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
)
)
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 operation.
select(
)
and
pselect(
)
return the total number of ready descriptors 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
structure, 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.
)
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,
including one due to an interrupted call,
the descriptor sets will be unmodified.
#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;
}
)
indicates:
EBADF
]
EFAULT
]
readfds
,
writefds
,
or
exceptfds
points outside the process's allocated address space.
EINTR
]
EINVAL
]
)
function call appeared in
4.2BSD.
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 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
>.
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.