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 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

       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

       EBADF  sockfd is not a valid file descriptor.

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

VERSIONS

       sockatmark() was added to glibc in version 2.2.4.

ATTRIBUTES

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

EXAMPLE

       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 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/.