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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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sockatmark(3) Library Functions Manual sockatmark(3)
sockatmark - determine whether socket is at out-of-band mark
Standard C library (libc, -lc)
#include <sys/socket.h>
int sockatmark(int sockfd);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
sockatmark():
_POSIX_C_SOURCE >= 200112L
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.
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.
EBADF sockfd is not a valid file descriptor.
EINVAL sockfd is not a file descriptor to which sockatmark() can
be applied.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ sockatmark() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
glibc 2.2.4. POSIX.1-2001.
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.
Prior to glibc 2.4, sockatmark() did not work.
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");
...
}
}
fcntl(2), recv(2), send(2), tcp(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 sockatmark(3)
Pages that refer to this page: recv(2), signal-safety(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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