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);

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).

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)