select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing
#include <sys/select.h> /* according to POSIX */
/* Earlier standards */ #include <sys/time.h> #include <sys/types.h> #include <sys/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, 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(2) and pselect(2) wait for a number of file descriptors to change status.
Their function is identical, with three differences:
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 */
};
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 descriptor sets, 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.
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, timeout is modified 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.
This is very slow if the number of fd's in the fdsets are large, especially if not many are active at any particular time. This can have the surprising effect of your program using 100% cpu time with a very low number of fd's (a few thousand), but still being able to run effectively for as much as 10x the number of fds. This is due to the fact that each time through your loop around select(2), you only have (on average) one fd to wake up initially, so you have a lot of overhead for that one fd. However as you get more fd's more and more of them occur during one pass through select(2) and your program starts doing more work per mainloop.
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)
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). It is found in glibc2.1 and later. Glibc2.0 has a function with this name, that however does not take a sigmask parameter.
accept(2), connect(2), poll(2), read(2), recv(2), send(2), sigprocmask(2), write(2)
26 pages link to select(2):
