1GETMSG(3P) POSIX Programmer's Manual GETMSG(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 getmsg, getpmsg — receive next message from a STREAMS file (STREAMS)
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15 #include <stropts.h>
16 int getmsg(int fildes, struct strbuf *restrict ctlptr,
18 struct strbuf *restrict dataptr, int *restrict flagsp);
19 int getpmsg(int fildes, struct strbuf *restrict ctlptr,
20 struct strbuf *restrict dataptr, int *restrict bandp,
21 int *restrict flagsp);
22
24 The getmsg() function shall retrieve the contents of a message located
25 at the head of the STREAM head read queue associated with a STREAMS
26 file and place the contents into one or more buffers. The message con‐
27 tains either a data part, a control part, or both. The data and control
28 parts of the message shall be placed into separate buffers, as
29 described below. The semantics of each part are defined by the origina‐
30 tor of the message.
31 The getpmsg() function shall be equivalent to getmsg(), except that it
33 provides finer control over the priority of the messages received.
34 Except where noted, all requirements on getmsg() also pertain to
35 getpmsg().
36 The fildes argument specifies a file descriptor referencing a STREAMS-
38 based file.
39 The ctlptr and dataptr arguments each point to a strbuf structure, in
41 which the buf member points to a buffer in which the data or control
42 information is to be placed, and the maxlen member indicates the maxi‐
43 mum number of bytes this buffer can hold. On return, the len member
44 shall contain the number of bytes of data or control information actu‐
45 ally received. The len member shall be set to 0 if there is a zero-
46 length control or data part and len shall be set to -1 if no data or
47 control information is present in the message.
48 When getmsg() is called, flagsp should point to an integer that indi‐
50 cates the type of message the process is able to receive. This is
51 described further below.
52 The ctlptr argument is used to hold the control part of the message,
54 and dataptr is used to hold the data part of the message. If ctlptr (or
55 dataptr) is a null pointer or the maxlen member is -1, the control (or
56 data) part of the message shall not be processed and shall be left on
57 the STREAM head read queue, and if the ctlptr (or dataptr) is not a
58 null pointer, len shall be set to -1. If the maxlen member is set to 0
59 and there is a zero-length control (or data) part, that zero-length
60 part shall be removed from the read queue and len shall be set to 0. If
61 the maxlen member is set to 0 and there are more than 0 bytes of con‐
62 trol (or data) information, that information shall be left on the read
63 queue and len shall be set to 0. If the maxlen member in ctlptr (or
64 dataptr) is less than the control (or data) part of the message, maxlen
65 bytes shall be retrieved. In this case, the remainder of the message
66 shall be left on the STREAM head read queue and a non-zero return value
67 shall be provided.
68 By default, getmsg() shall process the first available message on the
70 STREAM head read queue. However, a process may choose to retrieve only
71 high-priority messages by setting the integer pointed to by flagsp to
72 RS_HIPRI. In this case, getmsg() shall only process the next message if
73 it is a high-priority message. When the integer pointed to by flagsp
74 is 0, any available message shall be retrieved. In this case, on
75 return, the integer pointed to by flagsp shall be set to RS_HIPRI if a
76 high-priority message was retrieved, or 0 otherwise.
77 For getpmsg(), the flags are different. The flagsp argument points to a
79 bitmask with the following mutually-exclusive flags defined: MSG_HIPRI,
80 MSG_BAND, and MSG_ANY. Like getmsg(), getpmsg() shall process the
81 first available message on the STREAM head read queue. A process may
82 choose to retrieve only high-priority messages by setting the integer
83 pointed to by flagsp to MSG_HIPRI and the integer pointed to by bandp
84 to 0. In this case, getpmsg() shall only process the next message if it
85 is a high-priority message. In a similar manner, a process may choose
86 to retrieve a message from a particular priority band by setting the
87 integer pointed to by flagsp to MSG_BAND and the integer pointed to by
88 bandp to the priority band of interest. In this case, getpmsg() shall
89 only process the next message if it is in a priority band equal to, or
90 greater than, the integer pointed to by bandp, or if it is a high-pri‐
91 ority message. If a process wants to get the first message off the
92 queue, the integer pointed to by flagsp should be set to MSG_ANY and
93 the integer pointed to by bandp should be set to 0. On return, if the
94 message retrieved was a high-priority message, the integer pointed to
95 by flagsp shall be set to MSG_HIPRI and the integer pointed to by bandp
96 shall be set to 0. Otherwise, the integer pointed to by flagsp shall be
97 set to MSG_BAND and the integer pointed to by bandp shall be set to the
98 priority band of the message.
99 If O_NONBLOCK is not set, getmsg() and getpmsg() shall block until a
101 message of the type specified by flagsp is available at the front of
102 the STREAM head read queue. If O_NONBLOCK is set and a message of the
103 specified type is not present at the front of the read queue, getmsg()
104 and getpmsg() shall fail and set errno to [EAGAIN].
