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

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

       /* According to POSIX.1-2001, POSIX.1-2008 */
       #include <sys/select.h>

       /* According to earlier standards */
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.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);

       #include <sys/select.h>

       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() and pselect() allow a program to  monitor  multiple  file  de-
       scriptors,  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  considered  ready if it is possible to perform a corre-
       sponding I/O operation (e.g., read(2) without  blocking,  or  a  suffi-
       ciently small write(2)).

       select()  can  monitor only file descriptors numbers that are less than
       FD_SETSIZE; poll(2) does not have this limitation.  See BUGS.

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

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

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

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

       Three  independent  sets of file descriptors are watched.  The file de-
       scriptors listed in readfds will be watched to see if characters become
       available for reading (more precisely, to see if a read will not block;
       in particular, a file descriptor is also ready  on  end-of-file).   The
       file  descriptors in writefds will be watched to see if space is avail-
       able for write (though a large write may still block).   The  file  de-
       scriptors  in  exceptfds  will  be  watched for exceptional conditions.
       (For examples of some exceptional conditions,  see  the  discussion  of
       POLLPRI in poll(2).)

       On exit, each of the file descriptor sets is modified in place to indi-
       cate which file descriptors actually changed status.  (Thus,  if  using
       select()  within  a  loop,  the  sets must be reinitialized before each

       Each of the three file descriptor sets may be specified as NULL  if  no
       file  descriptors  are  to  be  watched  for the corresponding class of

       Four macros are provided to manipulate the sets.   FD_ZERO()  clears  a
       set.   FD_SET()  and  FD_CLR() respectively add and remove a given file
       descriptor from a set.  FD_ISSET() tests to see if a file descriptor is
       part of the set; this is useful after select() returns.

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

       The  timeout argument specifies the interval that select() should block
       waiting for a file descriptor to become ready.  The call will block un-
       til either:

       *  a file descriptor becomes ready;

       *  the call is interrupted by a signal handler; or

       *  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 inter-
       val  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 NULL (no timeout), select() can block in-

       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.

       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 time structures involved are defined in <sys/time.h> and look like

           struct timeval {
               long    tv_sec;         /* seconds */
               long    tv_usec;        /* microseconds */


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

       (However, see below on the POSIX.1 versions.)

       Some code calls select() with all three sets empty, nfds  zero,  and  a
       non-NULL  timeout as a fairly portable way to sleep with subsecond pre-

       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)  which
       may be zero if the timeout expires before anything interesting happens.
       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 exit, 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.

       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.

       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

       Concerning the types involved, the classical situation is that the  two
       fields  of  a timeval structure are typed as long (as shown above), and
       the structure is defined in <sys/time.h>.  The POSIX.1 situation is

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

       where the structure is defined in <sys/select.h>  and  the  data  types
       time_t and suseconds_t are defined in <sys/types.h>.

       Concerning  prototypes,  the classical situation is that one should in-
       clude <time.h> for select().  The POSIX.1 situation is that one  should
       include <sys/select.h> for select() and pselect().

       Under  glibc  2.0,  <sys/select.h>  gives  the wrong prototype for pse-
       lect().  Under glibc 2.1 to 2.2.1, it gives pselect() when  _GNU_SOURCE
       is  defined.   Since  glibc 2.2.2, the requirements are as shown in the

   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)):

                              /* Ready for reading */
                              /* Ready for writing */
           #define POLLEX_SET (POLLPRI)
                              /* 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 the file descriptor reopened between the time select() re-
       turned and the I/O operations was 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.

       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) 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

       Glibc  2.0  provided a version of pselect() that did not take a sigmask

       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.

       Under 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  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/time.h>
       #include <sys/types.h>
       #include <unistd.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 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

Linux                             2017-09-15                         SELECT(2)

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