exactly the same callback for exactly
the same port can be made from two different threads. This is for most
drivers not a problem, but it can be. Drivers that depend on all call-
backs that are called in the same thread, must be rewritten before they
are used in the runtime system with SMP support.
Note:
Regardless of locking scheme used, calls to driver callbacks can be
made from different threads.
Most functions in this API are not thread-safe, that is, they cannot be
called from arbitrary threads. Functions that are not documented as
thread-safe can only be called from driver callbacks or function calls
descending from a driver callback call. Notice that driver callbacks
can be called from different threads. This, however, is not a problem
for any function in this API, as the emulator has control over these
threads.
Warning:
Functions not explicitly documented as thread-safe are not thread safe.
Also notice that some functions are only thread-safe when used in a
runtime system with SMP support.
A function not explicitly documented as thread-safe can, at some point
in time, have a thread-safe implementation in the runtime system. Such
an implementation can however change to a thread unsafe implementation
at any time without any notice.
Only use functions explicitly documented as thread-safe from arbitrary
threads.
As mentioned in the warning text at the beginning of this section, it
is of vital importance that a driver callback returns relatively fast.
It is difficult to give an exact maximum amount of time that a driver
callback is allowed to work, but usually a well-behaving driver call-
back is to return within 1 millisecond. This can be achieved using dif-
ferent approaches. If you have full control over the code to execute in
the driver callback, the best approach is to divide the work into mul-
tiple chunks of work, and trigger multiple calls to the time-out call-
back using zero time-outs. Function erl_drv_consume_timeslice can be
useful to determine when to trigger such time-out callback calls. How-
ever, sometimes it cannot be implemented this way, for example when
calling third-party libraries. In this case, you typically want to dis-
patch the work to another thread. Information about thread primitives
is provided below.
FUNCTIONALITY
All functions that a driver needs to do with Erlang are performed
through driver API functions. Functions exist for the following func-
tionality:
Timer functions:
Control the timer that a driver can use. The timer has the emulator
call the timeout entry function after a specified time. Only one
timer is available for each driver instance.
Queue handling:
Every driver instance has an associated queue. This queue is a Sys-
IOVec, which works as a buffer. It is mostly used for the driver to
buffer data that is to be written to a device, it is a byte stream.
If the port owner process closes the driver, and the queue is not
empty, the driver is not closed. This enables the driver to flush
its buffers before closing.
The queue can be manipulated from any threads if a port data lock
is used. For more information, see ErlDrvPDL.
Output functions:
With these functions, the driver sends data back to the emulator.
The data is received as messages by the port owner process, see er-
lang:open_port/2. The vector function and the function taking a
driver binary are faster, as they avoid copying the data buffer.
There is also a fast way of sending terms from the driver, without
going through the binary term format.
Failure:
The driver can exit and signal errors up to Erlang. This is only
for severe errors, when the driver cannot possibly keep open.
Asynchronous calls:
Erlang/OTP R7B and later versions have provision for asynchronous
function calls, using a thread pool provided by Erlang. There is
also a select call, which can be used for asynchronous drivers.
Multi-threading:
A POSIX thread like API for multi-threading is provided. The Erlang
driver thread API only provides a subset of the functionality pro-
vided by the POSIX thread API. The subset provided is more or less
the basic functionality needed for multi-threaded programming:
* Threads
* Mutexes
*
Condition variables
*
Read/write locks
*
Thread-specific data
The Erlang driver thread API can be used in conjunction with the
POSIX thread API on UN-ices and with the Windows native thread API
on Windows. The Erlang driver thread API has the advantage of being
portable, but there can exist situations where you want to use
functionality from the POSIX thread API or the Windows native
thread API.
The Erlang driver thread API only returns error codes when it is
reasonable to recover from an error condition. If it is not reason-
able to recover from an error condition, the whole runtime system
is terminated. For example, if a create mutex operation fails, an
error code is returned, but if a lock operation on a mutex fails,
the whole runtime system is terminated.
Notice that there is no "condition variable wait with time-out" in
the Erlang driver thread API. This because of issues with
pthread_cond_timedwait. When the system clock suddenly is changed,
it is not always guaranteed that you will wake up from the call as
expected. An Erlang runtime system must be able to cope with sudden
changes of the system clock. Therefore, we have omitted it from the
Erlang driver thread API. In the Erlang driver case, time-outs can
and are to be handled with the timer functionality of the Erlang
driver API.
In order for the Erlang driver thread API to function, thread sup-
port must be enabled in the runtime system. An Erlang driver can
check if thread support is enabled by use of driver_system_info.
Notice that some functions in the Erlang driver API are thread-safe
only when the runtime system has SMP support, also this information
can be retrieved through driver_system_info. Also notice that many
functions in the Erlang driver API are not thread-safe, regardless
of whether SMP support is enabled or not. If a function is not doc-
umented as thread-safe, it is not thread-safe.
Note:
When executing in an emulator thread, it is very important that you
unlock all locks you have locked before letting the thread out of
your control; otherwise you are very likely to deadlock the whole em-
ulator.
If you need to use thread-specific data in an emulator thread, only
have the thread-specific data set while the thread is under your con-
trol, and clear the thread-specific data before you let the thread
out of your control.
In the future, debug functionality will probably be integrated with
the Erlang driver thread API. All functions that create entities
take a name argument. Currently the name argument is unused, but it
will be used when the debug functionality is implemented. If you
name all entities created well, the debug functionality will be
able to give you better error reports.
Adding/removing drivers:
A driver can add and later remove drivers.
Monitoring processes:
A driver can monitor a process that does not own a port.
Version management:
Version management is enabled for drivers that have set the ex-
tended_marker field of their driver_entry to ERL_DRV_EX-
TENDED_MARKER. erl_driver.h defines:
* ERL_DRV_EXTENDED_MARKER
* ERL_DRV_EXTENDED_MAJOR_VERSION, which is incremented when driver
incompatible changes are made to the Erlang runtime system. Nor-
mally it suffices to recompile drivers when ERL_DRV_EXTENDED_MA-
JOR_VERSION has changed, but it can, under rare circumstances,
mean that drivers must be slightly modified. If so, this will of
course be documented.
* ERL_DRV_EXTENDED_MINOR_VERSION, which is incremented when new
features are added. The runtime system uses the minor version of
the driver to determine what features to use.
The runtime system normally refuses to load a driver if the major
versions differ, or if the major versions are equal and the minor
version used by the driver is greater than the one used by the run-
time system. Old drivers with lower major versions are however al-
lowed after a bump of the major version during a transition period
of two major releases. Such old drivers can, however, fail if dep-
recated features are used.
The emulator refuses to load a driver that does not use the ex-
tended driver interface, to allow for 64-bit capable drivers, as
incompatible type changes for the callbacks output, control, and
call were introduced in Erlang/OTP R15B. A driver written with the
old types would compile with warnings and when called return
garbage sizes to the emulator, causing it to read random memory and
create huge incorrect result blobs.
Therefore it is not enough to only recompile drivers written with
version management for pre R15B types; the types must be changed in
the driver suggesting other rewrites, especially regarding size
variables. Investigate all warnings when recompiling.
Also, the API driver functions driver_output* and
driver_vec_to_buf, driver_alloc/realloc*, and the driver_* queue
functions were changed to have larger length arguments and return
values. This is a lesser problem, as code that passes smaller types
gets them auto-converted in the calls, and as long as the driver
does not handle sizes that overflow an int, all will work as be-
fore.
Time measurement:
Support for time measurement in drivers:
* ErlDrvTime
* ErlDrvTimeUnit
* erl_drv_monotonic_time
* erl_drv_time_offset
* erl_drv_convert_time_unit
REWRITES FOR 64-BIT DRIVER INTERFACE
ERTS 5.9 introduced two new integer types, ErlDrvSizeT and ErlDrvS-
SizeT, which can hold 64-bit sizes if necessary.
To not update a driver and only recompile, it probably works when
building for a 32-bit machine creating a false sense of security. Hope-
fully that will generate many important warnings. But when recompiling
the same driver later on for a 64-bit machine, there will be warnings
and almost certainly crashes. So it is a bad idea to postpone updating
the driver and not fixing the warnings.
When recompiling with gcc, use flag -Wstrict-prototypes to get better
warnings. Try to find a similar flag if you use another compiler.
The following is a checklist for rewriting a pre ERTS 5.9 driver, most
important first:
Return types for driver callbacks:
Rewrite driver callback control to use return type ErlDrvSSizeT in-
stead of int.
Rewrite driver callback call to use return type ErlDrvSSizeT in-
stead of int.
Note:
These changes are essential not to crash the emulator or worse cause
malfunction. Without them a driver can return garbage in the high 32
bits to the emulator, causing it to build a huge result from random
bytes, either crashing on memory allocation or succeeding with a ran-
dom result from the driver call.
Arguments to driver callbacks:
Driver callback output now gets ErlDrvSizeT as 3rd argument instead
of previously int.
Driver callback control now gets ErlDrvSizeT as 4th and 6th argu-
ments instead of previously int.
