select(2)

Name

select − synchronous I/O multiplexing

Syntax

#include <sys/types.h>
#include <sys/time.h>
int select (nfds, readfds, writefds, exceptfds, timeout)
int nfsd;
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

The select system call 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 exception condition pending. The nfds parameter is the number of bits to be checked in each bit mask that represent a file descriptor; the descriptors from 0 through nfds-1 in the descriptor sets are examined. Typically nfds has the value returned by getdtablesize() 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; otherwise the value is set to zero.

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 equal to the maximum number of descriptors that can be 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 call 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 call has been performed indicates that a subsequent accept call on that descriptor will not block.

Restrictions

The select call 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 the fcntl call.

Return Values

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

Diagnostics

[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’s 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. The t_sec range must be between 0 and 10^8, inclusive; t_usec must be greater than or equal to 0, and less than 10^6.

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