Originální popis anglicky:
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
multiplexing
Návod, kniha: Linux Programmer's Manual
/* According to POSIX 1003.1-2001 */
#include <sys/select.h>
/* According to earlier standards */
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int n, fd_set *readfds, fd_set
* writefds, fd_set *exceptfds, struct
timeval * timeout);
int pselect(int n, fd_set *readfds, fd_set
* writefds, fd_set *exceptfds, const struct
timespec * timeout, const sigset_t *sigmask);
FD_CLR(int fd, fd_set *set);
FD_ISSET(int fd, fd_set *set);
FD_SET(int fd, fd_set *set);
FD_ZERO(fd_set *set);
The functions
select and
pselect wait for a number of file
descriptors to change status.
Their function is identical, with three differences:
- (i)
- The select function uses a timeout that is a
struct timeval (with seconds and microseconds), while
pselect uses a struct timespec (with seconds and
nanoseconds).
- (ii)
- The select function may update the timeout
parameter to indicate how much time was left. The pselect function
does not change this parameter.
- (iii)
- The select function has no sigmask parameter,
and behaves as pselect called with NULL sigmask.
Three independent sets of descriptors are watched. Those listed in
readfds will be watched to see if characters become available for
reading (more precisely, to see if a read will not block - in particular, a
file descriptor is also ready on end-of-file), those in
writefds will
be watched to see if a write will not block, and those in
exceptfds
will be watched for exceptions. On exit, the sets are modified in place to
indicate which descriptors actually changed status.
Four macros are provided to manipulate the sets.
FD_ZERO will clear a
set.
FD_SET and
FD_CLR add or remove a given descriptor from a
set.
FD_ISSET tests to see if a descriptor is part of the set; this is
useful after
select returns.
n is the highest-numbered descriptor in any of the three sets, plus 1.
timeout is an upper bound on the amount of time elapsed before
select returns. It may be zero, causing
select to return
immediately. (This is useful for polling.) If
timeout is NULL (no
timeout),
select can block indefinitely.
sigmask is a pointer to a signal mask (see
sigprocmask(2)); if it
is not NULL, then
pselect first replaces the current signal mask by the
one pointed to by
sigmask, then does the `select' function, and then
restores the original signal mask again.
The idea of
pselect is that if one wants to wait for an event, either a
signal or something on a file descriptor, an atomic test is needed to prevent
race conditions. (Suppose the signal handler sets a global flag and returns.
Then a test of this global flag followed by a call of
select() could
hang indefinitely if the signal arrived just after the test but just before
the call. On the other hand,
pselect allows one to first block signals,
handle the signals that have come in, then call
pselect() with the
desired
sigmask, avoiding the race.) Since Linux today does not have a
pselect() system call, the current glibc2 routine still contains this
race.
The time structures involved are defined in
<sys/time.h> and look
like
struct timeval {
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
and
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
(However, see below on the POSIX 1003.1-2001 versions.)
Some code calls
select with all three sets empty,
n zero, and a
non-null
timeout as a fairly portable way to sleep with subsecond
precision.
On Linux, the function
select modifies
timeout to reflect the
amount of time not slept; most other implementations do not do this. This
causes problems both when Linux code which reads
timeout is ported to
other operating systems, and when code is ported to Linux that reuses a struct
timeval for multiple
selects in a loop without reinitializing it.
Consider
timeout to be undefined after
select returns.
On success,
select and
pselect return the number of descriptors
contained in the three returned descriptor sets (that is, the total number of
one bits in
readfds,
writefds,
exceptfds) which may be
zero if the timeout expires before anything interesting happens. On error, -1
is returned, and
errno is set appropriately; the sets and
timeout become undefined, so do not rely on their contents after an
error.
- EBADF
- An invalid file descriptor was given in one of the
sets.
- EINTR
- A non blocked signal was caught.
- EINVAL
- n is negative or the value contained within
timeout is invalid.
- ENOMEM
- select was unable to allocate memory for internal
tables.
#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int
main(void) {
fd_set rfds;
struct timeval tv;
int retval;
/* Watch stdin (fd 0) to see when it has input. */
FD_ZERO(&rfds);
FD_SET(0, &rfds);
/* Wait up to five seconds. */
tv.tv_sec = 5;
tv.tv_usec = 0;
retval = select(1, &rfds, NULL, NULL, &tv);
/* Don't rely on the value of tv now! */
if (retval == -1)
perror("select()");
else if (retval)
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
else
printf("No data within five seconds.\n");
return 0;
}
4.4BSD (the
select function first appeared in 4.2BSD). Generally portable
to/from non-BSD systems supporting clones of the BSD socket layer (including
System V variants). However, note that the System V variant typically sets the
timeout variable before exit, but the BSD variant does not.
The
pselect function is defined in IEEE Std 1003.1g-2000 (POSIX.1g), and
part of POSIX 1003.1-2001. It is found in glibc2.1 and later. Glibc2.0 has a
function with this name, that however does not take a
sigmask
parameter.
An fd_set is a fixed size buffer. Executing FD_CLR or FD_SET with a value of
fd that is negative or is equal to or larger than FD_SETSIZE will
result in undefined behavior. Moreover, POSIX requires
fd to be a valid
file descriptor.
Concerning the types involved, the classical situation is that the two fields of
a struct timeval are longs (as shown above), and the struct is defined in
<sys/time.h>. The POSIX 1003.1-2001 situation is
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
where the struct is defined in
<sys/select.h> and the data types
time_t and suseconds_t are defined in
<sys/types.h>.
Concerning prototypes, the classical situation is that one should include
<time.h> for
select. The POSIX 1003.1-2001 situation is
that one should include
<sys/select.h> for
select and
pselect. Libc4 and libc5 do not have a
<sys/select.h>
header; under glibc 2.0 and later this header exists. Under glibc 2.0 it
unconditionally gives the wrong prototype for
pselect, under glibc
2.1-2.2.1 it gives
pselect when
_GNU_SOURCE is defined, under
glibc 2.2.2-2.2.4 it gives it when
_XOPEN_SOURCE is defined and has a
value of 600 or larger. No doubt, since POSIX 1003.1-2001, it should give the
prototype by default.
pselect is currently emulated with a user-space wrapper that has a race
condition. For reliable (and more portable) signal trapping, use the self-pipe
trick. (Where a signal handler writes to a pipe whose other end is read by the
main loop.)
Under Linux,
select may report a socket file descriptor as "ready
for reading", while nevertheless a subsequent read blocks. This could for
example happen when data has arrived but upon examination has wrong checksum
and is discarded. There may be other circumstances. Thus it may be safer to
use O_NONBLOCK on sockets that should not block.
For a tutorial with discussion and examples, see
select_tut(2).
For vaguely related stuff, see
accept(2),
connect(2),
poll(2),
read(2),
recv(2),
send(2),
sigprocmask(2),
write(2)