Driver callback call now gets ErlDrvSizeT as 4th and 6th arguments
instead of previously int.
Sane compiler's calling conventions probably make these changes
necessary only for a driver to handle data chunks that require
64-bit size fields (mostly larger than 2 GB, as that is what an int
of 32 bits can hold). But it is possible to think of non-sane call-
ing conventions that would make the driver callbacks mix up the ar-
guments causing malfunction.
Note:
The argument type change is from signed to unsigned. This can cause
problems for, for example, loop termination conditions or error con-
ditions if you only change the types all over the place.
Larger size field in ErlIOVec:
The size field in ErlIOVec has been changed to ErlDrvSizeT from
int. Check all code that use that field.
Automatic type-casting probably makes these changes necessary only
for a driver that encounters sizes > 32 bits.
Note:
The size field changed from signed to unsigned. This can cause prob-
lems for, for example, loop termination conditions or error condi-
tions if you only change the types all over the place.
Arguments and return values in the driver API:
Many driver API functions have changed argument type and/or return
value to ErlDrvSizeT from mostly int. Automatic type-casting proba-
bly makes these changes necessary only for a driver that encounters
sizes > 32 bits.
driver_output:
3rd argument
driver_output2:
3rd and 5th arguments
driver_output_binary:
3rd, 5th, and 6th arguments
driver_outputv:
3rd and 5th arguments
driver_vec_to_buf:
3rd argument and return value
driver_alloc:
1st argument
driver_realloc:
2nd argument
driver_alloc_binary:
1st argument
driver_realloc_binary:
2nd argument
driver_enq:
3rd argument
driver_pushq:
3rd argument
driver_deq:
2nd argument and return value
driver_sizeq:
Return value
driver_enq_bin:
3rd and 4th arguments
driver_pushq_bin:
3rd and 4th arguments
driver_enqv:
3rd argument
driver_pushqv:
3rd argument
driver_peekqv:
Return value
Note:
This is a change from signed to unsigned. This can cause problems
for, for example, loop termination conditions and error conditions if
you only change the types all over the place.
DATA TYPES
ErlDrvSizeT:
An unsigned integer type to be used as size_t.
ErlDrvSSizeT:
A signed integer type, the size of ErlDrvSizeT.
ErlDrvSysInfo:
typedef struct ErlDrvSysInfo {
int driver_major_version;
int driver_minor_version;
char *erts_version;
char *otp_release;
int thread_support;
int smp_support;
int async_threads;
int scheduler_threads;
int nif_major_version;
int nif_minor_version;
int dirty_scheduler_support;
} ErlDrvSysInfo;
The ErlDrvSysInfo structure is used for storage of information
about the Erlang runtime system. driver_system_info writes the sys-
tem information when passed a reference to a ErlDrvSysInfo struc-
ture. The fields in the structure are as follows:
driver_major_version:
The value of ERL_DRV_EXTENDED_MAJOR_VERSION when the runtime sys-
tem was compiled. This value is the same as the value of
ERL_DRV_EXTENDED_MAJOR_VERSION used when compiling the driver;
otherwise the runtime system would have refused to load the
driver.
driver_minor_version:
The value of ERL_DRV_EXTENDED_MINOR_VERSION when the runtime sys-
tem was compiled. This value can differ from the value of
ERL_DRV_EXTENDED_MINOR_VERSION used when compiling the driver.
erts_version:
A string containing the version number of the runtime system (the
same as returned by erlang:system_info(version)).
otp_release:
A string containing the OTP release number (the same as returned
by erlang:system_info(otp_release)).
thread_support:
A value != 0 if the runtime system has thread support; otherwise
0.
smp_support:
A value != 0 if the runtime system has SMP support; otherwise 0.
async_threads:
The number of async threads in the async thread pool used by
driver_async (the same as returned by erlang:sys-
tem_info(thread_pool_size)).
scheduler_threads:
The number of scheduler threads used by the runtime system (the
same as returned by erlang:system_info(schedulers)).
nif_major_version:
The value of ERL_NIF_MAJOR_VERSION when the runtime system was
compiled.
nif_minor_version:
The value of ERL_NIF_MINOR_VERSION when the runtime system was
compiled.
dirty_scheduler_support:
A value != 0 if the runtime system has support for dirty sched-
uler threads; otherwise 0.
ErlDrvBinary:
typedef struct ErlDrvBinary {
ErlDrvSint orig_size;
char orig_bytes[];
} ErlDrvBinary;
The ErlDrvBinary structure is a binary, as sent between the emula-
tor and the driver. All binaries are reference counted; when
driver_binary_free is called, the reference count is decremented,
when it reaches zero, the binary is deallocated. orig_size is the
binary size and orig_bytes is the buffer. ErlDrvBinary has not a
fixed size, its size is orig_size + 2 * sizeof(int).
Note:
The refc field has been removed. The reference count of an ErlDrvBi-
nary is now stored elsewhere. The reference count of an ErlDrvBinary
can be accessed through driver_binary_get_refc, driver_bi-
nary_inc_refc, and driver_binary_dec_refc.
Some driver calls, such as driver_enq_binary, increment the driver
reference count, and others, such as driver_deq decrement it.
Using a driver binary instead of a normal buffer is often faster,
as the emulator needs not to copy the data, only the pointer is
used.
A driver binary allocated in the driver, with driver_alloc_binary,
is to be freed in the driver (unless otherwise stated) with
driver_free_binary. (Notice that this does not necessarily deallo-
cate it, if the driver is still referred in the emulator, the ref-
count will not go to zero.)
Driver binaries are used in the driver_output2 and driver_outputv
calls, and in the queue. Also the driver callback outputv uses
driver binaries.
If the driver for some reason wants to keep a driver binary around,
for example in a static variable, the reference count is to be in-
cremented, and the binary can later be freed in the stop callback,
with driver_free_binary.
Notice that as a driver binary is shared by the driver and the emu-
lator. A binary received from the emulator or sent to the emulator
must not be changed by the driver.
Since ERTS 5.5 (Erlang/OTP R11B), orig_bytes is guaranteed to be
properly aligned for storage of an array of doubles (usually 8-byte
aligned).
ErlDrvData:
A handle to driver-specific data, passed to the driver callbacks.
It is a pointer, and is most often type cast to a specific pointer
in the driver.
SysIOVec:
A system I/O vector, as used by writev on Unix and WSASend on
Win32. It is used in ErlIOVec.
ErlIOVec:
typedef struct ErlIOVec {
int vsize;
ErlDrvSizeT size;
SysIOVec* iov;
ErlDrvBinary** binv;
} ErlIOVec;
The I/O vector used by the emulator and drivers is a list of bina-
ries, with a SysIOVec pointing to the buffers of the binaries. It
is used in driver_outputv and the outputv driver callback. Also,
the driver queue is an ErlIOVec.
ErlDrvMonitor:
When a driver creates a monitor for a process, a ErlDrvMonitor is
filled in. This is an opaque data type that can be assigned to, but
not compared without using the supplied compare function (that is,
it behaves like a struct).
The driver writer is to provide the memory for storing the monitor
when calling driver_monitor_process. The address of the data is not
stored outside of the driver, so ErlDrvMonitor can be used as any
other data, it can be copied, moved in memory, forgotten, and so
on.
ErlDrvNowData:
The ErlDrvNowData structure holds a time stamp consisting of three
values measured from some arbitrary point in the past. The three
structure members are:
megasecs:
The number of whole megaseconds elapsed since the arbitrary point
in time
secs:
The number of whole seconds elapsed since the arbitrary point in
time
microsecs:
The number of whole microseconds elapsed since the arbitrary
point in time
ErlDrvPDL:
If certain port-specific data must be accessed from other threads
than those calling the driver callbacks, a port data lock can be
used to synchronize the operations on the data. Currently, the only
port-specific data that the emulator associates with the port data
lock is the driver queue.
Normally a driver instance has no port data lock. If the driver in-
stance wants to use a port data lock, it must create the port data
lock by calling driver_pdl_create.
Note:
Once the port data lock has been created, every access to data asso-
ciated with the port data lock must be done while the port data lock
is locked. The port data lock is locked and unlocked by
driver_pdl_lock, and driver_pdl_unlock, respectively.
A port data lock is reference counted, and when the reference count
reaches zero, it is destroyed. The emulator at least increments the
reference count once when the lock is created and decrements it
once the port associated with the lock terminates. The emulator
also increments the reference count when an async job is enqueued
and decrements it when an async job has been invoked. Also, the
driver is responsible for ensuring that the reference count does
not reach zero before the last use of the lock by the driver has
been made. The reference count can be read, incremented, and decre-
mented by driver_pdl_get_refc, driver_pdl_inc_refc, and
driver_pdl_dec_refc, respectively.
ErlDrvTid:
Thread identifier.
See also erl_drv_thread_create, erl_drv_thread_exit,
erl_drv_thread_join, erl_drv_thread_self, and erl_drv_equal_tids.
