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

       select,  pselect,  FD_CLR,  FD_ISSET, FD_SET, FD_ZERO - synchronous I/O

       #include <sys/select.h>

       int select(int nfds, fd_set *readfds, fd_set *writefds,
                  fd_set *exceptfds, struct timeval *timeout);

       void FD_CLR(int fd, fd_set *set);
       int  FD_ISSET(int fd, fd_set *set);
       void FD_SET(int fd, fd_set *set);
       void FD_ZERO(fd_set *set);

       int pselect(int nfds, fd_set *readfds, fd_set *writefds,
                   fd_set *exceptfds, const struct timespec *timeout,
                   const sigset_t *sigmask);

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

       pselect(): _POSIX_C_SOURCE >= 200112L

       select() allows a program to monitor multiple file descriptors, waiting
       until one or more of the file descriptors become "ready" for some class
       of I/O operation (e.g., input possible).  A file descriptor is  consid-
       ered  ready  if it is possible to perform a corresponding I/O operation
       (e.g., read(2), or a sufficiently small write(2)) without blocking.

       select() can monitor only file descriptors numbers that are  less  than
       FD_SETSIZE;  poll(2)  and  epoll(7)  do  not have this limitation.  See

   File descriptor sets
       The principal arguments of select() are three "sets" of  file  descrip-
       tors  (declared  with  the type fd_set), which allow the caller to wait
       for three classes of events on the specified set of  file  descriptors.
       Each  of  the  fd_set arguments may be specified as NULL if no file de-
       scriptors are to be watched for the corresponding class of events.

       Note well: Upon return, each of the file descriptor sets is modified in
       place  to indicate which file descriptors are currently "ready".  Thus,
       if using select() within a loop, the sets must be reinitialized  before
       each  call.  The implementation of the fd_set arguments as value-result
       arguments is a design error that is avoided in poll(2) and epoll(7).

       The contents of a file descriptor set can be manipulated using the fol-
       lowing macros:

              This  macro  clears (removes all file descriptors from) set.  It
              should be employed as the first step in initializing a file  de-
              scriptor set.

              This  macro  adds  the file descriptor fd to set.  Adding a file
              descriptor that is already present in the set is  a  no-op,  and
              does not produce an error.

              This  macro removes the file descriptor fd from set.  Removing a
              file descriptor that is not present in the set is a  no-op,  and
              does not produce an error.

              select()  modifies  the  contents  of  the sets according to the
              rules described below.  After calling select(),  the  FD_ISSET()
              macro  can be used to test if a file descriptor is still present
              in a set.  FD_ISSET() returns nonzero if the file descriptor  fd
              is present in set, and zero if it is not.

       The arguments of select() are as follows:

              The  file descriptors in this set are watched to see if they are
              ready for reading.  A file descriptor is ready for reading if  a
              read  operation will not block; in particular, a file descriptor
              is also ready on end-of-file.

              After select() has returned, readfds will be cleared of all file
              descriptors except for those that are ready for reading.

              The  file descriptors in this set are watched to see if they are
              ready for writing.  A file descriptor is ready for writing if  a
              write  operation  will  not  block.  However, even if a file de-
              scriptor indicates as writable, a large write may still block.

              After select() has returned, writefds will  be  cleared  of  all
              file descriptors except for those that are ready for writing.

              The  file  descriptors  in this set are watched for "exceptional
              conditions".  For examples of some exceptional  conditions,  see
              the discussion of POLLPRI in poll(2).

              After  select()  has  returned, exceptfds will be cleared of all
              file descriptors except for those for which an exceptional  con-
              dition has occurred.

       nfds   This  argument  should  be  set to the highest-numbered file de-
              scriptor in any of the three sets, plus 1.  The  indicated  file
              descriptors  in  each set are checked, up to this limit (but see

              The timeout argument is a timeval structure (shown  below)  that
              specifies  the interval that select() should block waiting for a
              file descriptor to become ready.  The call will block until  ei-

              o a file descriptor becomes ready;

              o the call is interrupted by a signal handler; or

              o the timeout expires.

              Note  that the timeout interval will be rounded up to the system
              clock granularity, and kernel scheduling delays  mean  that  the
              blocking interval may overrun by a small amount.

              If  both fields of the timeval structure are zero, then select()
              returns immediately.  (This is useful for polling.)

              If timeout is specified as NULL,  select()  blocks  indefinitely
              waiting for a file descriptor to become ready.

       The  pselect()  system  call allows an application to safely wait until
       either a file descriptor becomes ready or until a signal is caught.

       The operation of select() and pselect() is identical, other than  these
       three differences:

       o select()  uses  a  timeout that is a struct timeval (with seconds and
         microseconds), while pselect() uses a struct timespec  (with  seconds
         and nanoseconds).

       o select()  may  update  the timeout argument to indicate how much time
         was left.  pselect() does not change this argument.

       o select() has no sigmask argument, and  behaves  as  pselect()  called
         with NULL sigmask.

       sigmask  is  a  pointer to a signal mask (see sigprocmask(2)); if it is
       not NULL, then pselect() first replaces the current signal mask by  the
       one  pointed  to  by sigmask, then does the "select" function, and then
       restores the original signal mask.  (If sigmask  is  NULL,  the  signal
       mask is not modified during the pselect() call.)

