1MSGSND(3P) POSIX Programmer's Manual MSGSND(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.
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12 msgsnd - XSI message send operation
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15 #include <sys/msg.h>
16 int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
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21 The msgsnd() function operates on XSI message queues (see the Base Def‐
22 initions volume of IEEE Std 1003.1-2001, Section 3.224, Message Queue).
23 It is unspecified whether this function interoperates with the realtime
24 interprocess communication facilities defined in Realtime .
25 The msgsnd() function shall send a message to the queue associated with
27 the message queue identifier specified by msqid.
28 The application shall ensure that the argument msgp points to a user-
30 defined buffer that contains first a field of type long specifying the
31 type of the message, and then a data portion that holds the data bytes
32 of the message. The structure below is an example of what this user-
33 defined buffer might look like:
34 struct mymsg {
37 long mtype; /* Message type. */
38 char mtext[1]; /* Message text. */
39 }
40 The structure member mtype is a non-zero positive type long that can be
42 used by the receiving process for message selection.
43 The structure member mtext is any text of length msgsz bytes. The argu‐
45 ment msgsz can range from 0 to a system-imposed maximum.
46 The argument msgflg specifies the action to be taken if one or more of
48 the following is true:
49 * The number of bytes already on the queue is equal to msg_qbytes; see
51 <sys/msg.h>.
52 * The total number of messages on all queues system-wide is equal to
54 the system-imposed limit.
55 These actions are as follows:
57 * If (msgflg & IPC_NOWAIT) is non-zero, the message shall not be sent
59 and the calling thread shall return immediately.
60 * If (msgflg & IPC_NOWAIT) is 0, the calling thread shall suspend exe‐
62 cution until one of the following occurs:
63 * The condition responsible for the suspension no longer exists, in
65 which case the message is sent.
66 * The message queue identifier msqid is removed from the system;
68 when this occurs, errno shall be set equal to [EIDRM] and -1
69 shall be returned.
70 * The calling thread receives a signal that is to be caught; in
72 this case the message is not sent and the calling thread resumes
73 execution in the manner prescribed in sigaction().
74 Upon successful completion, the following actions are taken with
76 respect to the data structure associated with msqid; see <sys/msg.h>:
77 * msg_qnum shall be incremented by 1.
79 * msg_lspid shall be set equal to the process ID of the calling
81 process.
82 * msg_stime shall be set equal to the current time.
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86 Upon successful completion, msgsnd() shall return 0; otherwise, no mes‐
87 sage shall be sent, msgsnd() shall return -1, and errno shall be set to
88 indicate the error.
89
91 The msgsnd() function shall fail if:
92 EACCES Operation permission is denied to the calling process; see XSI
94 Interprocess Communication .
95 EAGAIN The message cannot be sent for one of the reasons cited above
97 and (msgflg & IPC_NOWAIT) is non-zero.
98 EIDRM The message queue identifier msqid is removed from the system.
100 EINTR The msgsnd() function was interrupted by a signal.
102 EINVAL The value of msqid is not a valid message queue identifier, or
104 the value of mtype is less than 1; or the value of msgsz is less
105 than 0 or greater than the system-imposed limit.
106 The following sections are informative.
109
111 Sending a Message
112 The following example sends a message to the queue identified by the
113 msqid argument (assuming that value has previously been set). This call
114 specifies that an error should be reported if no message is available.
115 The message size is calculated directly using the sizeof operator.
116 #include <sys/msg.h>
119 ...
120 int result;
121 int msqid;
122 struct message {
123 long type;
124 char text[20];
125 } msg;
126 msg.type = 1;
129 strcpy(msg.text, "This is message 1");
130 ...
131 result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), IPC_NOWAIT);
132
134 The POSIX Realtime Extension defines alternative interfaces for inter‐
135 process communication (IPC). Application developers who need to use IPC
136 should design their applications so that modules using the IPC routines
137 described in XSI Interprocess Communication can be easily modified to
138 use the alternative interfaces.
139
141 None.
142
144 None.
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147 XSI Interprocess Communication, Realtime, mq_close(), mq_getattr(),
148 mq_notify(), mq_open(), mq_receive(), mq_send(), mq_setattr(),
149 mq_unlink(), msgctl(), msgget(), msgrcv(), sigaction(), the Base Defi‐
150 nitions volume of IEEE Std 1003.1-2001, <sys/msg.h>
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153 Portions of this text are reprinted and reproduced in electronic form
154 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
155 -- Portable Operating System Interface (POSIX), The Open Group Base
156 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
157 Electrical and Electronics Engineers, Inc and The Open Group. In the
158 event of any discrepancy between this version and the original IEEE and
159 The Open Group Standard, the original IEEE and The Open Group Standard
160 is the referee document. The original Standard can be obtained online
161 at http://www.opengroup.org/unix/online.html .
162IEEE/The Open Group 2003 MSGSND(3P)