Manpage of SOCKATMARK

SOCKATMARK

Section: Linux Programmer's Manual (3)
Updated: 2016-03-15
Index
 

NAME

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

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 sockfdis 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 errnois set to indicate the error.  

ERRORS

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

VERSIONS

sockatmark() was added to glibc in version 2.2.4.  

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).
InterfaceAttributeValue
sockatmark() Thread safetyMT-Safe
 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.  

NOTES

If sockatmark() returns 1, then the out-of-band data can be read using the MSG_OOBflag 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 SIGURGsignal.

sockatmark() is implemented using the SIOCATMARKioctl(2) operation.  

BUGS

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

EXAMPLE

The following code can be used after receipt of a SIGURGsignal 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");
            ...
        }
    }
 

SEE ALSO

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


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
ATTRIBUTES
CONFORMING TO
NOTES
BUGS
EXAMPLE
SEE ALSO

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