select(2)                                               System Calls Manual                                               select(2)

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

       Standard C library (libc, -lc)

       #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.

       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  cur‐
       rently "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:

              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

              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

              The  file  descriptors in this set are watched for "exceptional conditions".  For examples of some exceptional condi‐
              tions, 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  excep‐
              tional 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 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.

       The pselect() system call allows an application to safely wait until either a file descriptor becomes ready or until a  sig‐
       nal 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 re‐
       turns.  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 operat‐
       ing systems, and when code is ported to Linux that reuses a struct timeval for multiple select()s in a loop  without  reini‐
       tializing 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 time‐
       out 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 be‐
       comes undefined.

       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.

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

       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 POSIX.1-2008.

       fd_set is defined in POSIX.1-2001 and later.

       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.

       On some other UNIX systems, select() can fail with the error EAGAIN if the system  fails  to  allocate  kernel-internal  re‐
       sources, 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 sys‐
       tems, 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  pse‐
       lect6().  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  be‐
       tween 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/se‐

       POSIX allows an implementation to define an upper limit, advertised via the constant FD_SETSIZE, on the range  of  file  de‐
       scriptors  that can be specified in a file descriptor set.  The Linux kernel imposes no fixed limit, but the glibc implemen‐
       tation 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

       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.

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

           int             retval;
           fd_set          rfds;
           struct timeval  tv;

           /* 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");


       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).

Linux man-pages 6.03                                         2023-02-05                                                   select(2)