Man Linux: Main Page and Category List


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


       /* According to POSIX.1-2001 */
       #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 || _XOPEN_SOURCE >= 600


       select() and  pselect()  allow  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  considered  ready  if  it  is  possible  to perform the
       corresponding I/O operation (e.g., read(2)) without blocking.

       The operation of  select()  and  pselect()  is  identical,  with  three

       (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.  Those 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),  those  in
       writefds will be watched to see if a write will not block, and those in
       exceptfds will be watched  for  exceptions.   On  exit,  the  sets  are
       modified  in  place to indicate which file descriptors actually changed
       status.  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 events.

       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  is the highest-numbered file descriptor in any of the three sets,
       plus 1.

       timeout is an upper bound on the amount of time elapsed before select()
       returns.   If  both  fields  of  the  timeval  stucture  are zero, then
       select() returns  immediately.   (This  is  useful  for  polling.)   If
       timeout is NULL (no timeout), select() can block indefinitely.

       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;

           sigprocmask(SIG_SETMASK, &sigmask, &origmask);
           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
           sigprocmask(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
       signal 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.1-2001 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

       On  Linux,  select() modifies timeout to reflect the amount of time not
       slept; most  other  implementations  do  not  do  this.   (POSIX.1-2001
       permits  either  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 select() returns.


       On   success,   select()  and  pselect()  return  the  number  of  file
       descriptors contained in the three returned descriptor sets  (that  is,
       the  total 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 appropriately; the
       sets and timeout become undefined, so do not  rely  on  their  contents
       after an error.


       EBADF  An  invalid  file  descriptor  was  given  in  one  of the sets.
              (Perhaps a file descriptor that was already closed,  or  one  on
              which an error has occurred.)

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

       EINVAL nfds  is  negative  or  the  value  contained  within timeout is

       ENOMEM unable to allocate memory for internal tables.


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


       select()  conforms  to POSIX.1-2001 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 variants).  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.


       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.

       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.1-2001  situation

           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
       include  <time.h> for select().  The POSIX.1-2001 situation is that one
       should include <sys/select.h> for select() and pselect().

       Libc4 and libc5 do not have a <sys/select.h> header;  under  glibc  2.0
       and later this header exists.  Under glibc 2.0 it unconditionally gives
       the wrong prototype for pselect().  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 SYNOPSIS.

   Linux Notes
       The  Linux  pselect()  system  call  modifies  its  timeout   argument.
       However,  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
       argument; this is the behavior required by POSIX.1-2001.


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

       Since version 2.1, glibc has provided an emulation of pselect() that is
       implemented using sigprocmask(2)  and  select().   This  implementation
       remains  vulnerable  to  the  very  race  condition  that pselect() was
       designed to prevent.  On systems that  lack  pselect(),  reliable  (and
       more  portable)  signal  trapping  can  be achieved using the self-pipe
       trick (where a signal handler writes a byte to a pipe whose  other  end
       is monitored by select() in the main program.)

       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.1-2001.   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");



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

       For vaguely related stuff, see accept(2), connect(2), poll(2), read(2),
       recv(2), send(2), sigprocmask(2), write(2), epoll(7), time(7)


       This page is part of release 3.24 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at