ErlDrvThreadOpts:
int suggested_stack_size;
Thread options structure passed to erl_drv_thread_create. The fol-
lowing field exists:
suggested_stack_size:
A suggestion, in kilowords, on how large a stack to use. A value
< 0 means default size.
See also erl_drv_thread_opts_create, erl_drv_thread_opts_destroy,
and erl_drv_thread_create.
ErlDrvMutex:
Mutual exclusion lock. Used for synchronizing access to shared
data. Only one thread at a time can lock a mutex.
See also erl_drv_mutex_create, erl_drv_mutex_destroy, erl_drv_mu-
tex_lock, erl_drv_mutex_trylock, and erl_drv_mutex_unlock.
ErlDrvCond:
Condition variable. Used when threads must wait for a specific con-
dition to appear before continuing execution. Condition variables
must be used with associated mutexes.
See also erl_drv_cond_create, erl_drv_cond_destroy,
erl_drv_cond_signal, erl_drv_cond_broadcast, and erl_drv_cond_wait.
ErlDrvRWLock:
Read/write lock. Used to allow multiple threads to read shared data
while only allowing one thread to write the same data. Multiple
threads can read lock an rwlock at the same time, while only one
thread can read/write lock an rwlock at a time.
See also erl_drv_rwlock_create, erl_drv_rwlock_destroy,
erl_drv_rwlock_rlock, erl_drv_rwlock_tryrlock, erl_drv_rwlock_run-
lock, erl_drv_rwlock_rwlock, erl_drv_rwlock_tryrwlock, and
erl_drv_rwlock_rwunlock.
ErlDrvTSDKey:
Key that thread-specific data can be associated with.
See also erl_drv_tsd_key_create, erl_drv_tsd_key_destroy,
erl_drv_tsd_set, and erl_drv_tsd_get.
ErlDrvTime:
A signed 64-bit integer type for time representation.
ErlDrvTimeUnit:
An enumeration of time units supported by the driver API:
ERL_DRV_SEC:
Seconds
ERL_DRV_MSEC:
Milliseconds
ERL_DRV_USEC:
Microseconds
ERL_DRV_NSEC:
Nanoseconds
EXPORTS
void add_driver_entry(ErlDrvEntry
*de)
Adds a driver entry to the list of drivers known by Erlang. The
init function of parameter de is called.
Note:
To use this function for adding drivers residing in dynamically
loaded code is dangerous. If the driver code for the added
driver resides in the same dynamically loaded module (that is,
.so file) as a normal dynamically loaded driver (loaded with the
erl_ddll interface), the caller is to call driver_lock_driver
before adding driver entries.
Use of this function is generally deprecated.
void *driver_alloc(ErlDrvSizeT size)
Allocates a memory block of the size specified in size, and re-
turns it. This fails only on out of memory, in which case NULL
is returned. (This is most often a wrapper for malloc).
Memory allocated must be explicitly freed with a corresponding
call to driver_free (unless otherwise stated).
This function is thread-safe.
ErlDrvBinary *driver_alloc_binary(ErlDrvSizeT size)
Allocates a driver binary with a memory block of at least size
bytes, and returns a pointer to it, or NULL on failure (out of
memory). When a driver binary has been sent to the emulator, it
must not be changed. Every allocated binary is to be freed by a
corresponding call to driver_free_binary (unless otherwise
stated).
Notice that a driver binary has an internal reference counter.
This means that calling driver_free_binary, it may not actually
dispose of it. If it is sent to the emulator, it can be refer-
enced there.
The driver binary has a field, orig_bytes, which marks the start
of the data in the binary.
This function is thread-safe.
long driver_async(ErlDrvPort port, unsigned
int* key, void (*async_invoke)(void*), void* async_data, void
(*async_free)(void*))
Performs an asynchronous call. The function async_invoke is in-
voked in a thread separate from the emulator thread. This en-
ables the driver to perform time-consuming, blocking operations
without blocking the emulator.
The async thread pool size can be set with command-line argument
+A in erl(1). If an async thread pool is unavailable, the call
is made synchronously in the thread calling driver_async. The
current number of async threads in the async thread pool can be
retrieved through driver_system_info.
If a thread pool is available, a thread is used. If argument key
is NULL, the threads from the pool are used in a round-robin
way, each call to driver_async uses the next thread in the pool.
With argument key set, this behavior is changed. The two same
values of *key always get the same thread.
To ensure that a driver instance always uses the same thread,
the following call can be used:
unsigned int myKey = driver_async_port_key(myPort);
r = driver_async(myPort, &myKey, myData, myFunc);
It is enough to initialize myKey once for each driver instance.
If a thread is already working, the calls are queued up and exe-
cuted in order. Using the same thread for each driver instance
ensures that the calls are made in sequence.
The async_data is the argument to the functions async_invoke and
async_free. It is typically a pointer to a structure containing
a pipe or event that can be used to signal that the async opera-
tion completed. The data is to be freed in async_free.
When the async operation is done, ready_async driver entry func-
tion is called. If ready_async is NULL in the driver entry, the
async_free function is called instead.
The return value is -1 if the driver_async call fails.
Note:
As from ERTS 5.5.4.3 the default stack size for threads in the
async-thread pool is 16 kilowords, that is, 64 kilobyte on
32-bit architectures. This small default size has been chosen
because the amount of async-threads can be quite large. The de-
fault stack size is enough for drivers delivered with Er-
lang/OTP, but is possibly not sufficiently large for other dy-
namically linked-in drivers that use the driver_async function-
ality. A suggested stack size for threads in the async-thread
pool can be configured through command-line argument +a in
erl(1).
unsigned int driver_async_port_key(ErlDrvPort
port)
Calculates a key for later use in driver_async. The keys are
evenly distributed so that a fair mapping between port IDs and
async thread IDs is achieved.
Note:
Before Erlang/OTP R16, the port ID could be used as a key with
proper casting, but after the rewrite of the port subsystem,
this is no longer the case. With this function, you can achieve
the same distribution based on port IDs as before Erlang/OTP
R16.
long driver_binary_dec_refc(ErlDrvBinary *bin)
Decrements the reference count on bin and returns the reference
count reached after the decrement.
This function is thread-safe.
Note:
The reference count of driver binary is normally to be decre-
mented by calling driver_free_binary.
driver_binary_dec_refc does not free the binary if the reference
count reaches zero. Only use driver_binary_dec_refc when you are
sure not to reach a reference count of zero.
long driver_binary_get_refc(ErlDrvBinary *bin)
Returns the current reference count on bin.
This function is thread-safe.
long driver_binary_inc_refc(ErlDrvBinary *bin)
Increments the reference count on bin and returns the reference
count reached after the increment.
This function is thread-safe.
ErlDrvTermData driver_caller(ErlDrvPort
port)
Returns the process ID of the process that made the current call
to the driver. The process ID can be used with driver_send_term
to send back data to the caller. driver_caller only returns
valid data when currently executing in one of the following
driver callbacks:
start:
Called from erlang:open_port/2.
output:
Called from erlang:send/2 and erlang:port_command/2.
outputv:
Called from erlang:send/2 and erlang:port_command/2.
control:
Called from erlang:port_control/3.
call:
Called from erlang:port_call/3.
Notice that this function is not thread-safe, not even when the
emulator with SMP support is used.
int driver_cancel_timer(ErlDrvPort port)
Cancels a timer set with driver_set_timer.
The return value is 0.
int driver_compare_monitors(const ErlDrvMonitor
*monitor1, const ErlDrvMonitor *monitor2)
Compares two ErlDrvMonitors. Can also be used to imply some ar-
tificial order on monitors, for whatever reason.
Returns 0 if monitor1 and monitor2 are equal, < 0 if monitor1 <
monitor2, and > 0 if monitor1 > monitor2.
ErlDrvTermData driver_connected(ErlDrvPort
port)
Returns the port owner process.
Notice that this function is not thread-safe, not even when the
emulator with SMP support is used.
ErlDrvPort driver_create_port(ErlDrvPort port,
ErlDrvTermData owner_pid, char* name,
ErlDrvData drv_data)
Creates a new port executing the same driver code as the port
creating the new port.
port:
The port handle of the port (driver instance) creating the
new port.
owner_pid:
The process ID of the Erlang process to become owner of the
new port. This process will be linked to the new port. You
usually want to use driver_caller(port) as owner_pid.
name:
The port name of the new port. You usually want to use the
same port name as the driver name (driver_name field of the
driver_entry).
drv_data:
The driver-defined handle that is passed in later calls to
driver callbacks. Notice that the driver start callback is
not called for this new driver instance. The driver-defined
handle is normally created in the driver start callback when
a port is created through erlang:open_port/2.
The caller of driver_create_port is allowed to manipulate the
newly created port when driver_create_port has returned. When
port level locking is used, the creating port is only allowed to
manipulate the newly created port until the current driver call-
back, which was called by the emulator, returns.
int driver_demonitor_process(ErlDrvPort port,
const ErlDrvMonitor *monitor)
Cancels a monitor created earlier.
Returns 0 if a monitor was removed and > 0 if the monitor no
longer exists.
