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SELECT(2) | System Calls Manual | SELECT(2) |
NAME¶
select
—
synchronous I/O multiplexing
LIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include
<sys/select.h>
int
select
(int
nfds, fd_set
*readfds, fd_set
*writefds,
fd_set
*exceptfds,
struct timeval
*timeout);
FD_SET
(fd,
&fdset);
FD_CLR
(fd,
&fdset);
FD_ISSET
(fd,
&fdset);
FD_ZERO
(&fdset);
DESCRIPTION¶
Theselect
() 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 select
() 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:
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 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 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.
RETURN VALUES¶
Theselect
() 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.
ERRORS¶
An error return fromselect
() indicates:
- [
EBADF
] - One of the descriptor sets specified an invalid descriptor.
- [
EFAULT
] - One of the arguments readfds, writefds, exceptfds, or timeout points to an invalid address.
- [
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.
- [
EINVAL
] - The nfds argument was invalid.
SEE ALSO¶
accept(2), connect(2), getdtablesize(2), gettimeofday(2), kqueue(2), poll(2), read(2), recv(2), send(2), write(2), clocks(7)NOTES¶
The default size ofFD_SETSIZE
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
FD_SETSIZE
before the inclusion of any
header which includes
<sys/types.h>
.
If nfds is greater than the number of open
files, select
() is not guaranteed to
examine the unused file descriptors. For historical reasons,
select
() will always examine the first 256
descriptors.
STANDARDS¶
Theselect
() system call and
FD_CLR
(),
FD_ISSET
(),
FD_SET
(), and
FD_ZERO
() macros conform with
IEEE Std 1003.1-2001
(“POSIX.1”).
HISTORY¶
Theselect
() system call appeared in
4.2BSD.
BUGS¶
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 theselect
() system call.
FreeBSD does not modify the return value, which can
cause problems for applications ported from other systems.November 17, 2002 | Debian |