sockatmark(3) Library Functions Manual sockatmark(3)

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

Standard C library (libc, -lc)

#include <sys/socket.h>
int sockatmark(int sockfd);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


    _POSIX_C_SOURCE >= 200112L

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.

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

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
sockatmark () Thread safety MT-Safe


glibc 2.2.4. 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) {
        if (atmark)
        s = read(sockfd, buf, BUF_LEN);
        if (s == -1)
        if (s <= 0)
    if (atmark == 1) {
        if (recv(sockfd, &oobdata, 1, MSG_OOB) == -1) {

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

2023-10-31 Linux man-pages 6.7