select(3C)
select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO --
synchronous I/O multiplexing
Synopsis
#include <sys/time.h>
#include <sys/types.h>
#include <sys/select.h>
int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *execptfds,
struct timeval *timeout);
void FD_SET(int fd, fd_set *fdset);
void FD_CLR(int fd, fd_set *fdset);
int FD_ISSET(int fd, fd_set *fdset);
void FD_ZERO(fd_set *fdset);
Description
select
examines the
I/O
descriptor sets whose addresses are passed in
readfds,
writefds,
and
execptfds
to see if any of their descriptors are ready for reading,
are ready for writing,
or have an exceptional condition pending, respectively.
nfds
is the number of bits to be checked in each bit mask that
represents a file descriptor; the descriptors from 0 to
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 return value from the call to select() is the number
of ready descriptors.
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.
FD_SETSIZE
is a constant defined in sys/select.h and 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 affect a poll, the
timeout
argument should be a non-NULL pointer,
pointing to a zero-valued timeval structure.
Any of
readfds,
writefds,
and
execptfds
may be given as NULL
pointers if no descriptors are of interest.
Return values
select
returns the number of ready descriptors contained in
the descriptor sets
or -1 if an error occurred.
If the time limit expires, then
select
returns 0.
Errors
An error return from select indicates:
- EBADF
-
One of the I/O descriptor sets specified an invalid I/O descriptor.
- EINTR
-
A signal was delivered before any of the
selected events occurred, or 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
8,
inclusive.
t_usec
must be greater-than or equal to 0, and less than
6.
- EINVAL
-
File descriptors must not refer to a STREAM or multiplexer linked
downstream from a multiplexer.
References
poll(2),
read(2),
write(2)
Notices
The default value for
FD_SETSIZE
(currently 1024) is larger than
the default limit on the number of open files.
In order to accommodate programs that may
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>.
In future versions of the system, select may return the
time remaining from the original timeout, if any,
by modifying the time value in place.
It is thus unwise to assume that the timeout value will be unmodified
by the
select
call.
The descriptor sets are always modified on return, even
if the call returns as the result of a timeout.
A file descriptor for a socket that is listening for connections will,
when connections are available, indicate that it is ready for reading.
A file descriptor for a socket that is connecting asynchronously will,
when a connection has been established, indicate that it is ready for
writing.
30 January 1998
© 1998 The Santa Cruz Operation, Inc. All rights reserved.