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+.\" Copyright (c) 2006, Michael Kerrisk (mtk-manpages@gmx.net)
+.\"
+.\" Permission is granted to make and distribute verbatim copies of this
+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
+.\"
+.\" Permission is granted to copy and distribute modified versions of this
+.\" manual under the conditions for verbatim copying, provided that the
+.\" entire resulting derived work is distributed under the terms of a
+.\" permission notice identical to this one.
+.\"
+.\" Since the Linux kernel and libraries are constantly changing, this
+.\" manual page may be incorrect or out-of-date.  The author(s) assume no
+.\" responsibility for errors or omissions, or for damages resulting from
+.\" the use of the information contained herein.
+.\"
+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.\"
+.TH SOCKATMARK 3 2006-04-24 "Linux" "Linux Programmer's Manual"
+.SH NAME
+sockatmark \- determine whether socket is at out-of-band mark
+.SH SYNOPSIS
+.B #include <sys/socket.h>
+.sp
+.BI "int sockatmark(int " fd );
+.SH DESCRIPTION
+.BR sockatmark ()
+returns a value indicating whether or not the socket referred 
+to by the file descriptor
+.I fd
+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.
+.SH "RETURN VALUE"
+A successful call to
+.BR 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 \fIerrno\fP is set to indicate the error.
+.SH ERRORS
+.TP
+.B EBADF
+.I fd
+is not a valid file descriptor.
+.TP
+.B EINVAL
+.\" POSIX.1 says ENOTTY for this case
+.I fd 
+is not a file descriptor to which
+.BR sockatmark ()
+can be applied.
+.SH NOTES
+If
+.BR sockatmark ()
+returns 1, then the out-of-band data can be read using the 
+.B MSG_OOB
+flag of
+.BR recv (2).
+
+Out-of-band data is only supported on some stream socket protocols.
+
+.BR sockatmark ()
+can safely be called from a handler for the SIGURG signal.
+
+.BR sockatmark ()
+is implemented using the 
+.B SIOCATMARK 
+.BR ioctl ()
+operation.
+.SH "CONFORMING TO"
+POSIX.1-2001
+.SH VERSIONS
+.BR sockatmark () 
+was added to glibc in version 2.2.4.
+.SH BUGS
+Prior to glibc 2.4,
+.BR sockatmark () 
+did not work.
+.SH 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:
+.nf
+
+    char buf[BUF_LEN];
+    char oobdata;
+    int atmark, s;
+
+    for (;;) {
+        atmark = sockatmark(fd);
+        if (atmark == \-1) {
+            perror("sockatmark");
+            break;
+        } 
+       
+        if (atmark)
+            break;
+        
+        s = read(fd, buf, BUF_LEN) <= 0);
+        if (s == -1)
+            perror("read");
+        if (s <= 0) 
+            break;
+    }
+    
+    if (atmark == 1) {
+        if (recv(fd, &oobdata, 1, MSG_OOB) == \-1) {
+            perror("recv");
+            ...
+        }
+    }
+.fi
+.SH "SEE ALSO"
+.BR fcntl (2),
+.BR recv (2),
+.BR send (2),
+.BR tcp (7)