1GETMSG(P) POSIX Programmer's Manual GETMSG(P)
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6 getmsg, getpmsg - receive next message from a STREAMS file (STREAMS)
7
9 #include <stropts.h>
10 int getmsg(int fildes, struct strbuf *restrict ctlptr,
12 struct strbuf *restrict dataptr, int *restrict flagsp);
13 int getpmsg(int fildes, struct strbuf *restrict ctlptr,
14 struct strbuf *restrict dataptr, int *restrict bandp,
15 int *restrict flagsp);
16
19 The getmsg() function shall retrieve the contents of a message located
20 at the head of the STREAM head read queue associated with a STREAMS
21 file and place the contents into one or more buffers. The message con‐
22 tains either a data part, a control part, or both. The data and control
23 parts of the message shall be placed into separate buffers, as
24 described below. The semantics of each part are defined by the origina‐
25 tor of the message.
26 The getpmsg() function shall be equivalent to getmsg(), except that it
28 provides finer control over the priority of the messages received.
29 Except where noted, all requirements on getmsg() also pertain to
30 getpmsg().
31 The fildes argument specifies a file descriptor referencing a STREAMS-
33 based file.
34 The ctlptr and dataptr arguments each point to a strbuf structure, in
36 which the buf member points to a buffer in which the data or control
37 information is to be placed, and the maxlen member indicates the maxi‐
38 mum number of bytes this buffer can hold. On return, the len member
39 shall contain the number of bytes of data or control information actu‐
40 ally received. The len member shall be set to 0 if there is a zero-
41 length control or data part and len shall be set to -1 if no data or
42 control information is present in the message.
43 When getmsg() is called, flagsp should point to an integer that indi‐
45 cates the type of message the process is able to receive. This is
46 described further below.
47 The ctlptr argument is used to hold the control part of the message,
49 and dataptr is used to hold the data part of the message. If ctlptr (or
50 dataptr) is a null pointer or the maxlen member is -1, the control (or
51 data) part of the message shall not be processed and shall be left on
52 the STREAM head read queue, and if the ctlptr (or dataptr) is not a
53 null pointer, len shall be set to -1. If the maxlen member is set to 0
54 and there is a zero-length control (or data) part, that zero-length
55 part shall be removed from the read queue and len shall be set to 0. If
56 the maxlen member is set to 0 and there are more than 0 bytes of con‐
57 trol (or data) information, that information shall be left on the read
58 queue and len shall be set to 0. If the maxlen member in ctlptr (or
59 dataptr) is less than the control (or data) part of the message, maxlen
60 bytes shall be retrieved. In this case, the remainder of the message
61 shall be left on the STREAM head read queue and a non-zero return value
62 shall be provided.
63 By default, getmsg() shall process the first available message on the
65 STREAM head read queue. However, a process may choose to retrieve only
66 high-priority messages by setting the integer pointed to by flagsp to
67 RS_HIPRI. In this case, getmsg() shall only process the next message if
68 it is a high-priority message. When the integer pointed to by flagsp is
69 0, any available message shall be retrieved. In this case, on return,
70 the integer pointed to by flagsp shall be set to RS_HIPRI if a high-
71 priority message was retrieved, or 0 otherwise.
72 For getpmsg(), the flags are different. The flagsp argument points to a
74 bitmask with the following mutually-exclusive flags defined: MSG_HIPRI,
75 MSG_BAND, and MSG_ANY. Like getmsg(), getpmsg() shall process the
76 first available message on the STREAM head read queue. A process may
77 choose to retrieve only high-priority messages by setting the integer
78 pointed to by flagsp to MSG_HIPRI and the integer pointed to by bandp
79 to 0. In this case, getpmsg() shall only process the next message if it
80 is a high-priority message. In a similar manner, a process may choose
81 to retrieve a message from a particular priority band by setting the
82 integer pointed to by flagsp to MSG_BAND and the integer pointed to by
83 bandp to the priority band of interest. In this case, getpmsg() shall
84 only process the next message if it is in a priority band equal to, or
85 greater than, the integer pointed to by bandp, or if it is a high-pri‐
86 ority message. If a process wants to get the first message off the
87 queue, the integer pointed to by flagsp should be set to MSG_ANY and
88 the integer pointed to by bandp should be set to 0. On return, if the
89 message retrieved was a high-priority message, the integer pointed to
90 by flagsp shall be set to MSG_HIPRI and the integer pointed to by bandp
91 shall be set to 0. Otherwise, the integer pointed to by flagsp shall be
92 set to MSG_BAND and the integer pointed to by bandp shall be set to the
93 priority band of the message.
