sockatmark (3)

#include <sys/socket.h>

int sockatmark(int sockfd);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

sockatmark(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600

sockatmark() returns a value indicating whether or not the socket referred to by the file descriptor sockfd 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.

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.

EBADF

sockfd is not a valid file descriptor.

EINVAL

sockfd is not a file descriptor to which sockatmark() can be applied.

sockatmark() was added to glibc in version 2.2.4.

Multithreading (see pthreads(7))

The sockatmark() function is thread-safe.

POSIX.1-2001.

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 supported only 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.

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

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(sockfd);
        if (atmark == -1) {
            perror("sockatmark");
            break;
        }

        if (atmark)
            break;

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

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

fcntl(2), recv(2), send(2), tcp(7)

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