| 1 |
786 |
skrzyp |
<html>
|
| 2 |
|
|
<body>
|
| 3 |
|
|
<pre>
|
| 4 |
|
|
NAME
|
| 5 |
|
|
select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous
|
| 6 |
|
|
I/O multiplexing
|
| 7 |
|
|
|
| 8 |
|
|
SYNOPSIS
|
| 9 |
|
|
#include <network.h>
|
| 10 |
|
|
|
| 11 |
|
|
int select(int n, fd_set *readfds, fd_set *writefds,
|
| 12 |
|
|
fd_set *exceptfds, struct timeval *timeout);
|
| 13 |
|
|
|
| 14 |
|
|
FD_CLR(int fd, fd_set *set);
|
| 15 |
|
|
FD_ISSET(int fd, fd_set *set);
|
| 16 |
|
|
FD_SET(int fd, fd_set *set);
|
| 17 |
|
|
FD_ZERO(fd_set *set);
|
| 18 |
|
|
|
| 19 |
|
|
DESCRIPTION
|
| 20 |
|
|
select waits for a number of file descriptors to change
|
| 21 |
|
|
status.
|
| 22 |
|
|
|
| 23 |
|
|
Three independent sets of descriptors are watched. Those
|
| 24 |
|
|
listed in readfds will be watched to see if characters
|
| 25 |
|
|
become available for reading, those in writefds will be
|
| 26 |
|
|
watched to see if it is ok to immediately write on them,
|
| 27 |
|
|
and those in exceptfds will be watched for exceptions. On
|
| 28 |
|
|
exit, the sets are modified in place to indicate which
|
| 29 |
|
|
descriptors actually changed status.
|
| 30 |
|
|
|
| 31 |
|
|
Four macros are provided to manipulate the sets. FD_ZERO
|
| 32 |
|
|
will clear a set. FD_SET and FD_CLR add or remove a given
|
| 33 |
|
|
descriptor from a set. FD_ISSET tests to see if a
|
| 34 |
|
|
descriptor is part of the set; this is useful after select
|
| 35 |
|
|
returns.
|
| 36 |
|
|
|
| 37 |
|
|
n is the highest-numbered descriptor in any of the three
|
| 38 |
|
|
sets, plus 1.
|
| 39 |
|
|
|
| 40 |
|
|
timeout is an upper bound on the amount of time elapsed
|
| 41 |
|
|
before select returns. It may be zero, causing select to
|
| 42 |
|
|
return immediately. If timeout is NULL (no timeout),
|
| 43 |
|
|
select can block indefinitely.
|
| 44 |
|
|
|
| 45 |
|
|
RETURN VALUE
|
| 46 |
|
|
On success, select returns the number of descriptors con-
|
| 47 |
|
|
tained in the descriptor sets, which may be zero if the
|
| 48 |
|
|
timeout expires before anything interesting happens. On
|
| 49 |
|
|
error, -1 is returned, and errno is set appropriately; the
|
| 50 |
|
|
sets and timeout become undefined, so do not rely on their
|
| 51 |
|
|
contents after an error.
|
| 52 |
|
|
|
| 53 |
|
|
ERRORS
|
| 54 |
|
|
EBADF An invalid file descriptor was given in one of the
|
| 55 |
|
|
sets.
|
| 56 |
|
|
|
| 57 |
|
|
EINTR A non blocked signal was caught.
|
| 58 |
|
|
|
| 59 |
|
|
EINVAL n is negative.
|
| 60 |
|
|
|
| 61 |
|
|
ENOMEM select was unable to allocate memory for internal
|
| 62 |
|
|
tables.
|
| 63 |
|
|
|
| 64 |
|
|
NOTES
|
| 65 |
|
|
Some code calls select with all three sets empty, n zero,
|
| 66 |
|
|
and a non-null timeout as a fairly portable way to sleep
|
| 67 |
|
|
with subsecond precision.
|
| 68 |
|
|
|
| 69 |
|
|
EXAMPLE
|
| 70 |
|
|
#include <stdio.h>
|
| 71 |
|
|
#include <sys/time.h>
|
| 72 |
|
|
#include <sys/types.h>
|
| 73 |
|
|
#include <unistd.h>
|
| 74 |
|
|
|
| 75 |
|
|
int
|
| 76 |
|
|
main(void)
|
| 77 |
|
|
{
|
| 78 |
|
|
fd_set rfds;
|
| 79 |
|
|
struct timeval tv;
|
| 80 |
|
|
int retval;
|
| 81 |
|
|
|
| 82 |
|
|
/* Watch stdin (fd 0) to see when it has input. */
|
| 83 |
|
|
FD_ZERO(&rfds);
|
| 84 |
|
|
FD_SET(0, &rfds);
|
| 85 |
|
|
/* Wait up to five seconds. */
|
| 86 |
|
|
tv.tv_sec = 5;
|
| 87 |
|
|
tv.tv_usec = 0;
|
| 88 |
|
|
|
| 89 |
|
|
retval = select(1, &rfds, NULL, NULL, &tv);
|
| 90 |
|
|
/* Don't rely on the value of tv now! */
|
| 91 |
|
|
|
| 92 |
|
|
if (retval)
|
| 93 |
|
|
printf("Data is available now.\n");
|
| 94 |
|
|
/* FD_ISSET(0, &rfds) will be true. */
|
| 95 |
|
|
else
|
| 96 |
|
|
printf("No data within five seconds.\n");
|
| 97 |
|
|
|
| 98 |
|
|
exit(0);
|
| 99 |
|
|
}
|
| 100 |
|
|
|
| 101 |
|
|
Generally portable to/from non-BSD systems supporting
|
| 102 |
|
|
clones of the BSD socket layer (including System V vari-
|
| 103 |
|
|
ants). However, note that the System V variant typically
|
| 104 |
|
|
sets the timeout variable before exit, but the BSD variant
|
| 105 |
|
|
does not.
|
| 106 |
|
|
</pre>
|
| 107 |
|
|
</body>
|
| 108 |
|
|
</html>
|
