SELECT(2)

NAME
select - synchronous I/O multiplexing

SYNOPSIS
#include <sys/types.h>
#include <sys/time.h>
#include <unistd.h>
int
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);
FD_SET(fd, &fdset);
FD_CLR(fd, &fdset);
FD_ISSET(fd, &fdset);
FD_ZERO(&fdset);

DESCRIPTION
select() examines 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. On return, select() replaces the given descriptor sets with subsets consisting of those descriptors that are ready for the requested operation. select() returns 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 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 effect a poll, the timeout argument should be non-null, pointing to a zero-valued timeval structure. 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().

Any of readfds, writefds, and exceptfds may be given as null pointers if no descriptors are of interest.

RETURN VALUES
select() returns the number of ready descriptors that are contained in the descriptor sets, or -1 is 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.

ERRORS
An error return from select() indicates:

[EFAULT]
    One or more of readfds, writefds, or exceptfds points outside the process's allocated address space.

[EBADF]
    One of the descriptor sets specified an invalid descriptor.

[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)

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.

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);

Alternatively, it is possible to use the poll(2) interface. poll(2) is more efficient when the size of select()'s fd_set bit-arrays are very large, and for fixed numbers of file descriptors one need not size and dynamically allocate a memory object.

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 brain-damaged 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. Given that, it is rather surprising to see that many daemons are written that way (i.e., httpd(8)).

HISTORY
The select() function call appeared in 4.2BSD.