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poll()

Test for events on multiple sockets simultaneously

Prototypes

#include <sys/poll.h>

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

Description

This function is very similar to select() in that they both watch sets of file descriptors for events, such as incoming data ready to recv(), socket ready to send() data to, out-of-band data ready to recv(), errors, etc.

The basic idea is that you pass an array of nfds struct pollfds in ufds, along with a timeout in milliseconds (1000 milliseconds in a second.) The timeout can be negative if you want to wait forever. If no event happen on any of the socket descriptors by the timeout, poll() will return.

Each element in the array of struct pollfds represents one socket descriptor, and contains the following fields:

struct pollfd {
    int fd;         // the socket descriptor
    short events;   // bitmap of events we're interested in
    short revents;  // when poll() returns, bitmap of events that occurred
};

Before calling poll(), load fd with the socket descriptor (if you set fd to a negative number, this struct pollfd is ignored and its revents field is set to zero) and then construct the events field by bitwise-ORing the following macros:

POLLIN

Alert me when data is ready to recv() on this socket.

POLLOUT

Alert me when I can send() data to this socket without blocking.

POLLPRI

Alert me when out-of-band data is ready to recv() on this socket.

Once the poll() call returns, the revents field will be constructed as a bitwise-OR of the above fields, telling you which descriptors actually have had that event occur. Additionally, these other fields might be present:

POLLERR

An error has occurred on this socket.

POLLHUP

The remote side of the connection hung up.

POLLNVAL

Something was wrong with the socket descriptor fd--maybe it's uninitialized?

Return Value

Returns the number of elements in the ufds array that have had event occur on them; this can be zero if the timeout occurred. Also returns -1 on error (and errno will be set accordingly.)

Example

int s1, s2;
int rv;
char buf1[256], buf2[256];
struct pollfd ufds[2];

s1 = socket(PF_INET, SOCK_STREAM, 0);
s2 = socket(PF_INET, SOCK_STREAM, 0);

// pretend we've connected both to a server at this point
//connect(s1, ...)...
//connect(s2, ...)...

// set up the array of file descriptors.
//
// in this example, we want to know when there's normal or out-of-band
// data ready to be recv()'d...

ufds[0].fd = s1;
ufds[0].events = POLLIN | POLLPRI; // check for normal or out-of-band

ufds[1] = s2;
ufds[1].events = POLLIN; // check for just normal data

// wait for events on the sockets, 3.5 second timeout
rv = poll(ufds, 2, 3500);

if (rv == -1) {
    perror("poll"); // error occurred in poll()
} else if (rv == 0) {
    printf("Timeout occurred!  No data after 3.5 seconds.\n");
} else {
    // check for events on s1:
    if (ufds[0].revents & POLLIN) {
        recv(s1, buf1, sizeof(buf1), 0); // receive normal data
    }
    if (ufds[0].revents & POLLPRI) {
        recv(s1, buf1, sizeof(buf1), MSG_OOB); // out-of-band data
    }

    // check for events on s2:
    if (ufds[1].revents & POLLIN) {
        recv(s1, buf2, sizeof(buf2), 0);
    }
}

See Also

select()

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