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


 /* According to POSIX.1-2001 */
#include <sys/select.h> /* According to earlier standards */
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h> int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); void FD_CLR(int fd, fd_set *set);
int FD_ISSET(int fd, fd_set *set);
void FD_SET(int fd, fd_set *set);
void FD_ZERO(fd_set *set); #define _XOPEN_SOURCE 600 #include <sys/select.h> int pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, const struct timespec *timeout, const sigset_t *sigmask);


select() and pselect() allow a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become "ready" for some class of I/O operation (e.g., input possible). A file descriptor is considered ready if it is possible to perform the corresponding I/O operation (e.g., read(2)) without blocking.

The operation of select() and pselect() is identical, with three differences:

select() uses a timeout that is a struct timeval (with seconds and microseconds), while pselect() uses a struct timespec (with seconds and nanoseconds).
select() may update the timeout argument to indicate how much time was left. pselect() does not change this argument.
select() has no sigmask argument, and behaves as pselect() called with NULL sigmask.

Three independent sets of file descriptors are watched. Those listed in readfds will be watched to see if characters become available for reading (more precisely, to see if a read will not block; in particular, a file descriptor is also ready on end-of-file), those in writefds will be watched to see if a write will not block, and those in exceptfds will be watched for exceptions. On exit, the sets are modified in place to indicate which file descriptors actually changed status. Each of the three file descriptor sets may be specified as NULL if no file descriptors are to be watched for the corresponding class of events.

Four macros are provided to manipulate the sets. FD_ZERO() clears a set. FD_SET() and FD_CLR() respectively add and remove a given file descriptor from a set. FD_ISSET() tests to see if a file descriptor is part of the set; this is useful after select() returns.

nfds is the highest-numbered file descriptor in any of the three sets, plus 1.

timeout is an upper bound on the amount of time elapsed before select() returns. It may be zero, causing select() to return immediately. (This is useful for polling.) If timeout is NULL (no timeout), select() can block indefinitely.

sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is not NULL, then pselect() first replaces the current signal mask by the one pointed to by sigmask, then does the `select' function, and then restores the original signal mask.

Other than the difference in the precision of the timeout argument, the following pselect() call:

     ready = pselect(nfds, &readfds, &writefds, &exceptfds, 
                     timeout, &sigmask);
is equivalent to atomically executing the following calls:
     sigset_t origmask;
     sigprocmask(SIG_SETMASK, &sigmask, &origmask);
     ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
     sigprocmask(SIG_SETMASK, &origmask, NULL);

The reason that pselect() is needed is that if one wants to wait for either a signal or for a file descriptor to become ready, then an atomic test is needed to prevent race conditions. (Suppose the signal handler sets a global flag and returns. Then a test of this global flag followed by a call of select() could hang indefinitely if the signal arrived just after the test but just before the call. By contrast, pselect() allows one to first block signals, handle the signals that have come in, then call pselect() with the desired sigmask, avoiding the race.)

The timeout

The time structures involved are defined in <sys/time.h> and look like
 struct timeval { 
     long    tv_sec;         /* seconds */
     long    tv_usec;        /* microseconds */


 struct timespec {
     long    tv_sec;         /* seconds */
     long    tv_nsec;        /* nanoseconds */

(However, see below on the POSIX.1-2001 versions.)

Some code calls select() with all three sets empty, n zero, and a non-NULL timeout as a fairly portable way to sleep with subsecond precision.

On Linux, select() modifies timeout to reflect the amount of time not slept; most other implementations do not do this. (POSIX.1-2001 permits either behaviour.) This causes problems both when Linux code which reads timeout is ported to other operating systems, and when code is ported to Linux that reuses a struct timeval for multiple select()s in a loop without reinitializing it. Consider timeout to be undefined after select() returns.


On success, select() and pselect() return the number of file descriptors contained in the three returned descriptor sets (that is, the total number of bits that are set in readfds, writefds, exceptfds) which may be zero if the timeout expires before anything interesting happens. On error, -1 is returned, and errno is set appropriately; the sets and timeout become undefined, so do not rely on their contents after an error.


