SELECT(2) SELECT(2)
NAME
select - synchronous input/output multiplexing
SYNOPSIS
#include <sys/types.h>
int select(nfds, readfds, writefds, milli);
fd_set *readfds, *writefds;
DESCRIPTION
Select examines a set of file descriptors to see if they
will block if read or written. Readfds points to an object
of type fd_set, which contains a set of descriptors to be
checked for reading; writefds similarly for writing. Only
descriptors 0 through nfds-1 are considered. The number of
ready descriptors is returned, and the fd_set pointed to by
readfds (writefds) is overwritten with a set of descriptors
ready to be read (written). The call waits until at least
one descriptor is ready, or until milli milliseconds have
passed.
Either readfds or writefds may be 0 if no descriptors are
interesting.
The fd_set type looks like
typedef struct {
unsigned int fds_bits[FDWORDS];
} fd_set;
FDWORDS is sufficient to contain as many file descriptors as
the system will allow (currently 128). If there are B bits
in an unsigned int, file descriptor n is represented by
1<<((n%B)-1) in word fds_bits[n/B].
These macros should be used to manipulate the contents of an
fd_set:
FD_ZERO(s) clear all bits in set s
FD_SET(n, s) set bit for file descriptor n in set s
FD_CLR(n, s) clear bit for file descriptor n in s
FD_ISSET(n, s) return a value of 1 if bit for file descrip-
tor n is set in s, 0 otherwise
EXAMPLES
int p[2];
fd_set wfs;
pipe(p);
do {
FD_SET(p[1], wfs);
write(p[1], ".", 1);
i++;
SELECT(2) SELECT(2)
} while(select(p[1]+1, (fd_set*)0, wfs, 0) == 1);
printf("Pipe capacity = %d\n", i);
SEE ALSO
read(2)
DIAGNOSTICS
EBADF, EFAULT, EINTR
BUGS
Milli is rounded up to the nearest second.
Select is intended for use with streams; file descriptors
referring to ordinary files or to non-stream special files
always appear ready. This is a lie for some special files.