SOCKATMARK(3) Linux Programmer's Manual SOCKATMARK(3)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 || _XOPEN_SOURCE >= 600
DESCRIPTIONsockatmark() 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.
VERSIONSsockatmark() was added to glibc in version 2.2.4.
CONFORMING TO
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.
EXAMPLE
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");
...
}
}
SEE ALSOfcntl(2), recv(2), send(2), tcp(7)COLOPHON
This page is part of release 3.55 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-03 SOCKATMARK(3)
