select.2 (2012 03)

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select(2) select(2)

NAME

select(), pselect(), FD_CLR(), FD_ISSET(), FD_SET(), FD_ZERO() - synchronous I/O multiplexing

SYNOPSIS

For UNIX 2003:

#include <sys/select.h>

int pselect(int nfds, fd_set *__restrict readfds,

fd_set *__restrict writefds, fd_set *__restrict errorfds,

const struct timespec *__restrict timeout,

const sigset_t *__restrict sigmask);

int select(int nfds, fd_set *__restrict readfds,

fd_set *__restrict writefds, fd_set *__restrict errorfds

struct timeval *__restrict timeout);

void FD_CLR(int fd, fd_set *fdset);

int FD_ISSET(int fd, fd_set *fdset);

void FD_SET(int fd, fd_set *fdset);

void FD_ZERO(fd_set *fdset);

For Standards prior to UNIX 2003:

#include <sys/time.h>

int select(int nfds, fd_set *readfds, fd_set *writefds,

fd_set *errorfds, struct timeval *timeout);

void FD_CLR(int fd, fd_set *fdset);

int FD_ISSET(int fd, fd_set *fdset);

void FD_SET(int fd, fd_set *fdset);

void FD_ZERO(fd_set *fdset);

For Backward Compatibility Only: (_XOPEN_SOURCE_EXTENDED not deﬁned)

#include <time.h>

int select(size_t nfds, int *readfds, int *writefds,

int *exceptds, const struct timeval *timeout);

DESCRIPTION

Prototypes for select(), FD_CLR(), FD_ISSET(), FD_SET(), FD_ZERO(), are available by includ-

ing <sys/time.h> as required by standards prior to UNIX 2003. The UNIX 2003 standard requires

including <sys/select.h> to expose these prototypes along with pselect(). Refer to the stan-

dards(5) man page for information on how to compile programs by using the correct namespace for

different UNIX standards.

The

pselect() and select() functions indicate which of the speciﬁed ﬁle descriptors is ready for

reading, ready for writing, or has an error condition pending. If the speciﬁed condition is false for all of

the speciﬁed ﬁle descriptors, pselect() and select() block, up to the speciﬁed timeout interval,

until the speciﬁed condition is true for at least one of the speciﬁed ﬁle descriptors.

The

pselect() and select() functions support regular ﬁles, terminal and pseudo-terminal devices,

STREAMS-based ﬁles, FIFOs and pipes. The behaviour of pselect() and select() on ﬁle descrip-

tors that refer to other ﬁle types is unspeciﬁed.

The nfds argument speciﬁes the range of ﬁle descriptors to be tested. The

pselect() and select()

functions test ﬁle descriptors in the range of 0 to nfds −1. File descriptor f is represented by the bit 1<<f

in the masks. More formally, a ﬁle descriptor is represented by:

fds[(f / BITS_PER_INT)] & (1 << (f % BITS_PER_INT))

HP-UX 11i Version 3: March 2012 − 1 − Hewlett-Packard Company 1

Summary of content (6 pages)

PAGE 1
select(2) select(2) NAME select(), pselect(), FD_CLR(), FD_ISSET(), FD_SET(), FD_ZERO() - synchronous I/O multiplexing SYNOPSIS For UNIX 2003: #include
PAGE 2
select(2) select(2) If the readfds argument is not a null pointer, it points to an object of type fd_set 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 writefds argument is not a null pointer, it points to an object of type fd_set 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.
PAGE 3
select(2) select(2) for the pipe, OR there are no readers left for the pipe. select() returns the same results for a pipe whether a file descriptor associated with the read-only end or the write-only end of the pipe is used, since both file descriptors refer to the same underlying pipe. So a select() of a read-only file descriptor that is associated with a pipe can return ready to write, even though that particular file descriptor cannot be written to.
PAGE 4
select(2) select(2) nfds argument is greater than FD_SETSIZE. [EINTR] The function was interrupted before any of the selected events occurred and before the timeout interval expired. If SA_RESTART has been set for the interrupting signal, it is implementation-dependent whether the function restarts or returns with [EINTR]. [EFAULT] One or more of the pointers was invalid. The reliable detection of this error is implementation dependent. [EINVAL] An invalid timeout interval was specified.
PAGE 5
select(2) select(2) The following example is the same as the previous example, except that it works for more than 32 open files. Definitions for howmany, fd_set, and NFDBITS are in . #include #include #include
PAGE 6
select(2) select(2) AUTHOR select() was developed by HP and the University of California, Berkeley. SEE ALSO fcntl(2), poll(2), read(2), sigprocmask(2), write(2), thread_safety(5), standards(5), .