SELECT(2)SELECT(2)NAMEselect - 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 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 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 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
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 normally 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 selection to complete. If timeout is a zero pointer, the
select blocks indefinitely. To effect 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 ALSOaccept(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 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 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 system. 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)
