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SELECT(2)		       System Calls Manual		       SELECT(2)

NAME
     select -- synchronous I/O multiplexing

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/select.h>

     int
     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);

     FD_ZERO(&fdset);

DESCRIPTION
     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  de-
     scriptors	are  ready for reading, are ready for writing, or have an excep-
     tional condition pending, respectively.  The only exceptional condition de-
     tectable is out-of-band data received on a socket.  The first nfds descrip-
     tors 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 maximum 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.

RETURN VALUES
     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,  in-
     cluding  one due to an interrupted system call, the descriptor sets will be
     unmodified.

ERRORS
     An error return from select() indicates:

     [EBADF]		One of the descriptor sets specified an invalid descrip-
			tor.

     [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 compo-
			nents is negative or too large.

     [EINVAL]		The nfds argument was invalid.

SEE ALSO
     accept(2),  connect(2),   getdtablesize(2),   gettimeofday(2),   kqueue(2),
     poll(2), pselect(2), read(2), recv(2), send(2), write(2), clocks(7)

NOTES
     The  default size of FD_SETSIZE is currently 1024.  In order to accommodate
     programs which might potentially use a larger number of open files with se-
     lect(), it is possible to increase this size by having the  program  define
     FD_SETSIZE before the inclusion of any header which includes <sys/types.h>.

     If  nfds  is greater than the number of open files, select() is not guaran-
     teed to examine the unused file descriptors.  For historical  reasons,  se-
     lect() will always examine the first 256 descriptors.

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

HISTORY
     The select() system call appeared in 4.2BSD.

BUGS
     Version 2 of the Single UNIX Specification ("SUSv2") allows systems to mod-
     ify the original timeout in place.  Thus, it is unwise to assume  that  the
     timeout value will be unmodified by the select() system call.  FreeBSD does
     not  modify  the  return  value,  which can cause problems for applications
     ported from other systems.

FreeBSD ports 15.quarterly	  June 25, 2020 		       SELECT(2)

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