SELECT(2) System Calls SELECT(2)
select - synchronous I/O multiplexing
int select (int nfds, fd_set *readfds, fd_set *writefds, fd_set
*exceptfds, struct timeval *timeout);
FD_SET (int fd, fd_set &fdset);
FD_CLR (int fd, fd_set &fdset);
FD_ISSET (int fd, fd_set &fdset);
FD_ZERO (fd_set &fdset);
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 descriptors are
checked in each set; i.e., the descriptors from 0 through (nfds) 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. Select 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(&fdsetx) 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 a non-NULL pointer, it specifies a maximum interval to
wait for the selection to complete. If timeout is a NULL pointer, the
select blocks indefinitely. To affect a poll, the timeout argument
should be non-NULL, pointing to a zero-valued timeval structure.
Any of readfds, writefds, and exceptfds may be given as NULL pointers
if no descriptors are of interest.
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, select returns 0. If select returns with an error, including
one due to an interrupted call, the descriptor sets will be unmodified.
An error return from select indicates:
EBADF One of the descriptor sets specified an invalid descrip-
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.
accept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2),
recv(2), send(2), write(2), clocks(7)
Although the provision of getdtablesize(2) was intended to allow user
programs to be written independent of the kernel limit on the number of
open files, the dimension of a sufficiently large bit field for select
remains a problem. The default size FD_SETSIZE (currently 32 for GNO)
is somewhat larger than the current kernel limit to the number of open
files. Unlike BSD systems, under GNO it is not possible for sites to
reconfigure this limit since it requires recompilation of the kernel.
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 system. Thus, it is unwise to
assume that the timeout value will be unmodified by the select call.
The select function call appeared in 4.2BSD. The first appearance in
GNO was in v2.0.5.
GNO 15 January 1997 SELECT(2)
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