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SELECT

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

NAME

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

SYNOPSIS

/* According to POSIX.1-2001, POSIX.1-2008 */
#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

DESCRIPTION

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)).

select() can monitor only file descriptors numbers that are less than FD_SETSIZE; poll(2) does not have this limitation. See BUGS.

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. The file descriptors 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). The file descriptors in writefds will be watched to see if space is available for write (though a large write may still block). The file descriptors in exceptfds will be watched for exceptional conditions. (For examples of some exceptional conditions, see the discussion of POLLPRI in poll(2).)

On exit, each of the file descriptor sets is modified in place to indicate which file descriptors actually changed status. (Thus, if using select() within a loop, the sets must be reinitialized before each call.)

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 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).

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 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 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) 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

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

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

CONFORMING TO

select() conforms to POSIX.1-2001, POSIX.1-2008, 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 and POSIX.1-2008.

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.

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.

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.)

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 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 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.

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 (POLLRDNORM | POLLRDBAND | POLLIN | POLLHUP |
                    POLLERR)
                   /* Ready for reading */ #define POLLOUT_SET (POLLWRBAND | POLLWRNORM | POLLOUT | POLLERR)
                   /* Ready for writing */ #define POLLEX_SET (POLLPRI)
                   /* 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 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 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.

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) instead.

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.

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.

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. 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

#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); }

COLOPHON

This page is part of release 4.13 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 https://www.kernel.org/doc/man-pages/.