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Linux / Unix Command: select
Command Library


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


/* 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);


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

Their function is identical, with three differences:

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


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.


On success, select and pselect return the number of descriptors contained in the descriptor sets, 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.


An invalid file descriptor was given in one of the sets.
A non blocked signal was caught.
n is negative.
select was unable to allocate memory for internal tables.


#include stdio.h>
#include sys/time.h>
#include sys/types.h>
#include unistd.h>

main(void) {
    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)
        printf("Data is available now.\n");
        /* FD_ISSET(0, &rfds) will be true. */
        printf("No data within five seconds.\n");

    return 0;


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.


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)

Important: Use the man command (% man) to see how a command is used on your particular computer.

>> Linux/Unix Command Library

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