SELECT(2)

SELECT(2) | System Calls Manual | SELECT(2)

# NAME

`select`, `pselect`, `FD_SET`, `FD_CLR`, `FD_ISSET`, `FD_ZERO` — synchronous I/O multiplexing

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <sys/select.h>`

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);

# DESCRIPTION

`select`() 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.

# NOTES

It is recommended to use the poll(2) interface instead, which tends to be more portable and efficient.

# RETURN VALUES

`select`() returns the number of ready descriptors that are contained in the descriptor sets, or returns -1 and sets `errno` 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.

# EXAMPLES

    #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 read 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 descriptor(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;
    }

# ERRORS

An error return from `select`() indicates:

[`EBADF`]
  One of the descriptor sets specified an invalid descriptor.
[`EFAULT`]
  One or more of readfds, writefds, or exceptfds points outside the process's allocated address space.
[`EINTR`]
  A signal was delivered before the time limit expired and before any of the selected events occurred.
[`EINVAL`]
  The specified time limit is invalid. One of its components is negative or too large.

# SEE ALSO

accept(2), connect(2), gettimeofday(2), poll(2), read(2), recv(2), send(2), write(2), getdtablesize(3)

# STANDARDS

The `select`() function, along with `FD_CLR`(), `FD_ISSET`(), `FD_SET`(), and `FD_ZERO`(), conforms to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).

The `pselect`() function conforms to IEEE Std 1003.1-2004 (“POSIX.1”).

# HISTORY

The `select`() function call appeared in 4.2BSD. The `pselect`() function call first appeared in NetBSD 3.0.

# BUGS

Although the provision of getdtablesize(3) was intended to allow user programs to be written independent of the kernel limit on the number of open files, the dimension of a sufficiently large bit field for select remains a problem. The default bit size of 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.

September 9, 2024 | NetBSD 11.0