105 If a hangup occurs on the STREAM from which messages are retrieved,
107 getmsg() and getpmsg() shall continue to operate normally, as described
108 above, until the STREAM head read queue is empty. Thereafter, they
109 shall return 0 in the len members of ctlptr and dataptr.
110
112 Upon successful completion, getmsg() and getpmsg() shall return a non-
113 negative value. A value of 0 indicates that a full message was read
114 successfully. A return value of MORECTL indicates that more control
115 information is waiting for retrieval. A return value of MOREDATA indi‐
116 cates that more data is waiting for retrieval. A return value of the
117 bitwise-logical OR of MORECTL and MOREDATA indicates that both types of
118 information remain. Subsequent getmsg() and getpmsg() calls shall
119 retrieve the remainder of the message. However, if a message of higher
120 priority has come in on the STREAM head read queue, the next call to
121 getmsg() or getpmsg() shall retrieve that higher-priority message
122 before retrieving the remainder of the previous message.
123 If the high priority control part of the message is consumed, the mes‐
125 sage shall be placed back on the queue as a normal message of band 0.
126 Subsequent getmsg() and getpmsg() calls shall retrieve the remainder of
127 the message. If, however, a priority message arrives or already exists
128 on the STREAM head, the subsequent call to getmsg() or getpmsg() shall
129 retrieve the higher-priority message before retrieving the remainder of
130 the message that was put back.
131 Upon failure, getmsg() and getpmsg() shall return -1 and set errno to
133 indicate the error.
134
136 The getmsg() and getpmsg() functions shall fail if:
137 EAGAIN The O_NONBLOCK flag is set and no messages are available.
139 EBADF The fildes argument is not a valid file descriptor open for
141 reading.
142 EBADMSG
144 The queued message to be read is not valid for getmsg() or
145 getpmsg() or a pending file descriptor is at the STREAM head.
146 EINTR A signal was caught during getmsg() or getpmsg().
148 EINVAL An illegal value was specified by flagsp, or the STREAM or mul‐
150 tiplexer referenced by fildes is linked (directly or indirectly)
151 downstream from a multiplexer.
152 ENOSTR A STREAM is not associated with fildes.
154 In addition, getmsg() and getpmsg() shall fail if the STREAM head had
156 processed an asynchronous error before the call. In this case, the
157 value of errno does not reflect the result of getmsg() or getpmsg() but
158 reflects the prior error.
159 The following sections are informative.
161
163 Getting Any Message
164 In the following example, the value of fd is assumed to refer to an
165 open STREAMS file. The call to getmsg() retrieves any available message
166 on the associated STREAM-head read queue, returning control and data
167 information to the buffers pointed to by ctrlbuf and databuf, respec‐
168 tively.
169 #include <stropts.h>
172 ...
173 int fd;
174 char ctrlbuf[128];
175 char databuf[512];
176 struct strbuf ctrl;
177 struct strbuf data;
178 int flags = 0;
179 int ret;
180 ctrl.buf = ctrlbuf;
182 ctrl.maxlen = sizeof(ctrlbuf);
183 data.buf = databuf;
185 data.maxlen = sizeof(databuf);
186 ret = getmsg (fd, &ctrl, &data, &flags);
188 Getting the First Message off the Queue
190 In the following example, the call to getpmsg() retrieves the first
191 available message on the associated STREAM-head read queue.
192 #include <stropts.h>
195 ...
196 int fd;
198 char ctrlbuf[128];
199 char databuf[512];
200 struct strbuf ctrl;
201 struct strbuf data;
202 int band = 0;
203 int flags = MSG_ANY;
204 int ret;
205 ctrl.buf = ctrlbuf;
207 ctrl.maxlen = sizeof(ctrlbuf);
208 data.buf = databuf;
210 data.maxlen = sizeof(databuf);
211 ret = getpmsg (fd, &ctrl, &data, &band, &flags);
213
215 None.
216
218 None.
219
221 The getmsg() and getpmsg() functions may be removed in a future ver‐
222 sion.
223
225 Section 2.6, STREAMS, poll(), putmsg(), read(), write()
226 The Base Definitions volume of POSIX.1‐2017, <stropts.h>
228
230 Portions of this text are reprinted and reproduced in electronic form
231 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
232 table Operating System Interface (POSIX), The Open Group Base Specifi‐
233 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
234 Electrical and Electronics Engineers, Inc and The Open Group. In the
235 event of any discrepancy between this version and the original IEEE and
236 The Open Group Standard, the original IEEE and The Open Group Standard
237 is the referee document. The original Standard can be obtained online
238 at http://www.opengroup.org/unix/online.html .
239 Any typographical or formatting errors that appear in this page are
241 most likely to have been introduced during the conversion of the source
242 files to man page format. To report such errors, see https://www.ker‐
243 nel.org/doc/man-pages/reporting_bugs.html .
244IEEE/The Open Group 2017 GETMSG(3P)