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SOCKATMARK(3)                           BSD Library Functions Manual                           SOCKATMARK(3)

NAME
     sockatmark -- determine whether the read pointer is at the OOB mark

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/socket.h>

     int
     sockatmark(int s);

DESCRIPTION
     To find out if the read pointer is currently pointing at the mark in the data stream, the sockatmark()
     function is provided.  If sockatmark() returns 1, the next read will return data after the mark.  Oth-erwise Otherwise
     erwise (assuming out of band data has arrived), the next read will provide data sent by the client
     prior to transmission of the out of band signal.  The routine used in the remote login process to flush
     output on receipt of an interrupt or quit signal is shown below.  It reads the normal data up to the
     mark (to discard it), then reads the out-of-band byte.

           #include <sys/socket.h>
           ...
           oob()
           {
                   int out = FWRITE, mark;
                   char waste[BUFSIZ];

                   /* flush local terminal output */
                   ioctl(1, TIOCFLUSH, (char *)&out);
                   for (;;) {
                           if ((mark = sockatmark(rem)) < 0) {
                                   perror("sockatmark");
                                   break;
                           }
                           if (mark)
                                   break;
                           (void) read(rem, waste, sizeof (waste));
                   }
                   if (recv(rem, &mark, 1, MSG_OOB) < 0) {
                           perror("recv");
                           ...
                   }
                   ...
           }

RETURN VALUES
     Upon successful completion, the sockatmark() function returns the value 1 if the read pointer is point-
     ing at the OOB mark, 0 if it is not.  Otherwise, the value -1 is returned and the global variable errno
     is set to indicate the error.

ERRORS
     The sockatmark() call fails if:

     [EBADF]            The s argument is not a valid descriptor.

     [ENOTTY]           The s argument is a descriptor for a file, not a socket.

SEE ALSO
     recv(2), send(2)

HISTORY
     The sockatmark() function was introduced by IEEE Std 1003.1-2001 (``POSIX.1''), to standardize the his-torical historical
     torical SIOCATMARK ioctl(2).  The ENOTTY error instead of the usual ENOTSOCK is to match the historical
     behavior of SIOCATMARK.

BSD                                           October 13, 2002                                           BSD