       Other than the difference in the precision of the timeout argument, the
       following pselect() call:

           ready = pselect(nfds, &readfds, &writefds, &exceptfds,
                           timeout, &sigmask);

       is equivalent to atomically executing the following calls:

           sigset_t origmask;

           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
           pthread_sigmask(SIG_SETMASK, &origmask, NULL);

       The reason that pselect() is needed is that if one wants  to  wait  for
       either  a  signal  or  for  a  file descriptor to become ready, then an
       atomic test is needed to prevent race conditions.  (Suppose the  signal
       handler  sets  a  global  flag and returns.  Then a test of this global
       flag followed by a call of select() could hang indefinitely if the sig-
       nal arrived just after the test but just before the call.  By contrast,
       pselect() allows one to first block signals, handle  the  signals  that
       have  come  in,  then call pselect() with the desired sigmask, avoiding
       the race.)

   The timeout
       The timeout argument for select() is a structure of the following type:

           struct timeval {
               time_t      tv_sec;         /* seconds */
               suseconds_t tv_usec;        /* microseconds */

       The corresponding argument for pselect() has the following type:

           struct timespec {
               time_t      tv_sec;         /* seconds */
               long        tv_nsec;        /* nanoseconds */

       On Linux, select() modifies timeout to reflect the amount of  time  not
       slept; most other implementations do not do this.  (POSIX.1 permits ei-
       ther behavior.)  This causes problems both when Linux code which  reads
       timeout  is  ported to other operating systems, and when code is ported
       to Linux that reuses a struct timeval for multiple select()s in a  loop
       without  reinitializing it.  Consider timeout to be undefined after se-
       lect() returns.

       On success, select() and pselect() return the number of  file  descrip-
       tors  contained in the three returned descriptor sets (that is, the to-
       tal number of bits that are set in readfds, writefds, exceptfds).   The
       return  value  may  be  zero if the timeout expired before any file de-
       scriptors became ready.

       On error, -1 is returned, and errno is set to indicate the  error;  the
       file descriptor sets are unmodified, and timeout becomes undefined.

       EBADF  An  invalid file descriptor was given in one of the sets.  (Per-
              haps a file descriptor that was already closed, or one on  which
              an error has occurred.)  However, see BUGS.

       EINTR  A signal was caught; see signal(7).

       EINVAL nfds  is  negative  or  exceeds the RLIMIT_NOFILE resource limit
              (see getrlimit(2)).

       EINVAL The value contained within timeout is invalid.

       ENOMEM Unable to allocate memory for internal tables.

       pselect() was added to Linux in kernel 2.6.16.   Prior  to  this,  pse-
       lect() was emulated in glibc (but see BUGS).

       select()  conforms  to POSIX.1-2001, POSIX.1-2008, and 4.4BSD (select()
       first appeared in 4.2BSD).  Generally portable to/from non-BSD  systems
       supporting  clones  of  the  BSD socket layer (including System V vari-
       ants).  However, note that the  System V  variant  typically  sets  the
       timeout variable before returning, but the BSD variant does not.

       pselect() is defined in POSIX.1g, and in POSIX.1-2001 and POSIX.1-2008.

       An  fd_set is a fixed size buffer.  Executing FD_CLR() or FD_SET() with
       a value of fd that is negative or is equal to or larger than FD_SETSIZE
       will result in undefined behavior.  Moreover, POSIX requires fd to be a
       valid file descriptor.

       The operation of select() and pselect() is not affected by  the  O_NON-
       BLOCK flag.

       On  some other UNIX systems, select() can fail with the error EAGAIN if
       the system fails to allocate  kernel-internal  resources,  rather  than
       ENOMEM  as Linux does.  POSIX specifies this error for poll(2), but not
       for select().  Portable programs may wish to check for EAGAIN and loop,
       just as with EINTR.

   The self-pipe trick
       On  systems  that  lack  pselect(), reliable (and more portable) signal
       trapping can be achieved using the self-pipe trick.  In this technique,
       a  signal  handler writes a byte to a pipe whose other end is monitored
       by select() in the main program.   (To  avoid  possibly  blocking  when
       writing  to  a pipe that may be full or reading from a pipe that may be
       empty, nonblocking I/O is used when reading from  and  writing  to  the

   Emulating usleep(3)
       Before  the  advent of usleep(3), some code employed a call to select()
       with all three sets empty, nfds zero,  and  a  non-NULL  timeout  as  a
       fairly portable way to sleep with subsecond precision.

