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sockatmark(3) — Linux manual page

sockatmark(3)            Library Functions Manualsockatmark(3)

NAME        top

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

LIBRARY        top

       Standard C library (libc,-lc)

SYNOPSIS        top

#include <sys/socket.h>int sockatmark(intsockfd);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):sockatmark():           _POSIX_C_SOURCE >= 200112L

DESCRIPTION        top

sockatmark() returns a value indicating whether or not the socket       referred to by the file descriptorsockfd 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        top

       A successful call tosockatmark() returns 1 if the socket is at       the out-of-band mark, or 0 if it is not.  On error, -1 is returned       anderrno is set to indicate the error.

ERRORS        top

EBADFsockfd is not a valid file descriptor.EINVALsockfd is not a file descriptor to whichsockatmark() can              be applied.

ATTRIBUTES        top

       For an explanation of the terms used in this section, seeattributes(7).       ┌──────────────────────────────────────┬───────────────┬─────────┐       │Interface│Attribute│Value│       ├──────────────────────────────────────┼───────────────┼─────────┤       │sockatmark()                         │ Thread safety │ MT-Safe │       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS        top

       POSIX.1-2008.

HISTORY        top

       glibc 2.2.4.  POSIX.1-2001.

NOTES        top

       Ifsockatmark() returns 1, then the out-of-band data can be read       using theMSG_OOBflag ofrecv(2).       Out-of-band data is supported only on some stream socket       protocols.sockatmark() can safely be called from a handler for theSIGURG       signal.sockatmark() is implemented using theSIOCATMARK ioctl(2)       operation.

BUGS        top

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

EXAMPLES        top

       The following code can be used after receipt of aSIGURGsignal 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        top

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

COLOPHON        top

       This page is part of theman-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       fetched from       ⟨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-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17sockatmark(3)

Pages that refer to this page:recv(2), signal-safety(7)

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