mirror of https://github.com/mkerrisk/man-pages
118 lines
4.1 KiB
Plaintext
118 lines
4.1 KiB
Plaintext
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "SOCKATMARK" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" sockatmark
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.SH NAME
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sockatmark \- determine whether a socket is at the out-of-band mark
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.SH SYNOPSIS
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.LP
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\fB#include <sys/socket.h>
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.br
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.sp
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int sockatmark(int\fP \fIs\fP\fB);
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIsockatmark\fP() function shall determine whether the socket
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specified by the descriptor \fIs\fP is at the out-of-band
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data mark (see the System Interfaces volume of IEEE\ Std\ 1003.1-2001,
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Section 2.10.12, Socket Out-of-Band State). If the protocol for the
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socket
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supports out-of-band data by marking the stream with an out-of-band
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data mark, the \fIsockatmark\fP() function shall return 1 when
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all data preceding the mark has been read and the out-of-band data
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mark is the first element in the receive queue. The
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\fIsockatmark\fP() function shall not remove the mark from the stream.
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.SH RETURN VALUE
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.LP
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Upon successful completion, the \fIsockatmark\fP() function shall
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return a value indicating whether the socket is at an
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out-of-band data mark. If the protocol has marked the data stream
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and all data preceding the mark has been read, the return value
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shall be 1; if there is no mark, or if data precedes the mark in the
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receive queue, the \fIsockatmark\fP() function shall return
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0. Otherwise, it shall return a value of -1 and set \fIerrno\fP to
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indicate the error.
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.SH ERRORS
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.LP
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The \fIsockatmark\fP() function shall fail if:
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.TP 7
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.B EBADF
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The \fIs\fP argument is not a valid file descriptor.
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.TP 7
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.B ENOTTY
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The \fIs\fP argument does not specify a descriptor for a socket.
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.sp
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.LP
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None.
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.SH APPLICATION USAGE
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.LP
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The use of this function between receive operations allows an application
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to determine which received data precedes the
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out-of-band data and which follows the out-of-band data.
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.LP
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There is an inherent race condition in the use of this function. On
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an empty receive queue, the current read of the location
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might well be at the "mark", but the system has no way of knowing
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that the next data segment that will arrive from the network
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will carry the mark, and \fIsockatmark\fP() will return false, and
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the next read operation will silently consume the mark.
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.LP
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Hence, this function can only be used reliably when the application
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already knows that the out-of-band data has been seen by the
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system or that it is known that there is data waiting to be read at
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the socket (via SIGURG or \fIselect\fP()). See \fISocket Receive Queue\fP
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, \fISocket Out-of-Band Data State\fP , \fISignals\fP , and \fIpselect\fP()
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for
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details.
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.SH RATIONALE
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.LP
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The \fIsockatmark\fP() function replaces the historical SIOCATMARK
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command to \fIioctl\fP() which implemented the same functionality
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on many implementations. Using a wrapper
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function follows the adopted conventions to avoid specifying commands
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to the \fIioctl\fP()
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function, other than those now included to support XSI STREAMS. The
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\fIsockatmark\fP() function could be implemented as
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follows:
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.sp
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.RS
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.nf
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\fB#include <sys/ioctl.h>
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.sp
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int sockatmark(int s)
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{
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int val;
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if (ioctl(s,SIOCATMARK,&val)==-1)
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return(-1);
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return(val);
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}
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\fP
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.fi
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.RE
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.LP
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The use of [ENOTTY] to indicate an incorrect descriptor type matches
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the historical behavior of SIOCATMARK.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIpselect\fP() , \fIrecv\fP() , \fIrecvmsg\fP() , the Base Definitions
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volume of IEEE\ Std\ 1003.1-2001, \fI<sys/socket.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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