POLL

Section: Linux Programmer's Manual (2)
Updated: 1997-12-07
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NAME

poll - wait for some event on a file descriptor

SYNOPSIS

#include <sys/poll.h>

int poll(struct pollfd *ufds, unsigned int nfds, int timeout);

DESCRIPTION

poll is a variation on the theme of select. It specifies an array of nfds structures of type

        struct pollfd {
                int fd;           /* file descriptor */
                short events;     /* requested events */
                short revents;    /* returned events */
        };

and a timeout in milliseconds. A negative value means infinite timeout. The field fd contains a file descriptor for an open file. The field events is an input parameter, a bitmask specifying the events the application is interested in. The field revents is an output parameter, filled by the kernel with the events that actually occurred, either of the type requested, or of one of the types POLLERR or POLLHUP or POLLNVAL. (These three bits are meaningless in the events field, and will be set in the revents field whenever the corresponding condition is true.) If none of the events requested (and no error) has occurred for any of the file descriptors, the kernel waits for timeout milliseconds for one of these events to occur. The following possible bits in these masks are defined in <sys/poll.h>

    #define POLLIN      0x0001    /* There is data to read */
    #define POLLPRI     0x0002    /* There is urgent data to read */
    #define POLLOUT     0x0004    /* Writing now will not block */
    #define POLLERR     0x0008    /* Error condition */
    #define POLLHUP     0x0010    /* Hung up */
    #define POLLNVAL    0x0020    /* Invalid request: fd not open */

In <asm/poll.h> also the values POLLRDNORM, POLLRDBAND, POLLWRNORM, POLLWRBAND and POLLMSG are defined.

RETURN VALUE

On success, a positive number is returned, where the number returned is the number of structures which have non-zero revents fields (in other words, those descriptors with events or errors reported). A value of 0 indicates that the call timed out and no file descriptors have been selected. On error, -1 is returned, and errno is set appropriately.

ERRORS

EBADF: An invalid file descriptor was given in one of the sets.
ENOMEM: There was no space to allocate file descriptor tables.
EFAULT: The array given as argument was not contained in the calling program's address space.
EINTR: A signal occurred before any requested event.

CONFORMING TO

XPG4-UNIX.

AVAILABILITY

The poll() systemcall was introduced in Linux 2.1.23. The poll() library call was introduced in libc 5.4.28 (and provides emulation using select if your kernel does not have a poll syscall).

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Time: 07:36:17 GMT, November 10, 2025