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 (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 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) 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(&fdsetx) 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 affect a poll, the timeout argument should be non-NULL, 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.

RETURN VALUES
Select returns the number of ready descriptors that are contained in the descriptor sets, or -1 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.

ERRORS
An error return from select indicates:

    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), getdtablesize(2), gettimeofday(2), read(2), recv(2), send(2), write(2), clocks(7)

BUGS
Although the provision of getdtablesize(2) 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 32 for GNO) is somewhat larger than the current kernel limit to the number of open files. Unlike BSD systems, under GNO it is not possible for sites to reconfigure this limit since it requires recompilation of the kernel.

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 value will be unmodified by the select call.

HISTORY
The select function call appeared in 4.2BSD. The first appearance in GNO was in v2.0.5.