poll() - Unix, Linux System Call

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Copyright © 2014 by tutorialspoint
#include <poll.h> 

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

#define _GNU_SOURCE 
#include <poll.h> 

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


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


#define _GNU_SOURCE 
#include <poll.h> 


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


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 */
    };



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




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
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 bits that may be set/returned in
events and
revents are defined in <poll.h>:


When compiling with
_XOPEN_SOURCE defined, one also has the following,
which convey no further information beyond the bits listed above:


Linux also knows about, but does not use
POLLMSG. 



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


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



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




    sigset_t origmask;

    sigprocmask(SIG_SETMASK, &sigmask, &origmask);
    ready = ppoll(&fds, nfds, timeout);
    sigprocmask(SIG_SETMASK, &origmask, NULL);


    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 */
};


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


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


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

select (2)
select (2)
select_tut (2)
select_tut (2)

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