1MSGRCV(P) POSIX Programmer's Manual MSGRCV(P)
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6 msgrcv - XSI message receive operation
7
9 #include <sys/msg.h>
10 ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
12 int msgflg);
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16 The msgrcv() function operates on XSI message queues (see the Base Def‐
17 initions volume of IEEE Std 1003.1-2001, Section 3.224, Message Queue).
18 It is unspecified whether this function interoperates with the realtime
19 interprocess communication facilities defined in Realtime .
20 The msgrcv() function shall read a message from the queue associated
22 with the message queue identifier specified by msqid and place it in
23 the user-defined buffer pointed to by msgp.
24 The application shall ensure that the argument msgp points to a user-
26 defined buffer that contains first a field of type long specifying the
27 type of the message, and then a data portion that holds the data bytes
28 of the message. The structure below is an example of what this user-
29 defined buffer might look like:
30 struct mymsg {
33 long mtype; /* Message type. */
34 char mtext[1]; /* Message text. */
35 }
36 The structure member mtype is the received message's type as specified
38 by the sending process.
39 The structure member mtext is the text of the message.
41 The argument msgsz specifies the size in bytes of mtext. The received
43 message shall be truncated to msgsz bytes if it is larger than msgsz
44 and (msgflg & MSG_NOERROR) is non-zero. The truncated part of the mes‐
45 sage shall be lost and no indication of the truncation shall be given
46 to the calling process.
47 If the value of msgsz is greater than {SSIZE_MAX}, the result is imple‐
49 mentation-defined.
50 The argument msgtyp specifies the type of message requested as follows:
52 * If msgtyp is 0, the first message on the queue shall be received.
54 * If msgtyp is greater than 0, the first message of type msgtyp shall
56 be received.
57 * If msgtyp is less than 0, the first message of the lowest type that
59 is less than or equal to the absolute value of msgtyp shall be
60 received.
61 The argument msgflg specifies the action to be taken if a message of
63 the desired type is not on the queue. These are as follows:
64 * If (msgflg & IPC_NOWAIT) is non-zero, the calling thread shall
66 return immediately with a return value of -1 and errno set to
67 [ENOMSG].
68 * If (msgflg & IPC_NOWAIT) is 0, the calling thread shall suspend exe‐
70 cution until one of the following occurs:
71 * A message of the desired type is placed on the queue.
73 * The message queue identifier msqid is removed from the system;
75 when this occurs, errno shall be set equal to [EIDRM] and -1
76 shall be returned.
77 * The calling thread receives a signal that is to be caught; in
79 this case a message is not received and the calling thread
80 resumes execution in the manner prescribed in sigaction() .
81 Upon successful completion, the following actions are taken with
83 respect to the data structure associated with msqid:
84 * msg_qnum shall be decremented by 1.
86 * msg_lrpid shall be set equal to the process ID of the calling
88 process.
89 * msg_rtime shall be set equal to the current time.
91
93 Upon successful completion, msgrcv() shall return a value equal to the
94 number of bytes actually placed into the buffer mtext. Otherwise, no
95 message shall be received, msgrcv() shall return (ssize_t)-1, and errno
96 shall be set to indicate the error.
97
99 The msgrcv() function shall fail if:
100 E2BIG The value of mtext is greater than msgsz and (msgflg & MSG_NOER‐
102 ROR) is 0.
103 EACCES Operation permission is denied to the calling process; see XSI
105 Interprocess Communication .
106 EIDRM The message queue identifier msqid is removed from the system.
108 EINTR The msgrcv() function was interrupted by a signal.
110 EINVAL msqid is not a valid message queue identifier.
112 ENOMSG The queue does not contain a message of the desired type and
114 (msgflg & IPC_NOWAIT) is non-zero.
115 The following sections are informative.
118
120 Receiving a Message
121 The following example receives the first message on the queue (based on
122 the value of the msgtyp argument, 0). The queue is identified by the
123 msqid argument (assuming that the value has previously been set). This
124 call specifies that an error should be reported if no message is avail‐
125 able, but not if the message is too large. The message size is calcu‐
126 lated directly using the sizeof operator.
127 #include <sys/msg.h>
130 ...
131 int result;
132 int msqid;
133 struct message {
134 long type;
135 char text[20];
136 } msg;
137 long msgtyp = 0;
138 ...
139 result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
140 msgtyp, MSG_NOERROR | IPC_NOWAIT);
141
143 The POSIX Realtime Extension defines alternative interfaces for inter‐
144 process communication (IPC). Application developers who need to use IPC
145 should design their applications so that modules using the IPC routines
146 described in XSI Interprocess Communication can be easily modified to
147 use the alternative interfaces.
148
150 None.
151
153 None.
154
156 XSI Interprocess Communication , Realtime , mq_close() , mq_getattr() ,
157 mq_notify() , mq_open() , mq_receive() , mq_send() , mq_setattr() ,
158 mq_unlink() , msgctl() , msgget() , msgsnd() , sigaction() , the Base
159 Definitions volume of IEEE Std 1003.1-2001, <sys/msg.h>
160
162 Portions of this text are reprinted and reproduced in electronic form
163 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
164 -- Portable Operating System Interface (POSIX), The Open Group Base
165 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
166 Electrical and Electronics Engineers, Inc and The Open Group. In the
167 event of any discrepancy between this version and the original IEEE and
168 The Open Group Standard, the original IEEE and The Open Group Standard
169 is the referee document. The original Standard can be obtained online
170 at http://www.opengroup.org/unix/online.html .
171IEEE/The Open Group 2003 MSGRCV(P)