ErlDrvSizeT driver_deq(ErlDrvPort port,
ErlDrvSizeT size)
Dequeues data by moving the head pointer forward in the driver
queue by size bytes. The data in the queue is deallocated.
Returns the number of bytes remaining in the queue on success,
otherwise -1.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
int driver_enq(ErlDrvPort port, char* buf,
ErlDrvSizeT len)
Enqueues data in the driver queue. The data in buf is copied
(len bytes) and placed at the end of the driver queue. The
driver queue is normally used in a FIFO way.
The driver queue is available to queue output from the emulator
to the driver (data from the driver to the emulator is queued by
the emulator in normal Erlang message queues). This can be use-
ful if the driver must wait for slow devices, and so on, and
wants to yield back to the emulator. The driver queue is imple-
mented as an ErlIOVec.
When the queue contains data, the driver does not close until
the queue is empty.
The return value is 0.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
int driver_enq_bin(ErlDrvPort port,
ErlDrvBinary *bin, ErlDrvSizeT offset, ErlDrvSizeT len)
Enqueues a driver binary in the driver queue. The data in bin at
offset with length len is placed at the end of the queue. This
function is most often faster than driver_enq, because no data
must be copied.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
The return value is 0.
int driver_enqv(ErlDrvPort port, ErlIOVec *ev,
ErlDrvSizeT skip)
Enqueues the data in ev, skipping the first skip bytes of it, at
the end of the driver queue. It is faster than driver_enq, be-
cause no data must be copied.
The return value is 0.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
int driver_failure(ErlDrvPort port, int
error)
int driver_failure_atom(ErlDrvPort port, char
*string)
int driver_failure_posix(ErlDrvPort port, int
error)
Signals to Erlang that the driver has encountered an error and
is to be closed. The port is closed and the tuple {'EXIT', er-
ror, Err} is sent to the port owner process, where error is an
error atom (driver_failure_atom and driver_failure_posix) or an
integer (driver_failure).
The driver is to fail only when in severe error situations, when
the driver cannot possibly keep open, for example, buffer allo-
cation gets out of memory. For normal errors it is more appro-
priate to send error codes with driver_output.
The return value is 0.
int driver_failure_eof(ErlDrvPort
port)
Signals to Erlang that the driver has encountered an EOF and is
to be closed, unless the port was opened with option eof, in
which case eof is sent to the port. Otherwise the port is closed
and an 'EXIT' message is sent to the port owner process.
The return value is 0.
void driver_free(void *ptr)
Frees the memory pointed to by ptr. The memory is to have been
allocated with driver_alloc. All allocated memory is to be de-
allocated, only once. There is no garbage collection in drivers.
This function is thread-safe.
void driver_free_binary(ErlDrvBinary *bin)
Frees a driver binary bin, allocated previously with driver_al-
loc_binary. As binaries in Erlang are reference counted, the bi-
nary can still be around.
This function is thread-safe.
ErlDrvTermData driver_get_monitored_process(ErlDrvPort port, const
ErlDrvMonitor *monitor)
Returns the process ID associated with a living monitor. It can
be used in the process_exit callback to get the process identi-
fication for the exiting process.
Returns driver_term_nil if the monitor no longer exists.
int driver_get_now(ErlDrvNowData *now)
Warning:
This function is deprecated. Do not use it. Use erl_drv_mono-
tonic_time (perhaps in combination with erl_drv_time_offset) in-
stead.
Reads a time stamp into the memory pointed to by parameter now.
For information about specific fields, see ErlDrvNowData.
The return value is 0, unless the now pointer is invalid, in
which case it is < 0.
int driver_lock_driver(ErlDrvPort
port)
Locks the driver used by the port port in memory for the rest of
the emulator process' lifetime. After this call, the driver be-
haves as one of Erlang's statically linked-in drivers.
ErlDrvTermData driver_mk_atom(char*
string)
Returns an atom given a name string. The atom is created and
does not change, so the return value can be saved and reused,
which is faster than looking up the atom several times.
Notice that this function is not thread-safe, not even when the
emulator with SMP support is used.
ErlDrvTermData driver_mk_port(ErlDrvPort
port)
Converts a port handle to the Erlang term format, usable in
erl_drv_output_term and erl_drv_send_term.
Notice that this function is not thread-safe, not even when the
emulator with SMP support is used.
int driver_monitor_process(ErlDrvPort port,
ErlDrvTermData process, ErlDrvMonitor *monitor)
Starts monitoring a process from a driver. When a process is
monitored, a process exit results in a call to the provided
process_exit callback in the ErlDrvEntry structure. The ErlDrv-
Monitor structure is filled in, for later removal or compare.
Parameter process is to be the return value of an earlier call
to driver_caller or driver_connected call.
Returns 0 on success, < 0 if no callback is provided, and > 0 if
the process is no longer alive.
int driver_output(ErlDrvPort port, char *buf,
ErlDrvSizeT len)
Sends data from the driver up to the emulator. The data is re-
ceived as terms or binary data, depending on how the driver port
was opened.
The data is queued in the port owner process' message queue. No-
tice that this does not yield to the emulator (as the driver and
the emulator run in the same thread).
Parameter buf points to the data to send, and len is the number
of bytes.
The return value for all output functions is 0 for normal use.
If the driver is used for distribution, it can fail and return
-1.
int driver_output_binary(ErlDrvPort port, char
*hbuf, ErlDrvSizeT hlen, ErlDrvBinary* bin, ErlDrvSizeT offset,
ErlDrvSizeT len)
Sends data to a port owner process from a driver binary. It has
a header buffer (hbuf and hlen) just like driver_output2. Param-
eter hbuf can be NULL.
Parameter offset is an offset into the binary and len is the
number of bytes to send.
Driver binaries are created with driver_alloc_binary.
The data in the header is sent as a list and the binary as an
Erlang binary in the tail of the list.
For example, if hlen is 2, the port owner process receives [H1,
H2 | <<T>>].
The return value is 0 for normal use.
Notice that, using the binary syntax in Erlang, the driver ap-
plication can match the header directly from the binary, so the
header can be put in the binary, and hlen can be set to 0.
int driver_output_term(ErlDrvPort port,
ErlDrvTermData* term, int n)
Warning:
This function is deprecated. Use erl_drv_output_terminstead.
Parameters term and n work as in erl_drv_output_term.
Notice that this function is not thread-safe, not even when the
emulator with SMP support is used.
int driver_output2(ErlDrvPort port, char *hbuf,
ErlDrvSizeT hlen, char *buf, ErlDrvSizeT len)
First sends hbuf (length in hlen) data as a list, regardless of
port settings. Then sends buf as a binary or list. For example,
if hlen is 3, the port owner process receives [H1, H2, H3 | T].
The point of sending data as a list header, is to facilitate
matching on the data received.
The return value is 0 for normal use.
int driver_outputv(ErlDrvPort port, char* hbuf,
ErlDrvSizeT hlen, ErlIOVec *ev, ErlDrvSizeT skip)
Sends data from an I/O vector, ev, to the port owner process. It
has a header buffer (hbuf and hlen), just like driver_output2.
Parameter skip is a number of bytes to skip of the ev vector
from the head.
You get vectors of ErlIOVec type from the driver queue (see be-
low), and the outputv driver entry function. You can also make
them yourself, if you want to send several ErlDrvBinary buffers
at once. Often it is faster to use driver_output or .
For example, if hlen is 2 and ev points to an array of three bi-
naries, the port owner process receives [H1, H2, <<B1>>, <<B2>>
| <<B3>>].
The return value is 0 for normal use.
The comment for driver_output_binary also applies for
driver_outputv.
ErlDrvPDL driver_pdl_create(ErlDrvPort port)
Creates a port data lock associated with the port.
Note:
Once a port data lock has been created, it must be locked during
all operations on the driver queue of the port.
Returns a newly created port data lock on success, otherwise
NULL. The function fails if port is invalid or if a port data
lock already has been associated with the port.
long driver_pdl_dec_refc(ErlDrvPDL
pdl)
Decrements the reference count of the port data lock passed as
argument (pdl).
The current reference count after the decrement has been per-
formed is returned.
This function is thread-safe.
long driver_pdl_get_refc(ErlDrvPDL pdl)
Returns the current reference count of the port data lock passed
as argument (pdl).
This function is thread-safe.
long driver_pdl_inc_refc(ErlDrvPDL pdl)
Increments the reference count of the port data lock passed as
argument (pdl).
The current reference count after the increment has been per-
formed is returned.
This function is thread-safe.
void driver_pdl_lock(ErlDrvPDL pdl)
Locks the port data lock passed as argument (pdl).
This function is thread-safe.
void driver_pdl_unlock(ErlDrvPDL pdl)
Unlocks the port data lock passed as argument (pdl).
This function is thread-safe.
SysIOVec *driver_peekq(ErlDrvPort port, int
*vlen)
Retrieves the driver queue as a pointer to an array of Sys-
IOVecs. It also returns the number of elements in vlen. This is
one of two ways to get data out of the queue.
