select(2)

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

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#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, const 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)):

pselect():
    _POSIX_C_SOURCE >= 200112L

DESCRIPTION
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.

RETURN VALUE
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.

ERRORS

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.) 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
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.

STANDARDS
POSIX.1-2024.

HISTORY

select()
    POSIX.1-2001, 4.4BSD (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 returning, but the BSD variant does not.

pselect()
    Linux 2.6.16. POSIX.1g, POSIX.1-2001.

    Prior to this, it was emulated in glibc (but see BUGS).

fd_set
    POSIX.1-2001.

NOTES
The following header also provides the fd_set type: <sys/time.h>.

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.

The operation of select() and pselect() is not affected by the O_NONBLOCK flag.

  The self-pipe trick
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.)

  Emulating usleep(3)
Before the advent of usleep(3), some code employed a call to select() with all three sets empty, nfds zero, and a non-NULL timeout as a fairly portable way to sleep with subsecond precision.

  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 provided by poll(2) and epoll(7):

    #define POLLIN_SET  (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN |
                         EPOLLHUP | EPOLLERR)
                       /* Ready for reading */
    #define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT |
                         EPOLLERR)
                       /* Ready for writing */
    #define POLLEX_SET  (EPOLLPRI)
                       /* 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 process reopens the file descriptor between the time select() returned and the I/O operation is 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 differences
The Linux kernel allows file descriptor sets of arbitrary size, determining 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. 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 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 support 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.

  Historical glibc details
glibc 2.0 provided an incorrect version of pselect() that did not take a sigmask argument.

From glibc 2.1 to glibc 2.2.1, one must define _GNU_SOURCE in order to obtain the declaration of pselect() from <sys/select.h>.

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) or epoll(7) instead.

The implementation of the fd_set arguments as value-result arguments is a design error that is avoided in poll(2) and epoll(7).

According to POSIX, select() should check all specified file descriptors in the three file descriptor sets, up to the limit nfds-1. However, 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 descriptor that is specified in one of the sets should result in the error EBADF.

Starting with glibc 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 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 the 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.

EXAMPLES
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/select.h>
    #include <sys/time.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);
    }

SEE ALSO
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).