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SOCKATMARK(3)		    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.
       Otherwise (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 pointing 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)

       Stuart  Sechrest,  An  Introductory  4.4BSD  Interprocess Communication
       Tutorial.

       Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil	Lapsley, Steve
       Miller, and Chris Torek,	An Advanced 4.4BSD Interprocess	 Communication
       Tutorial, https://docs.freebsd.org/44doc/psd/21.ipc/paper.pdf.

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

FreeBSD	13.4			 July 3, 2022			 SOCKATMARK(3)

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