sockatmark(3) — Linux manual pages

Name

sockatmark — determine whether socket is at out-of-band mark

Synopsis

#include <sys/socket.h>

int sockatmark( int fd);

DESCRIPTION

sockatmark() returns a value indicating whether or not the socket referred to by the file descriptor fd is at the out-of-band mark. If the socket is at the mark, then 1 is returned; if the socket is not at the mark, 0 is returned. This function does not remove the out-of-band mark.

RETURN VALUE

A successful call to sockatmark() returns 1 if the socket is at the out-of-band mark, or 0 if it is not. On error, −1 is returned and errno is set to indicate the error.

ERRORS

EBADF: fd is not a valid file descriptor.
EINVAL: fd is not a file descriptor to which sockatmark() can be applied.

VERSIONS

sockatmark() was added to glibc in version 2.2.4.

CONFORMING TO

POSIX.1-2001

NOTES

If sockatmark() returns 1, then the out-of-band data can be read using the MSG_OOB flag of recv(2).

Out-of-band data is only supported on some stream socket protocols.

sockatmark() can safely be called from a handler for the SIGURG signal.

sockatmark() is implemented using the SIOCATMARK ioctl(2) operation.

BUGS

Prior to glibc 2.4, sockatmark() did not work.

EXAMPLE

The following code can be used after receipt of a SIGURG signal to read (and discard) all data up to the mark, and then read the byte of data at the mark:

    char buf[BUF_LEN];
    char oobdata;
    int atmark, s;

    for (;;) {
        atmark = sockatmark(fd);
        if (atmark == −1) {
            perror("sockatmark");
            break;
        }

        if (atmark)
            break;

        s = read(fd, buf, BUF_LEN) <= 0);
        if (s == -1)
            perror("read");
        if (s <= 0)
            break;
    }

    if (atmark == 1) {
        if (recv(fd, &oobdata, 1, MSG_OOB) == −1) {
            perror("recv");
            ...
        }
    }

Copyright (c) 2006, Michael Kerrisk (mtk-manpages@gmx.net)

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