NAME

select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO – synchronous I/O multiplexing

SYNOPSIS

#include <sys/select.h>

int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);

FD_CLR(int fd, fd_set *set);
FD_ISSET(int fd, fd_set *set);
FD_SET(int fd, fd_set *set);
FD_ZERO(fd_set *set);

DESCRIPTION

The function select waits for a number of file descriptors to change status. The select function uses a timeout that is a struct timeval (with seconds and microseconds) and the function will update the timeout parameter to indicate how much time was left.

Three independent sets of 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 descriptors actually changed status.

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

n is the highest-numbered 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.

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 */
};

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, Unison and DSPnano, the function select modifies timeout to reflect the amount of time not slept; most other implementations do not do this. This causes problems both when code which reads timeout is ported to other operating systems, and when code is ported to Linux, Unison or DSPnano that reuses a struct timeval for multiple selects in a loop without reinitializing it. Consider timeout to be undefined after select returns.

RETURN VALUE

On success, select returns the number of descriptors contained in the descriptor sets, 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.

ERRORS

EBADF

An invalid file descriptor was given in one of the sets.

EINVAL

n is negative or the value contained within timeout is invalid.

ENOMEM

select was unable to allocate memory for internal tables.

EXAMPLE

#include < stdio.h >
#include < sys/time.h >
#include < sys/select.h >

THREAD Simple_Select(void *) {

    fd_set rfds;
    struct timeval tv;
    int retval;

    /* Watch stdin (fd 0) to see when it has input. */
    FD_ZERO(&rfds);
    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)
        perror("select()");
    else if (retval)
        printf("Data is available now.\n");
        /* FD_ISSET(0, &rfds) will be true. */
    else
        printf("No data within five seconds.\n");

    return 0;
}

NOTES

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. POSIX requires fd to be a valid file descriptor.

Concerning the types involved, the classical situation is that the two fields of a struct timeval are longs (as shown above), and the struct is defined in < sys/time.h >. The Linux, Unison, DSPnano and POSIX 1003.1-2001 situation is

struct timeval {
    time_t         tv_sec;     /* seconds */
    useconds_t     tv_usec;    /* microseconds */
};

where the struct is defined in < sys/time.h > and the data types time_t and useconds_t are defined in < sys/types.h >.

The Linux, Unison, DSPnano and POSIX 1003.1-2001 situation is that one should include < sys/select.h > for select.

SEE ALSO

For a tutorial with discussion and examples, see the Unison or DSPnano Programmer’s Guides and the demo directory.

For related calls, see accept, connect, read, recv, send, write