select(2)



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

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

SYNOPSIS
       /* 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

DESCRIPTION
       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 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
       descriptors 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
       available for write (though a large write may still block).   The  file
       descriptors  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
       call.)

       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  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
       until 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
       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;

           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 */
           };

       and

           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-
       cision.

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

RETURN VALUE
       On success, select() and pselect() return the number of  file  descrip-
       tors  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  to  indicate  the
       error;  the  file  descriptor  sets are unmodified, and timeout becomes
       undefined.

ERRORS
       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.

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

CONFORMING TO
       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.

NOTES
       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
       pipe.)

       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
       include  <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
       SYNOPSIS.

   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)):

           #define POLLIN_SET (POLLRDNORM | POLLRDBAND | POLLIN | POLLHUP |
                               POLLERR)
                              /* Ready for reading */
           #define POLLOUT_SET (POLLWRBAND | POLLWRNORM | POLLOUT | POLLERR)
                              /* 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()
       unblocks and returns, with an indication that the  file  descriptor  is
       ready  (a  subsequent  I/O  operation  will  likely fail with an error,
       unless another the file descriptor reopened between the  time  select()
       returned  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 par-
       ticular 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.

BUGS
       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
       according  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
       EBADF.

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

       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.

EXAMPLE
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       int
       main(void)
       {
           fd_set rfds;
           struct timeval tv;
           int retval;

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

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       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).

COLOPHON
       This  page  is  part of release 4.13 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                         SELECT(2)

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