Linux cli command sockatmark

➡ A Linux man page (short for manual page) is a form of software documentation found on Linux and Unix-like operating systems. This man-page explains the command sockatmark and provides detailed information about the command sockatmark, system calls, library functions, and other aspects of the system, including usage, options, and examples of _. You can access this man page by typing man followed by the sockatmark.

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NAME 🖥️ sockatmark 🖥️

determine whether socket is at out-of-band mark

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

#include <sys/socket.h>
int sockatmark(int sockfd);

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

sockatmark():

    _POSIX_C_SOURCE >= 200112L

DESCRIPTION

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.

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
sockfd is not a valid file descriptor.

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

ATTRIBUTES

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

InterfaceAttributeValue

sockatmark()

Thread safetyMT-Safe

STANDARDS

POSIX.1-2008.

HISTORY

glibc 2.2.4. 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 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.

BUGS

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

EXAMPLES

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);
        if (s == -1)
            perror("read");
        if (s <= 0)
            break;
    }
    if (atmark == 1) {
        if (recv(sockfd, &oobdata, 1, MSG_OOB) == -1) {
            perror("recv");
            ...
        }
    }

SEE ALSO

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

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