1SOCKATMARK(3P) POSIX Programmer's Manual SOCKATMARK(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 sockatmark — determine whether a socket is at the out-of-band mark
13
15 #include <sys/socket.h>
16 int sockatmark(int s);
18
20 The sockatmark() function shall determine whether the socket specified
21 by the descriptor s is at the out-of-band data mark (see Section
22 2.10.12, Socket Out-of-Band Data State). If the protocol for the
23 socket supports out-of-band data by marking the stream with an out-of-
24 band data mark, the sockatmark() function shall return 1 when all data
25 preceding the mark has been read and the out-of-band data mark is the
26 first element in the receive queue. The sockatmark() function shall not
27 remove the mark from the stream.
28
30 Upon successful completion, the sockatmark() function shall return a
31 value indicating whether the socket is at an out-of-band data mark. If
32 the protocol has marked the data stream and all data preceding the mark
33 has been read, the return value shall be 1; if there is no mark, or if
34 data precedes the mark in the receive queue, the sockatmark() function
35 shall return 0. Otherwise, it shall return a value of -1 and set errno
36 to indicate the error.
37
39 The sockatmark() function shall fail if:
40 EBADF The s argument is not a valid file descriptor.
42 ENOTTY The file associated with the s argument is not a socket.
44 The following sections are informative.
46
48 None.
49
51 The use of this function between receive operations allows an applica‐
52 tion to determine which received data precedes the out-of-band data and
53 which follows the out-of-band data.
54 There is an inherent race condition in the use of this function. On an
56 empty receive queue, the current read of the location might well be at
57 the ``mark'', but the system has no way of knowing that the next data
58 segment that will arrive from the network will carry the mark, and
59 sockatmark() will return false, and the next read operation will
60 silently consume the mark.
61 Hence, this function can only be used reliably when the application
63 already knows that the out-of-band data has been seen by the system or
64 that it is known that there is data waiting to be read at the socket
65 (via SIGURG or select()). See Section 2.10.11, Socket Receive Queue,
66 Section 2.10.12, Socket Out-of-Band Data State, Section 2.10.14, Sig‐
67 nals, and pselect() for details.
68
70 The sockatmark() function replaces the historical SIOCATMARK command to
71 ioctl() which implemented the same functionality on many implementa‐
72 tions. Using a wrapper function follows the adopted conventions to
73 avoid specifying commands to the ioctl() function, other than those now
74 included to support XSI STREAMS. The sockatmark() function could be
75 implemented as follows:
76 #include <sys/ioctl.h>
79 int sockatmark(int s)
81 {
82 int val;
83 if (ioctl(s,SIOCATMARK,&val)==-1)
84 return(-1);
85 return(val);
86 }
87 The use of [ENOTTY] to indicate an incorrect descriptor type matches
89 the historical behavior of SIOCATMARK.
90
92 None.
93
95 Section 2.10.12, Socket Out-of-Band Data State, pselect(), recv(),
96 recvmsg()
97 The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>
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101 Portions of this text are reprinted and reproduced in electronic form
102 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
103 table Operating System Interface (POSIX), The Open Group Base Specifi‐
104 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
105 Electrical and Electronics Engineers, Inc and The Open Group. In the
106 event of any discrepancy between this version and the original IEEE and
107 The Open Group Standard, the original IEEE and The Open Group Standard
108 is the referee document. The original Standard can be obtained online
109 at http://www.opengroup.org/unix/online.html .
110 Any typographical or formatting errors that appear in this page are
112 most likely to have been introduced during the conversion of the source
113 files to man page format. To report such errors, see https://www.ker‐
114 nel.org/doc/man-pages/reporting_bugs.html .
115IEEE/The Open Group 2017 SOCKATMARK(3P)