Nothing is removed from the queue by this function, that must be
done with driver_deq.
The returned array is suitable to use with the Unix system call
writev.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
ErlDrvSizeT driver_peekqv(ErlDrvPort port,
ErlIOVec *ev)
Retrieves the driver queue into a supplied ErlIOVec ev. It also
returns the queue size. This is one of two ways to get data out
of the queue.
If ev is NULL, all ones that is -1 type cast to ErlDrvSizeT are
returned.
Nothing is removed from the queue by this function, that must be
done with driver_deq.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
int driver_pushq(ErlDrvPort port, char* buf,
ErlDrvSizeT len)
Puts data at the head of the driver queue. The data in buf is
copied (len bytes) and placed at the beginning of the queue.
The return value is 0.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
int driver_pushq_bin(ErlDrvPort port,
ErlDrvBinary *bin, ErlDrvSizeT offset, ErlDrvSizeT len)
Puts data in the binary bin, at offset with length len at the
head of the driver queue. It is most often faster than
driver_pushq, because no data must be copied.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
The return value is 0.
int driver_pushqv(ErlDrvPort port, ErlIOVec
*ev, ErlDrvSizeT skip)
Puts the data in ev, skipping the first skip bytes of it, at the
head of the driver queue. It is faster than driver_pushq, be-
cause no data must be copied.
The return value is 0.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
int driver_read_timer(ErlDrvPort port, unsigned
long *time_left)
Reads the current time of a timer, and places the result in
time_left. This is the time in milliseconds, before the time-out
occurs.
The return value is 0.
void *driver_realloc(void *ptr, ErlDrvSizeT size)
Resizes a memory block, either in place, or by allocating a new
block, copying the data, and freeing the old block. A pointer is
returned to the reallocated memory. On failure (out of memory),
NULL is returned. (This is most often a wrapper for realloc.)
This function is thread-safe.
ErlDrvBinary *driver_realloc_binary(ErlDrvBinary *bin, ErlDrvSizeT
size)
Resizes a driver binary, while keeping the data.
Returns the resized driver binary on success. Returns NULL on
failure (out of memory).
This function is thread-safe.
int driver_select(ErlDrvPort port, ErlDrvEvent
event, int mode, int on)
This function is used by drivers to provide the emulator with
events to check for. This enables the emulator to call the
driver when something has occurred asynchronously.
Parameter event identifies an OS-specific event object. On Unix
systems, the functions select/poll are used. The event object
must be a socket or pipe (or other object that select/poll can
use). On Windows, the Win32 API function WaitForMultipleObjects
is used. This places other restrictions on the event object; see
the Win32 SDK documentation.
Parameter on is to be 1 for setting events and 0 for clearing
them.
Parameter mode is a bitwise OR combination of ERL_DRV_READ,
ERL_DRV_WRITE, and ERL_DRV_USE. The first two specify whether to
wait for read events and/or write events. A fired read event
calls ready_input and a fired write event calls ready_output.
Note:
Some OS (Windows) do not differentiate between read and write
events. The callback for a fired event then only depends on the
value of mode.
ERL_DRV_USE specifies if we are using the event object or if we
want to close it. On an emulator with SMP support, it is not
safe to clear all events and then close the event object after
driver_select has returned. Another thread can still be using
the event object internally. To safely close an event object,
call driver_select with ERL_DRV_USE and on==0, which clears all
events and then either calls stop_select or schedules it to be
called when it is safe to close the event object. ERL_DRV_USE is
to be set together with the first event for an event object. It
is harmless to set ERL_DRV_USE even if it already has been done.
Clearing all events but keeping ERL_DRV_USE set indicates that
we are using the event object and probably will set events for
it again.
Note:
ERL_DRV_USE was added in Erlang/OTP R13. Old drivers still work
as before, but it is recommended to update them to use
ERL_DRV_USE and stop_select to ensure that event objects are
closed in a safe way.
The return value is 0, unless ready_input/ready_output is NULL,
in which case it is -1.
int driver_send_term(ErlDrvPort port,
ErlDrvTermData receiver, ErlDrvTermData* term, int n)
Warning:
This function is deprecated. Use erl_drv_send_term instead.
Note:
The parameters of this function cannot be properly checked by
the runtime system when executed by arbitrary threads. This can
cause the function not to fail when it should.
Parameters term and n work as in erl_drv_output_term.
This function is only thread-safe when the emulator with SMP
support is used.
int driver_set_timer(ErlDrvPort port, unsigned
long time)
Sets a timer on the driver, which will count down and call the
driver when it is timed out. Parameter time is the time in mil-
liseconds before the timer expires.
When the timer reaches 0 and expires, the driver entry function
timeout is called.
Notice that only one timer exists on each driver instance; set-
ting a new timer replaces an older one.
Return value is 0, unless the timeout driver function is NULL,
in which case it is -1.
ErlDrvSizeT driver_sizeq(ErlDrvPort port)
Returns the number of bytes currently in the driver queue.
This function can be called from any thread if a port data lock
associated with the port is locked by the calling thread during
the call.
void driver_system_info(ErlDrvSysInfo
*sys_info_ptr, size_t size)
Writes information about the Erlang runtime system into the Erl-
DrvSysInfo structure referred to by the first argument. The sec-
ond argument is to be the size of the ErlDrvSysInfo structure,
that is, sizeof(ErlDrvSysInfo).
For information about specific fields, see ErlDrvSysInfo.
ErlDrvSizeT driver_vec_to_buf(ErlIOVec *ev,
char *buf, ErlDrvSizeT len)
Collects several segments of data, referenced by ev, by copying
them in order to the buffer buf, of the size len.
If the data is to be sent from the driver to the port owner
process, it is faster to use driver_outputv.
The return value is the space left in the buffer, that is, if ev
contains less than len bytes it is the difference, and if ev
contains len bytes or more, it is 0. This is faster if there is
more than one header byte, as the binary syntax can construct
integers directly from the binary.
void erl_drv_busy_msgq_limits(ErlDrvPort port,
ErlDrvSizeT *low, ErlDrvSizeT *high)
Sets and gets limits that will be used for controlling the busy
state of the port message queue.
The port message queue is set into a busy state when the amount
of command data queued on the message queue reaches the high
limit. The port message queue is set into a not busy state when
the amount of command data queued on the message queue falls be-
low the low limit. Command data is in this context data passed
to the port using either Port ! {Owner, {command, Data}} or
port_command/[2,3]. Notice that these limits only concerns com-
mand data that have not yet reached the port. The busy port fea-
ture can be used for data that has reached the port.
Valid limits are values in the range [ERL_DRV_BUSY_MSGQ_LIM_MIN,
ERL_DRV_BUSY_MSGQ_LIM_MAX]. Limits are automatically adjusted to
be sane. That is, the system adjusts values so that the low
limit used is lower than or equal to the high limit used. By de-
fault the high limit is 8 kB and the low limit is 4 kB.
By passing a pointer to an integer variable containing the value
ERL_DRV_BUSY_MSGQ_READ_ONLY, the currently used limit is read
and written back to the integer variable. A new limit can be set
by passing a pointer to an integer variable containing a valid
limit. The passed value is written to the internal limit. The
internal limit is then adjusted. After this the adjusted limit
is written back to the integer variable from which the new value
was read. Values are in bytes.
The busy message queue feature can be disabled either by setting
the ERL_DRV_FLAG_NO_BUSY_MSGQ driver flag in the driver_entry
used by the driver, or by calling this function with
ERL_DRV_BUSY_MSGQ_DISABLED as a limit (either low or high). When
this feature has been disabled, it cannot be enabled again. When
reading the limits, both are ERL_DRV_BUSY_MSGQ_DISABLED if this
feature has been disabled.
Processes sending command data to the port are suspended if ei-
ther the port is busy or if the port message queue is busy. Sus-
pended processes are resumed when neither the port or the port
message queue is busy.
For information about busy port functionality, see
set_busy_port.
void erl_drv_cond_broadcast(ErlDrvCond
*cnd)
Broadcasts on a condition variable. That is, if other threads
are waiting on the condition variable being broadcast on, all of
them are woken.
cnd is a pointer to a condition variable to broadcast on.
This function is thread-safe.
ErlDrvCond *erl_drv_cond_create(char
*name)
Creates a condition variable and returns a pointer to it.
name is a string identifying the created condition variable. It
is used to identify the condition variable in planned future de-
bug functionality.
Returns NULL on failure. The driver creating the condition vari-
able is responsible for destroying it before the driver is un-
loaded.
This function is thread-safe.
void erl_drv_cond_destroy(ErlDrvCond
*cnd)
Destroys a condition variable previously created by
erl_drv_cond_create.
cnd is a pointer to a condition variable to destroy.
This function is thread-safe.
char *erl_drv_cond_name(ErlDrvCond
*cnd)
Returns a pointer to the name of the condition.
cnd is a pointer to an initialized condition.
