sockatmark(3)

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

DESCRIPTION
       sockatmark()  returns  a value indicating whether or not the socket re-
       ferred 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  er-
       rno 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 ap-
              plied.

VERSIONS
       sockatmark() was added to glibc in version 2.2.4.

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(7).

       +-------------+---------------+---------+
       |Interface    | Attribute     | Value   |
       +-------------+---------------+---------+
       |sockatmark() | Thread safety | MT-Safe |
       +-------------+---------------+---------+
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

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.

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

COLOPHON
       This page is part of release 5.07 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                             2020-06-09                     SOCKATMARK(3)