— synchronous I/O
Standard C Library (libc, -lc)
, fd_set *readfds
struct timeval *timeout
() system call examines the I/O descriptor sets
whose addresses are passed in readfds
, 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
descriptors are checked in each set; i.e., the
descriptors from 0 through nfds-1
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
() system 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 descriptor sets:
) initializes a
descriptor set fdset
to the null set.
includes a particular descriptor fd
) removes fd
) is non-zero if fd
is a member
, zero otherwise. The behavior of these macros is
undefined if a descriptor value is less than zero or greater than or equal to
, which is normally at least equal to the
maximum number of descriptors supported by the system.
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.
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
may be given as null pointers if no descriptors
are of interest.
() 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
() returns with an error, including one due to an
interrupted system call, the descriptor sets will be unmodified.
An error return from select
- One of the descriptor sets specified an invalid
- 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
is currently 1024. In
order to accommodate programs which might potentially use a larger number of
open files with select
(), it is possible to increase this
size by having the program define
the inclusion of any header which includes
is greater than the number of open files,
() is not guaranteed to examine the unused file
descriptors. For historical reasons, select
() will always
examine the first 256 descriptors.
() system call and FD_CLR
() macros conform with IEEE Std 1003.1-2001
() 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.