Note:
This function is intended for debugging purposes only.
void erl_drv_cond_signal(ErlDrvCond
*cnd)
Signals on a condition variable. That is, if other threads are
waiting on the condition variable being signaled, one of them is
woken.
cnd is a pointer to a condition variable to signal on.
This function is thread-safe.
void erl_drv_cond_wait(ErlDrvCond *cnd,
ErlDrvMutex *mtx)
Waits on a condition variable. The calling thread is blocked un-
til another thread wakes it by signaling or broadcasting on the
condition variable. Before the calling thread is blocked, it un-
locks the mutex passed as argument. When the calling thread is
woken, it locks the same mutex before returning. That is, the
mutex currently must be locked by the calling thread when call-
ing this function.
cnd is a pointer to a condition variable to wait on. mtx is a
pointer to a mutex to unlock while waiting.
Note:
erl_drv_cond_wait can return even if no one has signaled or
broadcast on the condition variable. Code calling
erl_drv_cond_wait is always to be prepared for erl_drv_cond_wait
returning even if the condition that the thread was waiting for
has not occurred. That is, when returning from
erl_drv_cond_wait, always check if the condition has occurred,
and if not call erl_drv_cond_wait again.
This function is thread-safe.
int erl_drv_consume_timeslice(ErlDrvPort port,
int percent)
Gives the runtime system a hint about how much CPU time the cur-
rent driver callback call has consumed since the last hint, or
since the the start of the callback if no previous hint has been
given.
port:
Port handle of the executing port.
percent:
Approximate consumed fraction of a full time-slice in per-
cent.
The time is specified as a fraction, in percent, of a full time-
slice that a port is allowed to execute before it is to surren-
der the CPU to other runnable ports or processes. Valid range is
[1, 100]. The scheduling time-slice is not an exact entity, but
can usually be approximated to about 1 millisecond.
Notice that it is up to the runtime system to determine if and
how to use this information. Implementations on some platforms
can use other means to determine the consumed fraction of the
time-slice. Lengthy driver callbacks should, regardless of this,
frequently call this function to determine if it is allowed to
continue execution or not.
This function returns a non-zero value if the time-slice has
been exhausted, and zero if the callback is allowed to continue
execution. If a non-zero value is returned, the driver callback
is to return as soon as possible in order for the port to be
able to yield.
This function is provided to better support co-operative sched-
uling, improve system responsiveness, and to make it easier to
prevent misbehaviors of the VM because of a port monopolizing a
scheduler thread. It can be used when dividing lengthy work into
some repeated driver callback calls, without the need to use
threads.
See also the important warning text at the beginning of this
manual page.
ErlDrvTime erl_drv_convert_time_unit(ErlDrvTime
val, ErlDrvTimeUnit from, ErlDrvTimeUnit to)
Converts the val value of time unit from to the corresponding
value of time unit to. The result is rounded using the floor
function.
val:
Value to convert time unit for.
from:
Time unit of val.
to:
Time unit of returned value.
Returns ERL_DRV_TIME_ERROR if called with an invalid time unit
argument.
See also ErlDrvTime and ErlDrvTimeUnit.
int erl_drv_equal_tids(ErlDrvTid tid1,
ErlDrvTid tid2)
Compares two thread identifiers, tid1 and tid2, for equality.
Returns 0 it they are not equal, and a value not equal to 0 if
they are equal.
Note:
A thread identifier can be reused very quickly after a thread
has terminated. Therefore, if a thread corresponding to one of
the involved thread identifiers has terminated since the thread
identifier was saved, the result of erl_drv_equal_tids does pos-
sibly not give the expected result.
This function is thread-safe.
int erl_drv_getenv(const char *key, char
*value, size_t *value_size)
Retrieves the value of an environment variable.
key:
A NULL-terminated string containing the name of the environ-
ment variable.
value:
A pointer to an output buffer.
value_size:
A pointer to an integer. The integer is used both for pass-
ing input and output sizes (see below).
When this function is called, *value_size is to contain the size
of the value buffer.
On success, 0 is returned, the value of the environment variable
has been written to the value buffer, and *value_size contains
the string length (excluding the terminating NULL character) of
the value written to the value buffer.
On failure, that is, no such environment variable was found, a
value < 0 is returned. When the size of the value buffer is too
small, a value > 0 is returned and *value_size has been set to
the buffer size needed.
Warning:
This function reads the emulated environment used by os:getenv/1
and not the environment used by libc's getenv(3erl) or similar.
Drivers that require that these are in sync will need to do so
themselves, but keep in mind that they are segregated for a rea-
son; getenv(3erl) and its friends are not thread-safe and may
cause unrelated code to misbehave or crash the emulator.
This function is thread-safe.
void erl_drv_init_ack(ErlDrvPort port,
ErlDrvData res)
Acknowledges the start of the port.
port:
The port handle of the port (driver instance) doing the ac-
knowledgment.
res:
The result of the port initialization. Can be the same val-
ues as the return value of start, that is, any of the error
codes or the ErlDrvData that is to be used for this port.
When this function is called the initiating erlang:open_port
call is returned as if the start function had just been called.
It can only be used when flag ERL_DRV_FLAG_USE_INIT_ACK has been
set on the linked-in driver.
ErlDrvTime erl_drv_monotonic_time(ErlDrvTimeUnit time_unit)
Returns Erlang monotonic time. Notice that negative values are
not uncommon.
time_unit is time unit of returned value.
Returns ERL_DRV_TIME_ERROR if called with an invalid time unit
argument, or if called from a thread that is not a scheduler
thread.
See also ErlDrvTime and ErlDrvTimeUnit.
ErlDrvMutex *erl_drv_mutex_create(char
*name)
Creates a mutex and returns a pointer to it.
name is a string identifying the created mutex. It is used to
identify the mutex in debug functionality (see note).
Returns NULL on failure. The driver creating the mutex is re-
sponsible for destroying it before the driver is unloaded.
This function is thread-safe.
Note:
One such debug functionality is the lock checker, which can de-
tect locking order violations and thereby potential deadlock
bugs. For the lock checker to work the name should be on the
format "App.Type" or "App.Type[Instance]", where App is the name
of the application, Type is the name of the lock type and In-
stance is optional information about each lock instance.
"App.Type" should be a unique name for the lock checker to de-
tect lock order violations between locks of different types. The
Instance information is currently ignored.
For example, if we have mutexes of types "myapp.xtable" and
"myapp.xitem" then the lock checker will make sure either
"myapp.xtable" locks are never locked after "myapp.xitem" locks
or vice versa.
void erl_drv_mutex_destroy(ErlDrvMutex
*mtx)
Destroys a mutex previously created by erl_drv_mutex_create. The
mutex must be in an unlocked state before it is destroyed.
mtx is a pointer to a mutex to destroy.
This function is thread-safe.
void erl_drv_mutex_lock(ErlDrvMutex
*mtx)
Locks a mutex. The calling thread is blocked until the mutex has
been locked. A thread that has currently locked the mutex cannot
lock the same mutex again.
mtx is a pointer to a mutex to lock.
Warning:
If you leave a mutex locked in an emulator thread when you let
the thread out of your control, you will very likely deadlock
the whole emulator.
This function is thread-safe.
char *erl_drv_mutex_name(ErlDrvMutex
*mtx)
Returns a pointer to the mutex name.
mtx is a pointer to an initialized mutex.
Note:
This function is intended for debugging purposes only.
int erl_drv_mutex_trylock(ErlDrvMutex
*mtx)
Tries to lock a mutex. A thread that has currently locked the
mutex cannot try to lock the same mutex again.
mtx is a pointer to a mutex to try to lock.
Returns 0 on success, otherwise EBUSY.
Warning:
If you leave a mutex locked in an emulator thread when you let
the thread out of your control, you will very likely deadlock
the whole emulator.
This function is thread-safe.
void erl_drv_mutex_unlock(ErlDrvMutex
*mtx)
Unlocks a mutex. The mutex currently must be locked by the call-
ing thread.
mtx is a pointer to a mutex to unlock.
This function is thread-safe.
int erl_drv_output_term(ErlDrvTermData port,
ErlDrvTermData* term, int n)
Sends data in the special driver term format to the port owner
process. This is a fast way to deliver term data from a driver.
It needs no binary conversion, so the port owner process re-
ceives data as normal Erlang terms. The erl_drv_send_term func-
tions can be used for sending to any process on the local node.
Note:
Parameter port is not an ordinary port handle, but a port handle
converted using driver_mk_port.
Parameter term points to an array of ErlDrvTermData with n ele-
ments. This array contains terms described in the driver term
format. Every term consists of 1-4 elements in the array. The
first term has a term type and then arguments. Parameter port
specifies the sending port.
Tuples, maps, and lists (except strings, see below) are built in
reverse polish notation, so that to build a tuple, the elements
are specified first, and then the tuple term, with a count.
Likewise for lists and maps.
* A tuple must be specified with the number of elements. (The
elements precede the ERL_DRV_TUPLE term.)
* A map must be specified with the number of key-value pairs
N. The key-value pairs must precede the ERL_DRV_MAP in this
order: key1,value1,key2,value2,...,keyN,valueN. Duplicate
keys are not allowed.