94 If O_NONBLOCK is not set, getmsg() and getpmsg() shall block until a
96 message of the type specified by flagsp is available at the front of
97 the STREAM head read queue. If O_NONBLOCK is set and a message of the
98 specified type is not present at the front of the read queue, getmsg()
99 and getpmsg() shall fail and set errno to [EAGAIN].
100 If a hangup occurs on the STREAM from which messages are retrieved,
102 getmsg() and getpmsg() shall continue to operate normally, as described
103 above, until the STREAM head read queue is empty. Thereafter, they
104 shall return 0 in the len members of ctlptr and dataptr.
105
107 Upon successful completion, getmsg() and getpmsg() shall return a non-
108 negative value. A value of 0 indicates that a full message was read
109 successfully. A return value of MORECTL indicates that more control
110 information is waiting for retrieval. A return value of MOREDATA indi‐
111 cates that more data is waiting for retrieval. A return value of the
112 bitwise-logical OR of MORECTL and MOREDATA indicates that both types of
113 information remain. Subsequent getmsg() and getpmsg() calls shall
114 retrieve the remainder of the message. However, if a message of higher
115 priority has come in on the STREAM head read queue, the next call to
116 getmsg() or getpmsg() shall retrieve that higher-priority message
117 before retrieving the remainder of the previous message.
118 If the high priority control part of the message is consumed, the mes‐
120 sage shall be placed back on the queue as a normal message of band 0.
121 Subsequent getmsg() and getpmsg() calls shall retrieve the remainder of
122 the message. If, however, a priority message arrives or already exists
123 on the STREAM head, the subsequent call to getmsg() or getpmsg() shall
124 retrieve the higher-priority message before retrieving the remainder of
125 the message that was put back.
126 Upon failure, getmsg() and getpmsg() shall return -1 and set errno to
128 indicate the error.
129
131 The getmsg() and getpmsg() functions shall fail if:
132 EAGAIN The O_NONBLOCK flag is set and no messages are available.
134 EBADF The fildes argument is not a valid file descriptor open for
136 reading.
137 EBADMSG
139 The queued message to be read is not valid for getmsg() or
140 getpmsg() or a pending file descriptor is at the STREAM head.
141 EINTR A signal was caught during getmsg() or getpmsg().
143 EINVAL An illegal value was specified by flagsp, or the STREAM or mul‐
145 tiplexer referenced by fildes is linked (directly or indirectly)
146 downstream from a multiplexer.
147 ENOSTR A STREAM is not associated with fildes.
149 In addition, getmsg() and getpmsg() shall fail if the STREAM head had
152 processed an asynchronous error before the call. In this case, the
153 value of errno does not reflect the result of getmsg() or getpmsg() but
154 reflects the prior error.
155 The following sections are informative.
157
159 Getting Any Message
160 In the following example, the value of fd is assumed to refer to an
161 open STREAMS file. The call to getmsg() retrieves any available message
162 on the associated STREAM-head read queue, returning control and data
163 information to the buffers pointed to by ctrlbuf and databuf, respec‐
164 tively.
165 #include <stropts.h>
168 ...
169 int fd;
170 char ctrlbuf[128];
171 char databuf[512];
172 struct strbuf ctrl;
173 struct strbuf data;
174 int flags = 0;
175 int ret;
176 ctrl.buf = ctrlbuf;
179 ctrl.maxlen = sizeof(ctrlbuf);
180 data.buf = databuf;
183 data.maxlen = sizeof(databuf);
184 ret = getmsg (fd, &ctrl, &data, &flags);
187 Getting the First Message off the Queue
189 In the following example, the call to getpmsg() retrieves the first
190 available message on the associated STREAM-head read queue.
191 #include <stropts.h>
194 ...
195 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;
208 ctrl.maxlen = sizeof(ctrlbuf);
209 data.buf = databuf;
212 data.maxlen = sizeof(databuf);
213 ret = getpmsg (fd, &ctrl, &data, &band, &flags);
216
218 None.
219
221 None.
222
224 None.
225
227 STREAMS , poll() , putmsg() , read() , write() , the Base Definitions
228 volume of IEEE Std 1003.1-2001, <stropts.h>
229
231 Portions of this text are reprinted and reproduced in electronic form
232 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
233 -- Portable Operating System Interface (POSIX), The Open Group Base
234 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
235 Electrical and Electronics Engineers, Inc and The Open Group. In the
236 event of any discrepancy between this version and the original IEEE and
237 The Open Group Standard, the original IEEE and The Open Group Standard
238 is the referee document. The original Standard can be obtained online
239 at http://www.opengroup.org/unix/online.html .
240IEEE/The Open Group 2003 GETMSG(P)