SOCKATMARK(3)             Linux Programmer's Manual            SOCKATMARK(3)

NAME         top

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

SYNOPSIS         top

       #include <sys/socket.h>

       int sockatmark(int sockfd);

   Feature Test Macro Requirements for glibc (see feature_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 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         top

       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         top

       EBADF  sockfd is not a valid file descriptor.

       EINVAL sockfd is not a file descriptor to which sockatmark() can be
              applied.

VERSIONS         top

       sockatmark() was added to glibc in version 2.2.4.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌─────────────┬───────────────┬─────────┐
       │Interface    │ Attribute     │ Value   │
       ├─────────────┼───────────────┼─────────┤
       │sockatmark() │ Thread safety │ MT-Safe │
       └─────────────┴───────────────┴─────────┘

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

       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         top

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

EXAMPLE         top

       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         top

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

COLOPHON         top

       This page is part of release 4.15 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
       https://www.kernel.org/doc/man-pages/.

Linux                            2017-09-15                    SOCKATMARK(3)

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

Copyright and license for this manual page

HTML rendering created 2018-02-02 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH.