fd_set

A structure type that can represent a set of file descriptors. According to POSIX, the maximum number of file descriptors in an fd_set structure is the value of the macro FD_SETSIZE.

File descriptor sets

The principal arguments of select() are three "sets" of file descriptors (declared with the type fd_set), which allow the caller to wait for three classes of events on the specified set of file descriptors. Each of the fd_set arguments may be specified as NULL if no file descriptors are to be watched for the corresponding class of events.

Note well: Upon return, each of the file descriptor sets is modified in place to indicate which file descriptors are currently "ready". Thus, if using select() within a loop, the sets must be reinitialized before each call.

The contents of a file descriptor set can be manipulated using the following macros:

FD_ZERO()

This macro clears (removes all file descriptors from) set. It should be employed as the first step in initializing a file descriptor set.

FD_SET()

This macro adds the file descriptor fd to set. Adding a file descriptor that is already present in the set is a no-op, and does not produce an error.

FD_CLR()

This macro removes the file descriptor fd from set. Removing a file descriptor that is not present in the set is a no-op, and does not produce an error.

FD_ISSET()

select() modifies the contents of the sets according to the rules described below. After calling select(), the FD_ISSET() macro can be used to test if a file descriptor is still present in a set. FD_ISSET() returns nonzero if the file descriptor fd is present in set, and zero if it is not.

Arguments

The arguments of select() are as follows:

readfds

The file descriptors in this set are watched to see if they are ready for reading. A file descriptor is ready for reading if a read operation will not block; in particular, a file descriptor is also ready on end-of-file.

After select() has returned, readfds will be cleared of all file descriptors except for those that are ready for reading.

writefds

The file descriptors in this set are watched to see if they are ready for writing. A file descriptor is ready for writing if a write operation will not block. However, even if a file descriptor indicates as writable, a large write may still block.

After select() has returned, writefds will be cleared of all file descriptors except for those that are ready for writing.

exceptfds

The file descriptors in this set are watched for "exceptional conditions". For examples of some exceptional conditions, see the discussion of POLLPRI in poll(2).

After select() has returned, exceptfds will be cleared of all file descriptors except for those for which an exceptional condition has occurred.

nfds

This argument should be set to the highest-numbered file descriptor in any of the three sets, plus 1. The indicated file descriptors in each set are checked, up to this limit (but see BUGS).

timeout

The timeout argument is a timeval structure (shown below) that specifies the interval that select() should block waiting for a file descriptor to become ready. The call will block until either:

Note that the timeout interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the blocking interval may overrun by a small amount.

If both fields of the timeval structure are zero, then select() returns immediately. (This is useful for polling.)

If timeout is specified as NULL, select() blocks indefinitely waiting for a file descriptor to become ready.

pselect()

The pselect() system call allows an application to safely wait until either a file descriptor becomes ready or until a signal is caught.

The operation of select() and pselect() is identical, other than these 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.

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. (If sigmask is NULL, the signal mask is not modified during the pselect() call.)

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;
pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
pthread_sigmask(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 timeout argument for select() is a structure of the following type:

struct timeval {


    time_t      tv_sec;         /* seconds */


    suseconds_t tv_usec;        /* microseconds */
};

The corresponding argument for pselect() is a timespec(3) structure.

On Linux, select() modifies timeout to reflect the amount of time not slept; most other implementations do not do this. (POSIX.1 permits either behavior.) 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.

The self-pipe trick

On systems that lack pselect(), reliable (and more portable) signal trapping can be achieved using the self-pipe trick. In this technique, a signal handler writes a byte to a pipe whose other end is monitored by select() in the main program. (To avoid possibly blocking when writing to a pipe that may be full or reading from a pipe that may be empty, nonblocking I/O is used when reading from and writing to the pipe.)

Emulating usleep(3)

Before the advent of usleep(3), some code employed a call to select() with all three sets empty, nfds zero, and a non-NULL timeout as a fairly portable way to sleep with subsecond precision.

Correspondence between select() and poll() notifications

Within the Linux kernel source, we find the following definitions which show the correspondence between the readable, writable, and exceptional condition notifications of select() and the event notifications provided by poll(2) and epoll(7):

#define POLLIN_SET  (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN |


                     EPOLLHUP | EPOLLERR)


                   /* Ready for reading */
#define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT |


                     EPOLLERR)


                   /* Ready for writing */
#define POLLEX_SET  (EPOLLPRI)


                   /* Exceptional condition */

Multithreaded applications

If a file descriptor being monitored by select() is closed in another thread, the result is unspecified. On some UNIX systems, select() unblocks and returns, with an indication that the file descriptor is ready (a subsequent I/O operation will likely fail with an error, unless another process reopens the file descriptor between the time select() returned and the I/O operation is performed). On Linux (and some other systems), closing the file descriptor in another thread has no effect on select(). In summary, any application that relies on a particular behavior in this scenario must be considered buggy.

C library/kernel differences

The Linux kernel allows file descriptor sets of arbitrary size, determining the length of the sets to be checked from the value of nfds. However, in the glibc implementation, the fd_set type is fixed in size. See also BUGS.

The pselect() interface described in this page is implemented by glibc. The underlying Linux system call is named pselect6(). This system call has somewhat different behavior from the glibc wrapper function.

The Linux pselect6() system call modifies its timeout argument. However, the glibc wrapper function hides this behavior 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 behavior required by POSIX.1-2001.

The final argument of the pselect6() system call is not a sigset_t * pointer, but is instead a structure of the form:

struct {


    const kernel_sigset_t *ss;   /* Pointer to signal set */


    size_t ss_len;               /* Size (in bytes) of object


                                    pointed to by 'ss' */
};

This allows the system call to obtain both a pointer to the signal set and its size, while allowing for the fact that most architectures support a maximum of 6 arguments to a system call. See sigprocmask(2) for a discussion of the difference between the kernel and libc notion of the signal set.

Historical glibc details

glibc 2.0 provided an incorrect version of pselect() that did not take a sigmask argument.

From glibc 2.1 to glibc 2.2.1, one must define _GNU_SOURCE in order to obtain the declaration of pselect() from <sys/select.h>.