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


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


     /* 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


     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 corre-
     sponding I/O operation (e.g., read(2) without  blocking,  or  a  suffi-
     ciently 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 indi-
     cate which file descriptors actually changed status.  (Thus,  if  using
     select()  within  a  loop,  the  sets must be reinitialized before each
     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
     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 inter-
     val  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
     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 sig-
     nal 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 */ };
         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 pre-
     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.


     On success, select() and pselect() return the number of  file  descrip-
     tors  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


     EBADF  An invalid file descriptor was given in one of the sets.   (Per-
            haps  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  to  Linux in kernel 2.6.16.  Prior to this, pse-
     lect() 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  vari-
     ants).   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.


     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
     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 pse-
     lect().  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
 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  pro-
     vided by poll(2) (and epoll(7)):
                            /*  Ready  for  reading  */  #define POLLOUT_SET
                            /*  Ready  for  writing  */  #define  POLLEX_SET
                            /* 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  par-
     ticular behavior in this scenario must be considered buggy.
 C library/kernel differences
     The  Linux kernel allows file descriptor sets of arbitrary size, deter-
     mining 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.   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  sup-
     port 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.


     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 descrip-
     tors in the three file descriptor sets, up to the limit  nfds-1.   How-
     ever,  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 descrip-
     tor that is specified in one of the sets should  result  in  the  error
     Glibc  2.0  provided a version of pselect() that did not take a sigmask
     Starting with version 2.1, glibc provided  an  emulation  of  pselect()
     that was implemented using sigprocmask(2) and select().  This implemen-
     tation 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.


     #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_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");
         exit(EXIT_SUCCESS); }


     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).


     This  page  is  part of release 4.16 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 2017-09-15 SELECT(2)

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