select(3C) select(3C)
NAME
select, FDCLR, FDISSET, FDSET, FDZERO - synchronous I/O multiplex-
ing
SYNOPSIS
#include <sys/time.h>
#include <sys/types.h>
int select(int nfds, fdset *readfds, fdset *writefds,
fdset *errorfds, struct timeval *timeout);
void FDCLR(int fd, fdset *fdset);
int FDISSET(int fd, fdset *fdset);
void FDSET(int fd, fdset *fdset);
void FDZERO(fdset *fdset);
DESCRIPTION
The select() function indicates which of the specified file descrip-
tors is ready for reading, ready for writing, or has an error condi-
tion pending. If the specified condition is false for all of the
specified file descriptors, select() blocks, up to the specified
timeout interval, until the specified condition is true for at least
one of the specified file descriptors.
The select() function supports regular files, terminal and pseudo-
terminal devices, STREAMS-based files, FIFOs and pipes. The behavior
of select() on file descriptors that refer to other types of file is
unspecified.
The nfds argument specifies the range of file descriptors to be
tested. The select() function tests file descriptors in the range of 0
to nfds-1.
If the readfs argument is not a null pointer, it points to an object
of type fdset that on input specifies the file descriptors to be
checked for being ready to read, and on output indicates which file
descriptors are ready to read.
If the writefs argument is not a null pointer, it points to an object
of type fdset that on input specifies the file descriptors to be
checked for being ready to write, and on output indicates which file
descriptors are ready to write.
If the errorfds argument is not a null pointer, it points to an object
of type fdset that on input specifies the file descriptors to be
checked for error conditions pending, and on output indicates which
file descriptors have error conditions pending.
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select(3C) select(3C)
On successful completion, the objects pointed to by the readfs,
writefs, and errorfds arguments are modified to indicate which file
descriptors are ready for reading, ready for writing, or have an error
condition pending, respectively. For each file descriptor less than
nfds, the corresponding bit will be set on successful completion if it
was set on input and the associated condition is true for that file
descriptor.
If the timeout argument is not a null pointer, it points to an object
of type struct timeval that specifies a maximum interval to wait for
the selection to complete. If the timeout argument points to an object
of type struct timeval whose members are 0, select() does not block.
If the timeout argument is a null pointer, select() blocks until an
event causes one of the masks to be returned with a valid (non-zero)
value. If the time limit expires before any event occurs that would
cause one of the masks to be set to a non-zero value, select() com-
pletes successfully and returns 0.
Implementations may place limitations on the maximum timeout interval
supported. On all implementations, the maximum timeout interval sup-
ported will be at least 31 days. If the timeout argument specifies a
timeout interval greater than the implementation-dependent maximum
value, the maximum value will be used as the actual timeout value.
Implementations may also place limitations on the granularity of
timeout intervals. If the requested timeout interval requires a finer
granularity than the implementation supports, the actual timeout
interval will be rounded up to the next supported value.
If the readfs, writefs, and errorfds arguments are all null pointers
and the timeout argument is not a null pointer, select() blocks for
the time specified, or until interrupted by a signal. If the readfs,
writefs, and errorfds arguments are all null pointers and the timeout
argument is a null pointer, select() blocks until interrupted by a
signal.
File descriptors associated with regular files always select true for
ready to read, ready to write, and error conditions.
On failure, the objects pointed to by the readfs, writefs, and
errorfds arguments are not modified. If the timeout interval expires
without the specified condition being true for any of the specified
file descriptors, the objects pointed to by the readfs, writefs, and
errorfds arguments have all bits set to 0.
File descriptor masks of type fdset can be initialized. It is
unspecified whether each of these is a macro or a function. If a macro
definition is suppressed in order to access an actual function, or a
program defines an external identifier with any of these names, the
behavior is undefined.
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select(3C) select(3C)
FDCLR(fd, &fdset)
Clears the bit for the file descriptor fd in the file descriptor
set fdset.
FDISSET(fd, &fdset)
Returns a non-zero value if the bit for the file descriptor fd is
set in the file descriptor set pointed to by fdset, and 0 other-
wise.
FDSET(fd, &fdset)
Sets the bit for the file descriptor fd in the file descriptor
set fdset.
FDZERO(&fdset)
Initializes the file descriptor set fdset to have zero bits for
all file descriptors.
The behavior of these macros is undefined if the fd argument is less
than 0 or greater than or equal to FDSETSIZE.
RETURN VALUE
fd is set in the file descriptor set pointed to by fdset, and 0 other-
wise.
On successful completion, select() returns the total number of bits
set in the bit masks. Otherwise, -1 is returned, and errno is set to
indicate the error.
ERRORS
The following error code descriptions are function-specific. You will
find a general description in introprm2(2) or in errno(5).
Under the following conditions, select() fails and sets errno to:
EBADF One or more of the file descriptor sets specified a file
descriptor that is not a valid open file descriptor.
EINTR The select() function was interrupted before any of the
selected events occurred and before the timeout interval
expired.
If SARESTART has been set for the interrupting signal, it is
implementation-dependent whether select() restarts or returns with
EINTR.
EINVAL An invalid timeout interval was specified.
EINVAL The nfds argument is less than 0, or greater than or equal
to FDSETSIZE.
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select(3C) select(3C)
EINVAL One of the specified file descriptors refers to a STREAM or
multiplexer that is linked (directly or indirectly) down-
stream from a multiplexer.
APPLICATION USAGE
The use of a timeout does not affect any pending timers set up by
alarm(), ualarm() or settimer().
On successful completion, the object pointed to by the timeout argu-
ment may be modified.
SEE ALSO
fcntl(2), poll(2), read(2), write(2), systime(5), types(5).
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