sockatmark(3) — Linux manual page

sockatmark(3)           Library Functions Manual           sockatmark(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(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.

ATTRIBUTES         top

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

       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.

EXAMPLES         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)

Linux man-pages 6.04           2023-03-30                  sockatmark(3)

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

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