/* According to POSIX.1-2001 */ #include <sys/select.h> /* According to earlier standards */ #include <sys/time.h> #include <sys/types.h> #include <unistd.h> int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); void FD_CLR(int fd, fd_set *set); int FD_ISSET(int fd, fd_set *set); void FD_SET(int fd, fd_set *set); void FD_ZERO(fd_set *set); #define _XOPEN_SOURCE 600 #include <sys/select.h> int pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, const struct timespec *timeout, const sigset_t *sigmask); |
The operation of select() and pselect() is identical, with three differences:
| 標(biāo)簽 | 描述 |
|---|---|
| (i) | select() uses a timeout that is a struct timeval (with seconds and microseconds), while pselect() uses a struct timespec (with seconds and nanoseconds). |
| (ii) | select() may update the timeout argument to indicate how much time was left. pselect() does not change this argument. |
| (iii) | select() has no sigmask argument, and behaves as pselect() called with NULL sigmask. |
Four macros are provided to manipulate the sets. FD_ZERO() clears a set. FD_SET() and FD_CLR() respectively add and remove a given file descriptor from a set.FD_ISSET() tests to see if a file descriptor is part of the set; this is useful after select() returns.
nfds is the highest-numbered file 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.) Iftimeout is NULL (no timeout), select() can block indefinitely.
sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is not NULL, thenpselect() 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.
Other than the difference in the precision of the timeout argument, the followingpselect() call:
|
ready = pselect(nfds, &readfds, &writefds, &exceptfds, timeout, &sigmask); |
|
sigset_t origmask;
sigprocmask(SIG_SETMASK, &sigmask, &origmask); |
The reason that pselect() is needed is that if one wants to wait for either a signal or for a file descriptor to become ready, then 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. By contrast, pselect() allows one to first block signals, handle the signals that have come in, then call pselect() with the desiredsigmask, avoiding the race.)
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.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, select() modifies timeout to reflect the amount of time not slept; most other implementations do not do this. (POSIX.1-2001 permits either behaviour.) 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 multipleselect()s in a loop without reinitializing it. Consider timeout to be undefined afterselect() returns.
| 標(biāo)簽 | 描述 |
|---|---|
| EBADF | An invalid file descriptor was given in one of the sets. (Perhaps a file descriptor that was already closed, or one on which an error has occurred.) |
| EINTR | A signal was caught. |
| EINVAL | nfds is negative or the value contained within timeout is invalid. |
| ENOMEM | 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)
return 0; |
pselect() is defined in POSIX.1g, and in POSIX.1-2001.
Concerning the types involved, the classical situation is that the two fields of a timevalstructure are longs (as shown above), and the structure is defined in <sys/time.h>. The POSIX.1-2001 situation is
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
|
where the structure is defined in <sys/select.h> and the data types time_t andsuseconds_t are defined in <sys/types.h>.
Concerning prototypes, the classical situation is that one should include <time.h> forselect(). The POSIX.1-2001 situation is that one should include <sys/select.h> forselect() 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.1-2001, it should give the prototype by default.
Since version 2.1, glibc has provided an emulation of pselect() that is implemented using sigprocmask(2) and select(). This implementation remains vulnerable to the very race condition that pselect() was designed to prevent. On systems that lack pselect() reliable (and more portable) signal trapping can be achieved using the self-pipe trick (where a signal handler writes a byte to a pipe whose other end is monitored byselect() in the main program.)
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 in which a file descriptor is spuriously reported as ready. Thus it may be safer to use O_NONBLOCK on sockets that should not block.
For vaguely related stuff, see accept(2), connect(2), poll(2), read(2), recv(2), send(2),sigprocmask(2), write(2), epoll(7), feature_test_macros(7)
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