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select - synchronous I/O multiplexing



      select - synchronous I/O multiplexing


      Standard C Library (libc, -lc)


      #include <sys/select.h>
      select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
              struct timeval *timeout);
      FD_SET(fd, &fdset);
      FD_CLR(fd, &fdset);
      FD_ISSET(fd, &fdset);


      The select() system call examines the I/O descriptor sets whose addresses
      are passed in readfds, writefds, and exceptfds to see if some of their
      descriptors are ready for reading, are ready for writing, or have an
      exceptional condition pending, respectively.  The only exceptional condi‐
      tion detectable is out-of-band data received on a socket.  The first nfds
      descriptors are checked in each set; i.e., the descriptors from 0 through
      nfds-1 in the descriptor sets are examined.  On return, select() replaces
      the given descriptor sets with subsets consisting of those descriptors
      that are ready for the requested operation.  The select() system call
      returns the total number of ready descriptors in all the sets.
      The descriptor sets are stored as bit fields in arrays of integers.  The
      following macros are provided for manipulating such descriptor sets:
      FD_ZERO(&fdset) initializes a descriptor set fdset to the null set.
      FD_SET(fd, &fdset) includes a particular descriptor fd in fdset.
      FD_CLR(fd, &fdset) removes fd from fdset.  FD_ISSET(fd, &fdset) is non-
      zero if fd is a member of fdset, zero otherwise.  The behavior of these
      macros is undefined if a descriptor value is less than zero or greater
      than or equal to FD_SETSIZE, which is normally at least equal to the max‐
      imum number of descriptors supported by the system.
      If timeout is not a null pointer, it specifies the maximum interval to
      wait for the selection to complete.  System activity can lengthen the
      interval by an indeterminate amount.
      If timeout is a null pointer, the select blocks indefinitely.
      To effect a poll, the timeout argument should not be a null pointer, but
      it should point to a zero-valued timeval structure.
      Any of readfds, writefds, and exceptfds may be given as null pointers if
      no descriptors are of interest.
      The select() system call returns the number of ready descriptors that are
      contained in the descriptor sets, or -1 if an error occurred.  If the
      time limit expires, select() returns 0.  If select() returns with an
      error, including one due to an interrupted system call, the descriptor
      sets will be unmodified.


      An error return from select() indicates:
      [EBADF]            One of the descriptor sets specified an invalid
      [EFAULT]           One of the arguments readfds, writefds, exceptfds, or
                         timeout points to an invalid address.
      [EINTR]            A signal was delivered before the time limit expired
                         and before any of the selected events occurred.
      [EINVAL]           The specified time limit is invalid.  One of its com‐
                         ponents is negative or too large.
      [EINVAL]           The nfds argument was invalid.
      accept(2), connect(2), getdtablesize(2), gettimeofday(2), kqueue(2),
      poll(2), read(2), recv(2), send(2), write(2), clocks(7)


      The default size of FD_SETSIZE is currently 1024.  In order to accommo‐
      date programs which might potentially use a larger number of open files
      with select(), it is possible to increase this size by having the program
      define FD_SETSIZE before the inclusion of any header which includes
      If nfds is greater than the number of open files, select() is not guaran‐
      teed to examine the unused file descriptors.  For historical reasons,
      select() will always examine the first 256 descriptors.


      The select() system call and FD_CLR(), FD_ISSET(), FD_SET(), and
      FD_ZERO() macros conform with IEEE Std 1003.1-2001 (“POSIX.1”).


      The select() system call appeared in 4.2BSD.


      Version 2 of the Single UNIX Specification (“SUSv2”) allows systems to
      modify the original timeout in place.  Thus, it is unwise to assume that
      the timeout value will be unmodified by the select() system call.


Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.