An invalid file descriptor was given in one of the sets. (Perhaps a file descriptor that was already closed, or one on which an error has occurred.)
A signal was caught.
nfds is negative or the value contained within timeout is invalid.
unable to allocate memory for internal tables.


 #include <stdio.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <unistd.h>
 main(void) {
     fd_set rfds;
     struct timeval tv;
     int retval;
     /* Watch stdin (fd 0) to see when it has input. */
     FD_SET(0, &rfds);
     /* Wait up to five seconds. */
     tv.tv_sec = 5;
     tv.tv_usec = 0;
     retval = select(1, &rfds, NULL, NULL, &tv);
     /* Don't rely on the value of tv now! */
     if (retval == -1)
     else if (retval)
         printf("Data is available now.\n");
         /* FD_ISSET(0, &rfds) will be true. */
         printf("No data within five seconds.\n");
     return 0;


select() conforms to POSIX.1-2001 and 4.4BSD (select() first appeared in 4.2BSD). Generally portable to/from non-BSD systems supporting clones of the BSD socket layer (including System V variants). However, note that the System V variant typically sets the timeout variable before exit, but the BSD variant does not.

pselect() is defined in POSIX.1g, and in POSIX.1-2001.


An fd_set is a fixed size buffer. Executing FD_CLR() or FD_SET() with a value of fd that is negative or is equal to or larger than FD_SETSIZE will result in undefined behavior. Moreover, POSIX requires fd to be a valid file descriptor.

Concerning the types involved, the classical situation is that the two fields of a timeval structure are longs (as shown above), and the structure is defined in <sys/time.h>. The POSIX.1-2001 situation is

 struct timeval {
     time_t         tv_sec;     /* seconds */
     suseconds_t    tv_usec;    /* microseconds */

where the structure is defined in <sys/select.h> and the data types time_t and suseconds_t are defined in <sys/types.h>.

Concerning prototypes, the classical situation is that one should include <time.h> for select(). The POSIX.1-2001 situation is that one should include <sys/select.h> for select() and pselect(). Libc4 and libc5 do not have a <sys/select.h> header; under glibc 2.0 and later this header exists. Under glibc 2.0 it unconditionally gives the wrong prototype for pselect(), under glibc 2.1-2.2.1 it gives pselect() when _GNU_SOURCE is defined, under glibc 2.2.2-2.2.4 it gives it when _XOPEN_SOURCE is defined and has a value of 600 or larger. No doubt, since POSIX.1-2001, it should give the prototype by default.


pselect() was added to Linux in kernel 2.6.16. Prior to this, pselect() was emulated in glibc (but see BUGS).


The Linux pselect() system call modifies its timeout argument. However, the glibc wrapper function hides this behaviour by using a local variable for the timeout argument that is passed to the system call. Thus, the glibc pselect() function does not modify its timeout argument; this is the behaviour required by POSIX.1-2001.


Glibc 2.0 provided a version of pselect() that did not take a sigmask argument.

Since version 2.1, glibc has provided an emulation of pselect() that is implemented using sigprocmask(2) and select(). This implementation remains vulnerable to the very race condition that pselect() was designed to prevent. On systems that lack pselect() reliable (and more portable) signal trapping can be achieved using the self-pipe trick (where a signal handler writes a byte to a pipe whose other end is monitored by select() in the main program.)

Under Linux, select() may report a socket file descriptor as "ready for reading", while nevertheless a subsequent read blocks. This could for example happen when data has arrived but upon examination has wrong checksum and is discarded. There may be other circumstances in which a file descriptor is spuriously reported as ready. Thus it may be safer to use O_NONBLOCK on sockets that should not block.


For a tutorial with discussion and examples, see select_tut(2).

For vaguely related stuff, see accept(2), connect(2), poll(2), read(2), recv(2), send(2), sigprocmask(2), write(2), epoll(7), feature_test_macros(7)