* A list must be specified with the number of elements, in-
cluding the tail, which is the last term preceding
ERL_DRV_LIST.
The special term ERL_DRV_STRING_CONS is used to "splice" in a
string in a list, a string specified this way is not a list in
itself, but the elements are elements of the surrounding list.
Term type Arguments
--------- ---------
ERL_DRV_NIL
ERL_DRV_ATOM ErlDrvTermData atom (from driver_mk_atom(char *string))
ERL_DRV_INT ErlDrvSInt integer
ERL_DRV_UINT ErlDrvUInt integer
ERL_DRV_INT64 ErlDrvSInt64 *integer_ptr
ERL_DRV_UINT64 ErlDrvUInt64 *integer_ptr
ERL_DRV_PORT ErlDrvTermData port (from driver_mk_port(ErlDrvPort port))
ERL_DRV_BINARY ErlDrvBinary *bin, ErlDrvUInt len, ErlDrvUInt offset
ERL_DRV_BUF2BINARY char *buf, ErlDrvUInt len
ERL_DRV_STRING char *str, int len
ERL_DRV_TUPLE int sz
ERL_DRV_LIST int sz
ERL_DRV_PID ErlDrvTermData pid (from driver_connected(ErlDrvPort port)
or driver_caller(ErlDrvPort port))
ERL_DRV_STRING_CONS char *str, int len
ERL_DRV_FLOAT double *dbl
ERL_DRV_EXT2TERM char *buf, ErlDrvUInt len
ERL_DRV_MAP int sz
The unsigned integer data type ErlDrvUInt and the signed integer
data type ErlDrvSInt are 64 bits wide on a 64-bit runtime system
and 32 bits wide on a 32-bit runtime system. They were intro-
duced in ERTS 5.6 and replaced some of the int arguments in the
list above.
The unsigned integer data type ErlDrvUInt64 and the signed inte-
ger data type ErlDrvSInt64 are always 64 bits wide. They were
introduced in ERTS 5.7.4.
To build the tuple {tcp, Port, [100 | Binary]}, the following
call can be made.
ErlDrvBinary* bin = ...
ErlDrvPort port = ...
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("tcp"),
ERL_DRV_PORT, driver_mk_port(drvport),
ERL_DRV_INT, 100,
ERL_DRV_BINARY, bin, 50, 0,
ERL_DRV_LIST, 2,
ERL_DRV_TUPLE, 3,
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
Here bin is a driver binary of length at least 50 and drvport is
a port handle. Notice that ERL_DRV_LIST comes after the elements
of the list, likewise ERL_DRV_TUPLE.
The ERL_DRV_STRING_CONS term is a way to construct strings. It
works differently from how ERL_DRV_STRING works.
ERL_DRV_STRING_CONS builds a string list in reverse order (as
opposed to how ERL_DRV_LIST works), concatenating the strings
added to a list. The tail must be specified before
ERL_DRV_STRING_CONS.
ERL_DRV_STRING constructs a string, and ends it. (So it is the
same as ERL_DRV_NIL followed by ERL_DRV_STRING_CONS.)
/* to send [x, "abc", y] to the port: */
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("x"),
ERL_DRV_STRING, (ErlDrvTermData)"abc", 3,
ERL_DRV_ATOM, driver_mk_atom("y"),
ERL_DRV_NIL,
ERL_DRV_LIST, 4
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
/* to send "abc123" to the port: */
ErlDrvTermData spec[] = {
ERL_DRV_NIL, /* with STRING_CONS, the tail comes first */
ERL_DRV_STRING_CONS, (ErlDrvTermData)"123", 3,
ERL_DRV_STRING_CONS, (ErlDrvTermData)"abc", 3,
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
The ERL_DRV_EXT2TERM term type is used for passing a term en-
coded with the external format, that is, a term that has been
encoded by erlang:term_to_binary, erl_interface:ei(3erl), and so
on. For example, if binp is a pointer to an ErlDrvBinary that
contains term {17, 4711} encoded with the external format, and
you want to wrap it in a two-tuple with the tag my_tag, that is,
{my_tag, {17, 4711}}, you can do as follows:
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("my_tag"),
ERL_DRV_EXT2TERM, (ErlDrvTermData) binp->orig_bytes, binp->orig_size
ERL_DRV_TUPLE, 2,
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
To build the map #{key1 => 100, key2 => {200, 300}}, the follow-
ing call can be made.
ErlDrvPort port = ...
ErlDrvTermData spec[] = {
ERL_DRV_ATOM, driver_mk_atom("key1"),
ERL_DRV_INT, 100,
ERL_DRV_ATOM, driver_mk_atom("key2"),
ERL_DRV_INT, 200,
ERL_DRV_INT, 300,
ERL_DRV_TUPLE, 2,
ERL_DRV_MAP, 2
};
erl_drv_output_term(driver_mk_port(drvport), spec, sizeof(spec) / sizeof(spec[0]));
If you want to pass a binary and do not already have the content
of the binary in an ErlDrvBinary, you can benefit from using
ERL_DRV_BUF2BINARY instead of creating an ErlDrvBinary through
driver_alloc_binary and then pass the binary through ERL_DRV_BI-
NARY. The runtime system often allocates binaries smarter if
ERL_DRV_BUF2BINARY is used. However, if the content of the bi-
nary to pass already resides in an ErlDrvBinary, it is normally
better to pass the binary using ERL_DRV_BINARY and the ErlDrvBi-
nary in question.
The ERL_DRV_UINT, ERL_DRV_BUF2BINARY, and ERL_DRV_EXT2TERM term
types were introduced in ERTS 5.6.
This function is only thread-safe when the emulator with SMP
support is used.
int erl_drv_putenv(const char *key, char
*value)
Sets the value of an environment variable.
key is a NULL-terminated string containing the name of the envi-
ronment variable.
value is a NULL-terminated string containing the new value of
the environment variable.
Returns 0 on success, otherwise a value != 0.
Note:
The result of passing the empty string ("") as a value is plat-
form-dependent. On some platforms the variable value is set to
the empty string, on others the environment variable is removed.
Warning:
This function modifies the emulated environment used by
os:putenv/2 and not the environment used by libc's putenv(3erl)
or similar. Drivers that require that these are in sync will
need to do so themselves, but keep in mind that they are segre-
gated for a reason; putenv(3erl) and its friends are not thread-
safe and may cause unrelated code to misbehave or crash the emu-
lator.
This function is thread-safe.
ErlDrvRWLock *erl_drv_rwlock_create(char
*name)
Creates an rwlock and returns a pointer to it.
name is a string identifying the created rwlock. It is used to
identify the rwlock in debug functionality (see note about the
lock checker).
Returns NULL on failure. The driver creating the rwlock is re-
sponsible for destroying it before the driver is unloaded.
This function is thread-safe.
void erl_drv_rwlock_destroy(ErlDrvRWLock
*rwlck)
Destroys an rwlock previously created by erl_drv_rwlock_create.
The rwlock must be in an unlocked state before it is destroyed.
rwlck is a pointer to an rwlock to destroy.
This function is thread-safe.
char *erl_drv_rwlock_name(ErlDrvRWLock
*rwlck)
Returns a pointer to the name of the rwlock.
rwlck is a pointer to an initialized rwlock.
Note:
This function is intended for debugging purposes only.
void erl_drv_rwlock_rlock(ErlDrvRWLock
*rwlck)
Read locks an rwlock. The calling thread is blocked until the
rwlock has been read locked. A thread that currently has read or
read/write locked the rwlock cannot lock the same rwlock again.
rwlck is a pointer to the rwlock to read lock.
Warning:
If you leave an rwlock locked in an emulator thread when you let
the thread out of your control, you will very likely deadlock
the whole emulator.
This function is thread-safe.
void erl_drv_rwlock_runlock(ErlDrvRWLock
*rwlck)
Read unlocks an rwlock. The rwlock currently must be read locked
by the calling thread.
rwlck is a pointer to an rwlock to read unlock.
This function is thread-safe.
void erl_drv_rwlock_rwlock(ErlDrvRWLock
*rwlck)
Read/write locks an rwlock. The calling thread is blocked until
the rwlock has been read/write locked. A thread that currently
has read or read/write locked the rwlock cannot lock the same
rwlock again.
rwlck is a pointer to an rwlock to read/write lock.
Warning:
If you leave an rwlock locked in an emulator thread when you let
the thread out of your control, you will very likely deadlock
the whole emulator.
This function is thread-safe.
void erl_drv_rwlock_rwunlock(ErlDrvRWLock
*rwlck)
Read/write unlocks an rwlock. The rwlock currently must be
read/write locked by the calling thread.
rwlck is a pointer to an rwlock to read/write unlock.
This function is thread-safe.
int erl_drv_rwlock_tryrlock(ErlDrvRWLock
*rwlck)
Tries to read lock an rwlock.
rwlck is a pointer to an rwlock to try to read lock.
Returns 0 on success, otherwise EBUSY. A thread that currently
has read or read/write locked the rwlock cannot try to lock the
same rwlock again.