   Correspondence between select() and poll() notifications
       Within the Linux kernel source, we find the following definitions which
       show the correspondence between the readable, writable, and exceptional
       condition  notifications  of  select() and the event notifications pro-
       vided by poll(2) and epoll(7):

                                EPOLLHUP | EPOLLERR)
                              /* Ready for reading */
                              /* Ready for writing */
           #define POLLEX_SET  (EPOLLPRI)
                              /* Exceptional condition */

   Multithreaded applications
       If a file descriptor being monitored by select() is closed  in  another
       thread,  the result is unspecified.  On some UNIX systems, select() un-
       blocks and returns, with an indication  that  the  file  descriptor  is
       ready  (a  subsequent I/O operation will likely fail with an error, un-
       less another process reopens file descriptor between the time  select()
       returned and the I/O operation is performed).  On Linux (and some other
       systems), closing the file descriptor in another thread has  no  effect
       on  select().   In summary, any application that relies on a particular
       behavior in this scenario must be considered buggy.

   C library/kernel differences
       The Linux kernel allows file descriptor sets of arbitrary size,  deter-
       mining  the  length  of  the sets to be checked from the value of nfds.
       However, in the glibc implementation, the fd_set type is fixed in size.
       See also BUGS.

       The pselect() interface described in this page is implemented by glibc.
       The underlying Linux system call is named pselect6().  This system call
       has somewhat different behavior from the glibc wrapper function.

       The  Linux  pselect6() system call modifies its timeout argument.  How-
       ever, the glibc wrapper function hides this behavior by using  a  local
       variable  for  the  timeout argument that is passed to the system call.
       Thus, the glibc pselect() function does not modify  its  timeout  argu-
       ment; this is the behavior required by POSIX.1-2001.

       The  final  argument  of the pselect6() system call is not a sigset_t *
       pointer, but is instead a structure of the form:

           struct {
               const kernel_sigset_t *ss;   /* Pointer to signal set */
               size_t ss_len;               /* Size (in bytes) of object
                                               pointed to by 'ss' */

       This allows the system call to obtain both a pointer to the signal  set
       and  its size, while allowing for the fact that most architectures sup-
       port a maximum of 6 arguments to a system call.  See sigprocmask(2) for
       a  discussion  of  the difference between the kernel and libc notion of
       the signal set.

   Historical glibc details
       Glibc 2.0 provided an incorrect version of pselect() that did not  take
       a sigmask argument.

       In glibc versions 2.1 to 2.2.1, one must define _GNU_SOURCE in order to
       obtain the declaration of pselect() from <sys/select.h>.

       POSIX allows an implementation to define an upper limit, advertised via
       the  constant  FD_SETSIZE, on the range of file descriptors that can be
       specified in a file descriptor set.  The Linux kernel imposes no  fixed
       limit,  but  the  glibc  implementation makes fd_set a fixed-size type,
       with FD_SETSIZE defined as 1024, and the FD_*()  macros  operating  ac-
       cording  to that limit.  To monitor file descriptors greater than 1023,
       use poll(2) or epoll(7) instead.

       According to POSIX, select() should check all specified  file  descrip-
       tors  in  the three file descriptor sets, up to the limit nfds-1.  How-
       ever, the current implementation ignores any file descriptor  in  these
       sets  that  is greater than the maximum file descriptor number that the
       process currently has open.  According to POSIX, any such file descrip-
       tor  that  is  specified  in one of the sets should result in the error

       Starting with version 2.1, glibc provided  an  emulation  of  pselect()
       that was implemented using sigprocmask(2) and select().  This implemen-
       tation remained vulnerable to the very race  condition  that  pselect()
       was  designed to prevent.  Modern versions of glibc use the (race-free)
       pselect() system call on kernels where it is provided.

       On Linux, select() may report a socket file descriptor  as  "ready  for
       reading",  while nevertheless a subsequent read blocks.  This could for
       example happen when data has arrived but upon examination has the wrong
       checksum and is discarded.  There may be other circumstances in which a
       file descriptor is spuriously reported as ready.  Thus it may be  safer
       to use O_NONBLOCK on sockets that should not block.

       On  Linux, select() also modifies timeout if the call is interrupted by
       a signal handler (i.e., the EINTR error return).  This is not permitted
       by POSIX.1.  The Linux pselect() system call has the same behavior, but
       the glibc wrapper hides this behavior by internally copying the timeout
       to a local variable and passing that variable to the system call.

       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/select.h>

           fd_set rfds;
           struct timeval tv;
           int retval;

           /* Watch stdin (fd 0) to see when it has input. */

           FD_SET(0, &rfds);

           /* Wait up to five seconds. */

           tv.tv_sec = 5;
           tv.tv_usec = 0;

           retval = select(1, &rfds, NULL, NULL, &tv);
           /* Don't rely on the value of tv now! */

           if (retval == -1)
           else if (retval)
               printf("Data is available now.\n");
               /* FD_ISSET(0, &rfds) will be true. */
               printf("No data within five seconds.\n");


       accept(2),  connect(2),  poll(2), read(2), recv(2), restart_syscall(2),
       send(2), sigprocmask(2), write(2), epoll(7), time(7)

       For a tutorial with discussion and examples, see select_tut(2).

       This page is part of release 5.07 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

Linux                             2020-04-11                         SELECT(2)

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