select

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



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

       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() 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.

       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.

       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 process reopens 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.

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.

       The implementation of the fd_set arguments as value-result arguments
       means that they must be reinitialized on each call to select().  This
       design error is avoided by poll(2), which uses separate structure
       fields for the input and output of the call.

       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);
       }

SEE ALSO
       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
       This page is part of release 5.02 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/.



Linux                             2019-03-06                         SELECT(2)