Warning:
If you leave an rwlock locked in an emulator thread when you let
the thread out of your control, you will very likely deadlock
the whole emulator.
This function is thread-safe.
int erl_drv_rwlock_tryrwlock(ErlDrvRWLock
*rwlck)
Tries to read/write lock an rwlock. A thread that currently has
read or read/write locked the rwlock cannot try to lock the same
rwlock again.
rwlckis pointer to an rwlock to try to read/write lock.
Returns 0 on success, otherwise EBUSY.
Warning:
If you leave an rwlock locked in an emulator thread when you let
the thread out of your control, you will very likely deadlock
the whole emulator.
This function is thread-safe.
int erl_drv_send_term(ErlDrvTermData port,
ErlDrvTermData receiver, ErlDrvTermData* term, int n)
This function is the only way for a driver to send data to other
processes than the port owner process. Parameter receiver speci-
fies the process to receive the data.
Note:
Parameter port is not an ordinary port handle, but a port handle
converted using driver_mk_port.
Parameters port, term, and n work as in erl_drv_output_term.
This function is only thread-safe when the emulator with SMP
support is used.
void erl_drv_set_os_pid(ErlDrvPort port,
ErlDrvSInt pid)
Sets the os_pid seen when doing erlang:port_info/2 on this port.
port is the port handle of the port (driver instance) to set the
pid on. pidis the pid to set.
int erl_drv_thread_create(char *name, ErlDrvTid
*tid, void * (*func)(void *), void *arg, ErlDrvThreadOpts
*opts)
Creates a new thread.
name:
A string identifying the created thread. It is used to iden-
tify the thread in planned future debug functionality.
tid:
A pointer to a thread identifier variable.
func:
A pointer to a function to execute in the created thread.
arg:
A pointer to argument to the func function.
opts:
A pointer to thread options to use or NULL.
Returns 0 on success, otherwise an errno value is returned to
indicate the error. The newly created thread begins executing in
the function pointed to by func, and func is passed arg as argu-
ment. When erl_drv_thread_create returns, the thread identifier
of the newly created thread is available in *tid. opts can be
either a NULL pointer, or a pointer to an ErlDrvThreadOpts
structure. If opts is a NULL pointer, default options are used,
otherwise the passed options are used.
Warning:
You are not allowed to allocate the ErlDrvThreadOpts structure
by yourself. It must be allocated and initialized by
erl_drv_thread_opts_create.
The created thread terminates either when func returns or if
erl_drv_thread_exit is called by the thread. The exit value of
the thread is either returned from func or passed as argument to
erl_drv_thread_exit. The driver creating the thread is responsi-
ble for joining the thread, through erl_drv_thread_join, before
the driver is unloaded. "Detached" threads cannot be created,
that is, threads that do not need to be joined.
Warning:
All created threads must be joined by the driver before it is
unloaded. If the driver fails to join all threads created before
it is unloaded, the runtime system most likely crashes when the
driver code is unloaded.
This function is thread-safe.
void erl_drv_thread_exit(void
*exit_value)
Terminates the calling thread with the exit value passed as ar-
gument. exit_value is a pointer to an exit value or NULL.
You are only allowed to terminate threads created with
erl_drv_thread_create.
The exit value can later be retrieved by another thread through
erl_drv_thread_join.
This function is thread-safe.
int erl_drv_thread_join(ErlDrvTid tid, void
**exit_value)
Joins the calling thread with another thread, that is, the call-
ing thread is blocked until the thread identified by tid has
terminated.
tid is the thread identifier of the thread to join. exit_value
is a pointer to a pointer to an exit value, or NULL.
Returns 0 on success, otherwise an errno value is returned to
indicate the error.
A thread can only be joined once. The behavior of joining more
than once is undefined, an emulator crash is likely. If
exit_value == NULL, the exit value of the terminated thread is
ignored, otherwise the exit value of the terminated thread is
stored at *exit_value.
This function is thread-safe.
char *erl_drv_thread_name(ErlDrvTid
tid)
Returns a pointer to the name of the thread.
tid is a thread identifier.
Note:
This function is intended for debugging purposes only.
ErlDrvThreadOpts *erl_drv_thread_opts_create(char *name)
Allocates and initializes a thread option structure.
name is a string identifying the created thread options. It is
used to identify the thread options in planned future debug
functionality.
Returns NULL on failure. A thread option structure is used for
passing options to erl_drv_thread_create. If the structure is
not modified before it is passed to erl_drv_thread_create, the
default values are used.
Warning:
You are not allowed to allocate the ErlDrvThreadOpts structure
by yourself. It must be allocated and initialized by
erl_drv_thread_opts_create.
This function is thread-safe.
void erl_drv_thread_opts_destroy(ErlDrvThreadOpts *opts)
Destroys thread options previously created by
erl_drv_thread_opts_create.
opts is a pointer to thread options to destroy.
This function is thread-safe.
ErlDrvTid erl_drv_thread_self(void)
Returns the thread identifier of the calling thread.
This function is thread-safe.
ErlDrvTime erl_drv_time_offset(ErlDrvTimeUnit
time_unit)
Returns the current time offset between Erlang monotonic time
and Erlang system time converted into the time_unit passed as
argument.
time_unit is time unit of returned value.
Returns ERL_DRV_TIME_ERROR if called with an invalid time unit
argument, or if called from a thread that is not a scheduler
thread.
See also ErlDrvTime and ErlDrvTimeUnit.
void *erl_drv_tsd_get(ErlDrvTSDKey
key)
Returns the thread-specific data associated with key for the
calling thread.
key is a thread-specific data key.
Returns NULL if no data has been associated with key for the
calling thread.
This function is thread-safe.
int erl_drv_tsd_key_create(char *name,
ErlDrvTSDKey *key)
Creates a thread-specific data key.
name is a string identifying the created key. It is used to
identify the key in planned future debug functionality.
key is a pointer to a thread-specific data key variable.
Returns 0 on success, otherwise an errno value is returned to
indicate the error. The driver creating the key is responsible
for destroying it before the driver is unloaded.
This function is thread-safe.
void erl_drv_tsd_key_destroy(ErlDrvTSDKey
key)
Destroys a thread-specific data key previously created by
erl_drv_tsd_key_create. All thread-specific data using this key
in all threads must be cleared (see erl_drv_tsd_set) before the
call to erl_drv_tsd_key_destroy.
key is a thread-specific data key to destroy.
Warning:
A destroyed key is very likely to be reused soon. Therefore, if
you fail to clear the thread-specific data using this key in a
thread before destroying the key, you will very likely get unex-
pected errors in other parts of the system.
This function is thread-safe.
void erl_drv_tsd_set(ErlDrvTSDKey key, void
*data)
Sets thread-specific data associated with key for the calling
thread. You are only allowed to set thread-specific data for
threads while they are fully under your control. For example, if
you set thread-specific data in a thread calling a driver call-
back function, it must be cleared, that is, set to NULL, before
returning from the driver callback function.
key is a thread-specific data key.
data is a pointer to data to associate with key in the calling
thread.
Warning:
If you fail to clear thread-specific data in an emulator thread
before letting it out of your control, you might never be able
to clear this data with later unexpected errors in other parts
of the system as a result.
This function is thread-safe.
char *erl_errno_id(int error)
Returns the atom name of the Erlang error, given the error num-
ber in error. The error atoms are einval, enoent, and so on. It
can be used to make error terms from the driver.
int remove_driver_entry(ErlDrvEntry
*de)
Removes a driver entry de previously added with add_driver_en-
try.
Driver entries added by the erl_ddll Erlang interface cannot be
removed by using this interface.
void set_busy_port(ErlDrvPort port, int
on)
Sets and unsets the busy state of the port. If on is non-zero,
the port is set to busy. If it is zero, the port is set to not
busy. You typically want to combine this feature with the busy
port message queue functionality.
Processes sending command data to the port are suspended if ei-
ther the port or the port message queue is busy. Suspended pro-
cesses are resumed when neither the port or the port message
queue is busy. Command data is in this context data passed to
the port using either Port ! {Owner, {command, Data}} or
port_command/[2,3].
If the ERL_DRV_FLAG_SOFT_BUSY has been set in the driver_entry,
data can be forced into the driver through erlang:port_com-
mand(Port, Data, [force]) even if the driver has signaled that
it is busy.
For information about busy port message queue functionality, see
erl_drv_busy_msgq_limits.
void set_port_control_flags(ErlDrvPort port,
int flags)
Sets flags for how the control driver entry function will return
data to the port owner process. (The control function is called
from erlang:port_control/3.)
Currently there are only two meaningful values for flags: 0
means that data is returned in a list, and PORT_CONTROL_FLAG_BI-
NARY means data is returned as a binary from control.
SEE ALSO
driver_entry(3erl), erlang(3erl), erl_ddll(3erl), section How to Imple-
ment an Alternative Carrier for the Erlang Distribution in the User's
Guide
Ericsson AB erts 11.0.2 erl_driver(3erl)