SELECT(2) System Calls Manual SELECT(2)
select - synchronous I/O multiplexing
int select (width, readfds, writefds, exceptfds, timeout)
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;
FD_SET (fd, &&fdset)
FD_CLR (fd, &&fdset)
FD_ISSET (fd, &&fdset)
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 condi-
tion 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 descrip-
tors. On return, select() replaces the given descriptor sets with sub-
sets consisting of those descriptors that are ready for the requested
operation. The total number of ready descriptors in all the sets is
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 struc-
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 lis-
ten(2) call has been performed indicates that a subsequent accept(2)
call on that descriptor will not block.
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 fail-
ure, select() returns -1, sets errno to indicate the error, and the
descriptor sets are not changed.
EBADF One of the descriptor sets specified an invalid descrip-
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 10^8,
inclusive. t_usec must be greater than or equal to 0,
and less than 10^6.
accept(2), connect(2), fcntl(2V), ulimit(3C), gettimeofday(2), lis-
ten(2), read(2V), recv(2), send(2), write(2V)
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).
Although the provision of ulimit(3C) was intended to allow user pro-
grams 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 poten-
tially 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()
21 January 1990 SELECT(2)