NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       /* 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

DESCRIPTION         top

       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
       corresponding I/O operation (e.g., read(2) without blocking, or a
       sufficiently small write(2)).

       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.  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 space is available for write
       (though a large write may still 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.

       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
       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 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 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 */
           };

       and

           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
       precision.

       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.

RETURN VALUE         top

       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 to
       indicate the error; the file descriptor sets are unmodified, and
       timeout becomes undefined.

ERRORS         top

       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
              invalid.

       ENOMEM unable to allocate memory for internal tables.

VERSIONS         top

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

CONFORMING TO         top

       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.

NOTES         top

       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 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.1-2001 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
       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.

   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
       particular behavior in this scenario must be considered buggy.

   C library/kernel ABI differences
       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.
       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.

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

           struct {
               const 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
       support a maximum of 6 arguments to a system call.

BUGS         top

       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
       implementation 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 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.)

       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.

EXAMPLE         top

       #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         top

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

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

COLOPHON         top

       This page is part of release 3.73 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-08-19                        SELECT(2)