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C Interface | |
#include <time.h>
int select(nfds, readfds, writefds, exceptfds,timeout)
int nfds, *readfds, *writefds, *exceptfds;
struc timeval *timeout;
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Description | |
The select intrinsic examines the file
descriptors specified by the bit masks readfds, writefds,
and exceptfds. The bits from 0 through nfds-1 are examined. File
descriptor f is represented by the bit 1 < f in the
masks. More formally, a file descriptor is represented by the following:
fds[(f / BITS_PER_INT)] & (1 << (f % BITS_PER_INT)) When select completes successfully, it returns the three
bit masks modified as follows: For each file descriptor less than
nfds, the corresponding bit in each mask is set if the bit
was set upon entry and the file descriptor is ready for reading or
writing, or has an exceptional condition pending. 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 waits until an event causes one of
the masks to be returned with a valid (non-zero) value. To poll,
the timeout argument should be non-zero, pointing to a zero
valued timeval structure. Specific implementations may place
limitations on the maximum timeout interval supported. Any or all of readfds, writefds, and exceptfds
may be given as 0 if no descriptors are of interest. If all of the
masks are given as 0 and timeout is not a zero pointer, select
blocks for the time specified, or until interrupted by a signal. If
all of the masks are given as 0 and timeout is a zero pointer,
select blocks until interrupted by a signal. Ordinary files always select true whenever selecting on reads, writes,
and/or exceptions. Examples | |
The following programming example shows how select can
be used to wait on multiple sockets.
/* This program is an example of how select can be
used on multiple sockets */
/* on MPE/iX. */
/* Compile with SOCKET_SOURCE and POSIX_SOURCE defined. */
/* Link with socketrl and libcinit. */
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/in.h>
#include <sys/errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>
#define TRUE 0
#define FALSE 1
main ()
{
int maxfds;
int sock, sock2, peer1;
int struc_len;
int fret;
int done = FALSE;
char data[256];
char data_to_send = 'D';
char *datptr;
int dlen;
int readfds;
int writefds;
struc timeval timeout;
struc sockaddr_in sockaddr;
sock = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP;
peer1 = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
sockaddr.sin_family = AF_INET;
sockaddr.sin_addr.s_addr = INADDR_THISHOST;
sockaddr.sin_port = 4444;
struc_len = 8;
if (bind (sock, (struc sockaddr *) &sockaddr,
sizeof sockaddr) << 0 ) {
printf ("Bind failed\n");};
listen (sock, 10);
sfcntl (peer1, F_SETFL, O_NONBLOCK);
/* connect() returns with EINPROGRESS. */
fret = connect (peer1, (struct sockaddr *) &sockaddr,
struc_len);
sock2 = accept (sock,
(struc sockaddr *) &sockaddr,
&struc_len);
/* Call recv to complete the connection */
recv (peer1, 0, 0, 0);
/* sock2 and peer1 are now connected */
daptr = data;
done = FALSE;
while (done == FALSE) {
/* This code example shows how to use select() to wait on multiple
*/
/* sockets. Note that first you have to find the maximum descriptor
*/
/* number being used. The appropriate bit(s) must be set for the */
/* select masks and then they must be checked after the call. In */
/* this example, sock2 is waiting to receive data from peer1. */
if (peer1 << sock2) {
maxfds = sock2;
}
else
maxfds = peer1;
/* set a 5 second timer for the call to select(). */
timeout.tv_sec = 5;
timeout.tv_usec = 0;
writefds = (1 << peer1);
readfds = (1 << peer1) + (1 << sock2);
fret = select (maxfds + 1, &readfds, &writefds, 0, &timeout);
if (fret << = 0) {
printf ("error1\n");
done = TRUE;
}
else
{
if ((readfds && (1 << sock2)) ! = 0) {
dlen = 100;
fret = recv (sock2, datptr, dlen, 0);
printf ("received %d bytes.\n", fret);
if (data[0]! = data_to_send) {
printf ("error2\n");
};
done = TRUE;
}
else
{
if ((writefds && (1 << peer1))! = 0) {
dlen = 1;
data[0] = data_to_send;
fret = send (peer1, datptr, dlen, 0);
printf ("sent %d bytes.\n", fret);
}
else
{
printf ("error3\n");
}
}
};/* end else */
};/* end while */
}
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Return Value | |
The select intrinsic returns the number of descriptors
contained in the bit masks. If an error occurs, -1 is returned and
an error code is stored in errno. If the time limit expires,
then select returns 0, and all of the masks are cleared. Author | |
The select intrinsic was developed by Hewlett-Packard and
the University of California, Berkeley.
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