POLL(2) Linux Programmer's Manual POLL(2)

poll, ppoll - wait for some event on a file descriptor

#include <sys/poll.h>

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

#define _GNU_SOURCE
#include <sys/poll.h>

int ppoll(struct pollfd *fds, nfds_t nfds, 
        const struct timespec *timeout, const sigset_t *sigmask);

poll() performs a similar task to select(2): it waits for one of a set of file descriptors to become ready to perform I/O.

The set of file descriptors to be monitored is specified in the fds argument, which is an array of nfds structures of the following form:



    struct pollfd {

        int   fd;         /* file descriptor */

        short events;     /* requested events */

        short revents;    /* returned events */

    };
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. The bits returned in revents can include any of those specified in events, or one of the values POLLERR, 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, then poll() blocks until one of the events occurs. The bits that may be set/returned in events and revents 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 (output only) */

    #define POLLHUP     0x0010    /* Hung up (output only) */

    #define POLLNVAL    0x0020    /* Invalid request: fd not open 

                                     (output only) */

When compiling XPG4.2 source one also has: 



    #ifdef _XOPEN_SOURCE

    #define POLLRDNORM  0x0040    /* Normal data may be read */

    #define POLLRDBAND  0x0080    /* Priority data may be read */

    #define POLLWRNORM  0x0100    /* Writing now will not block */

    #define POLLWRBAND  0x0200    /* Priority data may be written */

    #endif

Finally, Linux knows about: 



    #ifdef _GNU_SOURCE

    #define POLLMSG     0x0400

    #endif

The timeout argument specifies an upper limit on the time for which poll() will block, in milliseconds. Specifying a negative value in timeout means an infinite timeout.

The relationship between poll() and ppoll() is analogous to the relationship between select() and pselect(): like pselect(), ppoll() allows an application to safely wait until either a file descriptor becomes ready or until a signal is caught.

Other than the difference in the timeout argument, the following ppoll() call:



    ready = ppoll(&fds, nfds, timeout, &sigmask);
is equivalent to atomically executing the following calls: 


    sigset_t origmask;

    sigprocmask(SIG_SETMASK, &sigmask, &origmask);

    ready = ppoll(&fds, nfds, timeout);

    sigprocmask(SIG_SETMASK, &origmask, NULL);

See the description of pselect(2) for an explanation of why ppoll() is necessary.

The timeout argument specifies an upper limit on the amount of time that ppoll() will block. This argument is a pointer to a structure of the following form: 


struct timespec {

    long    tv_sec;         /* seconds */

    long    tv_nsec;        /* nanoseconds */
};

If timeout is specified as NULL, then ppoll() can block indefinitely.

On success, a positive number is returned; this 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 were ready. On error, -1 is returned, and errno is set appropriately.

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

The Linux ppoll() 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 ppoll() function does not modify its timeout argument.

See the discussion of spurious readiness notifications under the BUGS section of select(2).

poll() conforms to POSIX.1-2001. ppoll() is Linux specific.

The poll() system call 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() system call).

The ppoll() system call was added to Linux in kernel 2.6.16. The ppoll() library call was added in glibc 2.4.

Some implementations define the non-standard constant INFTIM with the value -1 for use as a timeout. This constant is not provided in glibc.

select(2), select_tut(2)

2006-03-13 Linux 2.6.16

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