FD_CLR(3) — Linux manual page


SELECT(2)                 Linux Programmer's Manual                SELECT(2)

NAME         top

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

SYNOPSIS         top

       #include <sys/select.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);

       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         top

       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.

       select() can monitor only file descriptors numbers that are less than
       FD_SETSIZE; poll(2) and epoll(7) do not have this limitation.  See

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

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

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

              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.

              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.

       The arguments of select() are as follows:

              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.

              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

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

              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).

              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

              • 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.

       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 sig‐
       nal 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 sig‐
       mask, avoiding the race.)

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

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

       The corresponding argument for pselect() has the following type:

           struct timespec {
               time_t      tv_sec;         /* seconds */
               long        tv_nsec;        /* nanoseconds */

       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 unde‐
       fined after select() returns.

RETURN VALUE         top

       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         top

       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         top

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

CONFORMING TO         top

       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 returning, but the BSD variant does not.

       pselect() is defined in POSIX.1g, and in POSIX.1-2001 and

NOTES         top

       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.

       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.

   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):

                                EPOLLHUP | EPOLLERR)
                              /* Ready for reading */
                              /* 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, un‐
       less another process reopens file descriptor between the time se‐
       lect() 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, de‐
       termining 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

       The Linux pselect6() system call modifies its timeout argument.  How‐
       ever, 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 argu‐
       ment; 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.

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

BUGS         top

       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.

       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 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.

       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         top

       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/select.h>

           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 == -1)
           else if (retval)
               printf("Data is available now.\n");
               /* FD_ISSET(0, &rfds) will be true. */
               printf("No data within five seconds.\n");


SEE ALSO         top

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

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2020-11-01                        SELECT(2)