|SELECT(2)||System Calls Manual||SELECT(2)|
synchronous I/O multiplexing
Standard C Library (libc, -lc)
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, are ready for writing, or have an exceptional condition
pending, respectively. The only exceptional condition detectable is
out-of-band data received on a socket. 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. The
call 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
initializes a descriptor set fdset to the null set.
&fdset) includes a particular descriptor
fd in fdset.
&fdset) removes fd from
&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
If timeout is not a null pointer, it specifies the maximum interval to wait for the selection to complete. System activity can lengthen the interval by an indeterminate amount.
If timeout is a null pointer, the select blocks indefinitely.
To effect a poll, the timeout argument should not be a null pointer, but it should point to a zero-valued timeval structure.
Any of readfds, writefds, and exceptfds may be given as null pointers if no descriptors are of interest.
select() system call 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 system call, the descriptor sets will be unmodified.
An error return from
- One of the descriptor sets specified an invalid descriptor.
- One of the arguments readfds, writefds, exceptfds, or timeout points to an invalid address.
- A signal was delivered before the time limit expired and before any of the selected events occurred.
- The specified time limit is invalid. One of its components is negative or too large.
- The nfds argument was invalid.
The default size of
currently 1024. In order to accommodate programs which might potentially use
a larger number of open files with
it is possible to increase this size by having the program define
FD_SETSIZE before the inclusion of any header which
If nfds is greater than the
number of open files,
is not guaranteed to examine the unused file descriptors. For historical
select() will always examine the first 256
select() system call and
macros conform with IEEE Std 1003.1-2001
select() system call appeared in
Version 2 of the Single UNIX Specification
(“SUSv2”) allows systems to modify the original timeout
in place. Thus, it is unwise to assume that the timeout value will be
unmodified by the
select() system call.
FreeBSD does not modify the return value, which can
cause problems for applications ported from other systems.
|June 25, 2020||Debian|