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

#include <sys/select.h>
typedef /* ... */ fd_set;
int select(int nfds, fd_set *_Nullable restrict readfds,
 fd_set *_Nullable restrict writefds,
 fd_set *_Nullable restrict exceptfds,
 struct timeval *_Nullable restrict 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);
int pselect(int nfds, fd_set *_Nullable restrict readfds,
 fd_set *_Nullable restrict writefds,
 fd_set *_Nullable restrict exceptfds,
 const struct timespec *_Nullable restrict timeout,
 const sigset_t *_Nullable restrict sigmask);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    _POSIX_C_SOURCE >= 200112L

WARNING: select() can monitor only file descriptors numbers that are less than FD_SETSIZE (1024)—an unreasonably low limit for many modern applications—and this limitation will not change. All modern applications should instead use poll(2) or epoll(7), which do not suffer this limitation.

select() allows 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), or a sufficiently small write(2)) without blocking.

fd_set

A structure type that can represent a set of file descriptors. According to POSIX, the maximum number of file descriptors in an fd_set structure is the value of the macro FD_SETSIZE.

File descriptor sets

The principal arguments of select() are three "sets" of file descriptors (declared with the type fd_set), which allow the caller to wait for three classes of events on the specified set of file descriptors. Each of the fd_set arguments may be specified as NULL if no file descriptors are to be watched for the corresponding class of events.

Note well: Upon return, each of the file descriptor sets is modified in place to indicate which file descriptors are currently "ready". Thus, if using select() within a loop, the sets must be reinitialized before each call.

The contents of a file descriptor set can be manipulated using the following macros:

FD_ZERO()

This macro clears (removes all file descriptors from) set. It should be employed as the first step in initializing a file descriptor set.

FD_SET()

This macro adds the file descriptor fd to set. Adding a file descriptor that is already present in the set is a no-op, and does not produce an error.

FD_CLR()

This macro removes the file descriptor fd from set. Removing a file descriptor that is not present in the set is a no-op, and does not produce an error.

FD_ISSET()

select() modifies the contents of the sets according to the rules described below. After calling select(), the FD_ISSET() macro can be used to test if a file descriptor is still present in a set. FD_ISSET() returns nonzero if the file descriptor fd is present in set, and zero if it is not.

Arguments

The arguments of select() are as follows:

readfds

The file descriptors in this set are watched to see if they are ready for reading. A file descriptor is ready for reading if a read operation will not block; in particular, a file descriptor is also ready on end-of-file.

After select() has returned, readfds will be cleared of all file descriptors except for those that are ready for reading.

writefds

The file descriptors in this set are watched to see if they are ready for writing. A file descriptor is ready for writing if a write operation will not block. However, even if a file descriptor indicates as writable, a large write may still block.

After select() has returned, writefds will be cleared of all file descriptors except for those that are ready for writing.

exceptfds

The file descriptors in this set are watched for "exceptional conditions". For examples of some exceptional conditions, see the discussion of POLLPRI in poll(2).

After select() has returned, exceptfds will be cleared of all file descriptors except for those for which an exceptional condition has occurred.

nfds

This argument 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).

timeout

The timeout argument is a timeval structure (shown below) that 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 specified as NULL, select() blocks indefinitely waiting for a file descriptor to become ready.

pselect()

The pselect() system call allows an application to safely wait until either a file descriptor becomes ready or until a signal is caught.

The operation of select() and pselect() is identical, other than these three differences:

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

• select() may update the timeout argument to indicate how much time was left. pselect() does not change this argument.

• select() has no sigmask argument, and behaves as pselect() called with NULL sigmask.

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. (If sigmask is NULL, the signal mask is not modified during the pselect() call.)

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 timeout argument for select() is a structure of the following type:

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

The corresponding argument for pselect() is a timespec(3) structure.

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.

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). The return value may be zero if the timeout expired before any file descriptors became ready.

On error, -1 is returned, and errno is set to indicate the error; the file descriptor sets are unmodified, and timeout becomes undefined.

#include <stdio.h>
#include <stdlib.h>
#include <sys/select.h>
int
main(void)
{
    int             retval;
    fd_set          rfds;
    struct timeval  tv;
    /* 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);
}

accept(2), connect(2), poll(2), read(2), recv(2), restart_syscall(2), send(2), sigprocmask(2), write(2), timespec(3), epoll(7), time(7)

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