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BSD 2.11 - man page for select (bsd section 2)

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SELECT(2)										SELECT(2)

NAME
       select - synchronous I/O multiplexing

SYNOPSIS
       #include <sys/types.h>
       #include <sys/time.h>

       nfound = select(nfds, readfds, writefds, exceptfds, timeout)
       int nfound, nfds;
       fd_set *readfds, *writefds, *exceptfds;
       struct timeval *timeout;

       FD_SET(fd, &fdset)
       FD_CLR(fd, &fdset)
       FD_ISSET(fd, &fdset)
       FD_ZERO(&fdset)
       int fd;
       fd_set fdset;

DESCRIPTION
       Select  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 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 con-
       sisting of those descriptors that are ready for the requested operation.  The total number
       of ready descriptors in all the sets is returned in nfound.

       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 descrip-
       tor  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 nonzero  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  nor-
       mally at least equal to the maximum number of descriptors supported by the system.

       If  timeout  is a non-zero pointer, it specifies a maximum interval to wait for the selec-
       tion to complete.  If timeout is a zero	pointer,  the  select  blocks  indefinitely.   To
       affect  a poll, the timeout argument should be non-zero, pointing to a zero-valued timeval
       structure.

       Any of readfds, writefds, and exceptfds may be given as zero pointers  if  no  descriptors
       are of interest.

RETURN VALUE
       Select  returns the number of ready descriptors that are contained in the descriptor sets,
       or -1 if an error occurred.  If the time limit expires then select returns 0.   If  select
       returns	with an error, including one due to an interrupted call, the descriptor sets will
       be unmodified.

ERRORS
       An error return from select indicates:

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

       [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 components is negative or
		      too large.

SEE ALSO
       accept(2), connect(2), read(2), write(2), recv(2), send(2), getdtablesize(2)

BUGS
       Although the provision of getdtablesize(2) was intended to allow user programs to be writ-
       ten independent of the kernel limit on the number of open files, the dimension of a suffi-
       ciently large bit field for select remains a problem.  The default size	FD_SETSIZE  (cur-
       rently  256) is somewhat larger than the current kernel limit to the number of open files.
       However, in order to accommodate programs which might potentially use a larger  number  of
       open files with select, it is possible to increase this size within a program by providing
       a larger definition of FD_SETSIZE before the inclusion of <sys/types.h>.

       Select should probably return the time remaining from the original  timeout,  if  any,  by
       modifying the time value in place.  This may be implemented in future versions of the sys-
       tem.  Thus, it is unwise to assume that the timeout value will be unmodified by the select
       call.

4.2 Berkeley Distribution		   May 15, 1986 				SELECT(2)
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