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Manual page for SELECT(2)

select - synchronous I/O multiplexing

SYNOPSIS

#include <sys/types.h>
#include <sys/time.h>

int select (width, readfds, writefds, exceptfds, timeout)
int width;
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;

FD_SET (fd, &fdset)	
FD_CLR (fd, &fdset)	
FD_ISSET (fd, &fdset)	
FD_ZERO (&fdset)	
int fd;
fd_set 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, ready for writing, or have an exceptional condition pending. width is the number of bits to be checked in each bit mask that represent a file descriptor; the descriptors from 0 through width-1 in the descriptor sets are examined. Typically width has the value returned by ulimit.3c for the maximum number of file descriptors. On return, select() replaces the given descriptor sets with subsets consisting of those descriptors that are ready for the requested operation. The total number of ready descriptors in all the sets is returned.

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 nonzero 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 not a 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 a non-NULL pointer, pointing to a zero-valued timeval structure.

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

Selecting true for reading on a socket descriptor upon which a listen.2 call has been performed indicates that a subsequent accept.2 call on that descriptor will not block.

RETURN VALUES

select() returns a non-negative value on success. A positive value indicates the number of ready descriptors in the descriptor sets. 0 indicates that the time limit referred to by timeout expired. On failure, select() returns -1, sets errno to indicate the error, and the descriptor sets are not changed.

ERRORS

EBADF: One of the descriptor sets specified an invalid descriptor.
EFAULT: One of the pointers given in the call referred to a non-existent portion of the process' address space.
EINTR: A signal was delivered before any of the selected events occurred, or before the time limit expired.
EINVAL: A component of the pointed-to time limit is outside the acceptable range: t_sec must be between 0 and 10, inclusive. t_usec must be greater than or equal to 0, and less than 10.

NOTES

Under rare circumstances, select() may indicate that a descriptor is ready for writing when in fact an attempt to write would block. This can happen if system resources necessary for a write are exhausted or otherwise unavailable. If an application deems it critical that writes to a file descriptor not block, it should set the descriptor for non-blocking I/O using the F_SETFL request to fcntl.2v

BUGS

Although the provision of ulimit.3c 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 size FD_SETSIZE (currently 256) is somewhat larger 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>.

select() should probably return the time remaining from the original timeout, if any, by modifying the time value in place. This may be implemented in future versions of the system. Thus, it is unwise to assume that the timeout pointer will be unmodified by the select() call.