SELECT(2) | System Calls Manual | SELECT(2) |
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);
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.
#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; }
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.
October 18, 2008 | NetBSD 6.1 |