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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
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.
VERSIONS
sockatmark() was added to glibc in version 2.2.4.
ATTRIBUTES
Multithreading (see pthreads(7))
The sockatmark() function is thread-safe.
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 ALSO
fcntl(2), recv(2), send(2), tcp(7)
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2014-02-28 SOCKATMARK(3)
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