process_vm_writev(2)



PROCESS_VM_READV(2)        Linux Programmer's Manual       PROCESS_VM_READV(2)

NAME
       process_vm_readv, process_vm_writev - transfer data between process ad-
       dress spaces

SYNOPSIS
       #include <sys/uio.h>

       ssize_t process_vm_readv(pid_t pid,
                                const struct iovec *local_iov,
                                unsigned long liovcnt,
                                const struct iovec *remote_iov,
                                unsigned long riovcnt,
                                unsigned long flags);

       ssize_t process_vm_writev(pid_t pid,
                                 const struct iovec *local_iov,
                                 unsigned long liovcnt,
                                 const struct iovec *remote_iov,
                                 unsigned long riovcnt,
                                 unsigned long flags);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       process_vm_readv(), process_vm_writev():
           _GNU_SOURCE

DESCRIPTION
       These system calls transfer data between the address space of the call-
       ing  process  ("the  local  process") and the process identified by pid
       ("the remote process").  The data moves directly  between  the  address
       spaces of the two processes, without passing through kernel space.

       The  process_vm_readv()  system  call  transfers  data  from the remote
       process to the local process.  The data to be transferred is identified
       by remote_iov and riovcnt: remote_iov is a pointer to an array describ-
       ing address ranges in the process pid, and riovcnt specifies the number
       of  elements  in  remote_iov.  The data is transferred to the locations
       specified by local_iov and liovcnt: local_iov is a pointer to an  array
       describing address ranges in the calling process, and liovcnt specifies
       the number of elements in local_iov.

       The   process_vm_writev()   system   call   is    the    converse    of
       process_vm_readv()--it transfers data from the local process to the re-
       mote process.  Other than the direction of the transfer, the  arguments
       liovcnt,  local_iov,  riovcnt,  and remote_iov have the same meaning as
       for process_vm_readv().

       The local_iov and remote_iov arguments  point  to  an  array  of  iovec
       structures, defined in <sys/uio.h> as:

           struct iovec {
               void  *iov_base;    /* Starting address */
               size_t iov_len;     /* Number of bytes to transfer */
           };

       Buffers    are   processed   in   array   order.    This   means   that
       process_vm_readv() completely fills local_iov[0] before  proceeding  to
       local_iov[1],  and  so  on.  Likewise, remote_iov[0] is completely read
       before proceeding to remote_iov[1], and so on.

       Similarly, process_vm_writev() writes out the entire  contents  of  lo-
       cal_iov[0]  before  proceeding to local_iov[1], and it completely fills
       remote_iov[0] before proceeding to remote_iov[1].

       The lengths of remote_iov[i].iov_len and  local_iov[i].iov_len  do  not
       have to be the same.  Thus, it is possible to split a single local buf-
       fer into multiple remote buffers, or vice versa.

       The flags argument is currently unused and must be set to 0.

       The values specified in the liovcnt and riovcnt arguments must be  less
       than  or  equal to IOV_MAX (defined in <limits.h> or accessible via the
       call sysconf(_SC_IOV_MAX)).

       The count arguments and local_iov are checked before doing  any  trans-
       fers.   If  the counts are too big, or local_iov is invalid, or the ad-
       dresses refer to regions that are inaccessible to  the  local  process,
       none of the vectors will be processed and an error will be returned im-
       mediately.

       Note, however, that these system calls do not check the memory  regions
       in  the  remote process until just before doing the read/write.  Conse-
       quently, a partial read/write (see RETURN VALUE) may result if  one  of
       the  remote_iov  elements points to an invalid memory region in the re-
       mote process.  No further reads/writes will be  attempted  beyond  that
       point.   Keep  this  in  mind  when  attempting to read data of unknown
       length (such as C strings  that  are  null-terminated)  from  a  remote
       process,  by avoiding spanning memory pages (typically 4 KiB) in a sin-
       gle remote iovec element.  (Instead, split the remote read into two re-
       mote_iov  elements  and  have  them  merge back into a single write lo-
       cal_iov entry.  The first read entry goes  up  to  the  page  boundary,
       while the second starts on the next page boundary.)

       Permission  to  read  from or write to another process is governed by a
       ptrace access mode PTRACE_MODE_ATTACH_REALCREDS check; see ptrace(2).

RETURN VALUE
       On success, process_vm_readv() returns the number  of  bytes  read  and
       process_vm_writev()  returns  the number of bytes written.  This return
       value may be less than the total number of requested bytes, if  a  par-
       tial  read/write occurred.  (Partial transfers apply at the granularity
       of iovec elements.  These system calls won't perform a partial transfer
       that  splits  a single iovec element.)  The caller should check the re-
       turn value to determine whether a partial read/write occurred.

       On error, -1 is returned and errno is set appropriately.

ERRORS
       EFAULT The memory described by local_iov is outside the caller's acces-
              sible address space.

       EFAULT The memory described by remote_iov is outside the accessible ad-
              dress space of the process pid.

       EINVAL The sum of the iov_len values of either local_iov or  remote_iov
              overflows a ssize_t value.

       EINVAL flags is not 0.

       EINVAL liovcnt or riovcnt is too large.

       ENOMEM Could  not  allocate  memory  for  internal  copies of the iovec
              structures.

       EPERM  The caller does not have permission to access the address  space
              of the process pid.

       ESRCH  No process with ID pid exists.

VERSIONS
       These  system  calls  were  added in Linux 3.2.  Support is provided in
       glibc since version 2.15.

CONFORMING TO
       These system calls are nonstandard Linux extensions.

NOTES
       The   data    transfers    performed    by    process_vm_readv()    and
       process_vm_writev() are not guaranteed to be atomic in any way.

       These  system calls were designed to permit fast message passing by al-
       lowing messages to be exchanged with a single  copy  operation  (rather
       than  the  double  copy that would be required when using, for example,
       shared memory or pipes).

EXAMPLE
       The following code sample demonstrates the use  of  process_vm_readv().
       It  reads  20 bytes at the address 0x10000 from the process with PID 10
       and writes the first 10 bytes into buf1 and the second  10  bytes  into
       buf2.

       #include <sys/uio.h>

       int
       main(void)
       {
           struct iovec local[2];
           struct iovec remote[1];
           char buf1[10];
           char buf2[10];
           ssize_t nread;
           pid_t pid = 10;             /* PID of remote process */

           local[0].iov_base = buf1;
           local[0].iov_len = 10;
           local[1].iov_base = buf2;
           local[1].iov_len = 10;
           remote[0].iov_base = (void *) 0x10000;
           remote[0].iov_len = 20;

           nread = process_vm_readv(pid, local, 2, remote, 1, 0);
           if (nread != 20)
               return 1;
           else
               return 0;
       }

SEE ALSO
       readv(2), writev(2)

COLOPHON
       This  page  is  part of release 4.16 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15               PROCESS_VM_READV(2)

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