SELECT(2)

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

SYNOPSIS
/* According to POSIX 1003.1-2001 */
#include <sys/select.h>

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

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

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

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

DESCRIPTION
The functions select and pselect wait for a number of file descriptors to change status.

Their function is identical, with three differences:

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

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

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

Three independent sets of 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 descriptors actually changed status.

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

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

timeout is an upper bound on the amount of time elapsed before select returns. It may be zero, causing select to return 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 again.

The idea of pselect is that if one wants to wait for an event, either a signal or something on a file descriptor, 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. On the other hand, pselect allows one to first block signals, handle the signals that have come in, then call pselect() with the desired sigmask, avoiding the race.) Since Linux today does not have a pselect() system call, the current glibc2 routine still contains this 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 1003.1-2001 versions.)

Some code calls select with all three sets empty, n zero, and a non-null timeout as a fairly portable way to sleep with subsecond precision.

On Linux, the function select modifies timeout to reflect the amount of time not slept; most other implementations do not do this. 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 selects 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 descriptors contained in the three returned descriptor sets (that is, the total number of one bits 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.

ERRORS

EBADF
    An invalid file descriptor was given in one of the sets.

EINTR
    A non blocked signal was caught.

EINVAL
    n is negative or the value contained within timeout is invalid.

ENOMEM
    select was unable to allocate memory for internal tables.

EXAMPLE
#include <stdio.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");

    return 0;
}

CONFORMING TO
4.4BSD (the select function 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.

The pselect function is defined in IEEE Std 1003.1g-2000 (POSIX.1g), and part of POSIX 1003.1-2001. It is found in glibc2.1 and later. Glibc2.0 has a function with this name, that however does not take a sigmask parameter.

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.

Concerning the types involved, the classical situation is that the two fields of a struct timeval are longs (as shown above), and the struct is defined in <sys/time.h>. The POSIX 1003.1-2001 situation is

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

where the struct 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 1003.1-2001 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 2.1-2.2.1 it gives pselect when _GNU_SOURCE is defined, under glibc 2.2.2-2.2.4 it gives it when _XOPEN_SOURCE is defined and has a value of 600 or larger. No doubt, since POSIX 1003.1-2001, it should give the prototype by default.

BUGS
pselect is currently emulated with a user-space wrapper that has a race condition. For reliable (and more portable) signal trapping, use the self-pipe trick. (Where a signal handler writes to a pipe whose other end is read by the main loop.)

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. Thus it may be safer to use O_NONBLOCK on sockets that should not block.

SEE ALSO
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)