SELECT(2) BSD System Calls Manual SELECT(2)NAME
select, pselect — synchronous I/O multiplexing
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/select.h>
int
select(int nfds, fd_set * restrict readfds, fd_set * restrict writefds,
fd_set * restrict exceptfds, struct timeval * restrict timeout);
int
pselect(int nfds, fd_set * restrict readfds, fd_set * restrict writefds,
fd_set * restrict exceptfds, const struct timespec *restrict timeout,
const sigset_t * restrict sigmask);
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);
DESCRIPTIONselect() and pselect() examine 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. This means that nfds must be set to
the highest file descriptor of the three sets, plus one. On return,
select() and pselect() replace the given descriptor sets with subsets
consisting of those descriptors that are ready for the requested opera‐
tion. select() and pselect() return the total number of ready descrip‐
tors 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(fdset) initializes a descriptor set pointed to by 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 poll without blocking, the timeout argument
should be non-null, pointing to a zero-valued timeval or timespec struc‐
ture, as appropriate. timeout is not changed by select(), and may be
reused on subsequent calls; however, it is good style to re-initialize it
before each invocation of select().
If sigmask is a non-null pointer, then the pselect() function shall
replace the signal mask of the caller by the set of signals pointed to by
sigmask before examining the descriptors, and shall restore the signal
mask of the calling thread before returning.
Any of readfds, writefds, and exceptfds may be given as null pointers if
no descriptors are of interest.
NOTES
It is recommended to use the poll(2) interface instead, which tends to be
more portable and efficient.
RETURN VALUESselect() 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, includ‐
ing one due to an interrupted call, the descriptor sets will be unmodi‐
fied.
EXAMPLES
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <err.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/time.h>
int
main(int argc, char **argv)
{
fd_set read_set;
struct timeval timeout;
int ret, fd, i;
/* file descriptor 1 is stdout */
fd = 1;
/* Wait for ten seconds. */
timeout.tv_sec = 10;
timeout.tv_usec = 0;
/* Initialize the read set to null */
FD_ZERO(&read_set);
/* Add file descriptor 1 to read_set */
FD_SET(fd, &read_set);
/*
* Check if data is ready to be readen on
* file descriptor 1, give up after 10 seconds.
*/
ret = select(fd + 1, &read_set, NULL, NULL, &timeout);
/*
* Returned value is the number of file
* descriptors ready for I/O, or -1 on error.
*/
switch (ret) {
case -1:
err(EXIT_FAILURE, "select() failed");
break;
case 0:
printf("Timeout, no data received.\n");
break;
default:
printf("Data received on %d file desciptor(s)\n", ret);
/*
* select(2) hands back a file descriptor set where
* only descriptors ready for I/O are set. These can
* be tested using FD_ISSET
*/
for (i = 0; i <= fd; i++) {
if (FD_ISSET(i, &read_set)) {
printf("Data on file descriptor %d\n", i);
/* Remove the file descriptor from the set */
FD_CLR(fd, &read_set);
}
}
break;
}
return 0;
}
ERRORS
An error return from select() indicates:
[EBADF] One of the descriptor sets specified an invalid
descriptor.
[EFAULT] One or more of readfds, writefds, or exceptfds points
outside the process's allocated address space.
[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 com‐
ponents is negative or too large.
SEE ALSOaccept(2), connect(2), gettimeofday(2), poll(2), read(2), recv(2),
send(2), write(2), getdtablesize(3)HISTORY
The select() function call appeared in 4.2BSD.
BUGS
Although the provision of getdtablesize(3) 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 bit size of fd_set is based on the symbol
FD_SETSIZE (currently 256), but that is somewhat smaller than the current
kernel limit to the number of open files. However, in order to accommo‐
date 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>. The kernel will cope, and the userland libraries provided
with the system are also ready for large numbers of file descriptors.
Note: rpc(3) library uses fd_set with the default FD_SETSIZE as part of
its ABI. Therefore, programs that use rpc(3) routines cannot change
FD_SETSIZE.
Alternatively, to be really safe, it is possible to allocate fd_set bit-
arrays dynamically. The idea is to permit a program to work properly
even if it is execve(2)'d with 4000 file descriptors pre-allocated. The
following illustrates the technique which is used by userland libraries:
fd_set *fdsr;
int max = fd;
fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS),
sizeof(fd_mask));
if (fdsr == NULL) {
...
return (-1);
}
FD_SET(fd, fdsr);
n = select(max+1, fdsr, NULL, NULL, &tv);
...
free(fdsr);
select() should probably have been designed to return the time remaining
from the original timeout, if any, by modifying the time value in place.
Even though some systems stupidly act in this different way, it is
unlikely this semantic will ever be commonly implemented, as the change
causes massive source code compatibility problems. Furthermore, recent
new standards have dictated the current behaviour. In general, due to
the existence of those non-conforming systems, it is unwise to assume
that the timeout value will be unmodified by the select() call, and the
caller should reinitialize it on each invocation. Calculating the delta
is easily done by calling gettimeofday(2) before and after the call to
select(), and using timersub() (as described in getitimer(2)).
Internally to the kernel, select() works poorly if multiple processes
wait on the same file descriptor.
BSD October 18, 2008 BSD
