msgrcv(3p)

1MSGRCV(3P)                 POSIX Programmer's Manual                MSGRCV(3P)
2
3
4

PROLOG

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

NAME

12       msgrcv - XSI message receive operation
13

SYNOPSIS

15       #include <sys/msg.h>
16
17       ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
18              int msgflg);
19
20

DESCRIPTION

22       The msgrcv() function operates on XSI message queues (see the Base Def‐
23       initions volume of IEEE Std 1003.1-2001, Section 3.224, Message Queue).
24       It is unspecified whether this function interoperates with the realtime
25       interprocess communication facilities defined in Realtime .
26
27       The  msgrcv()  function  shall read a message from the queue associated
28       with the message queue identifier specified by msqid and  place  it  in
29       the user-defined buffer pointed to by msgp.
30
31       The  application  shall ensure that the argument msgp points to a user-
32       defined buffer that contains first a field of type long specifying  the
33       type  of the message, and then a data portion that holds the data bytes
34       of the message. The structure below is an example of  what  this  user-
35       defined buffer might look like:
36
37
38              struct mymsg {
39                  long    mtype;     /* Message type. */
40                  char    mtext[1];  /* Message text. */
41              }
42
43       The  structure member mtype is the received message's type as specified
44       by the sending process.
45
46       The structure member mtext is the text of the message.
47
48       The argument msgsz specifies the size in bytes of mtext.  The  received
49       message  shall  be  truncated to msgsz bytes if it is larger than msgsz
50       and (msgflg & MSG_NOERROR) is non-zero. The truncated part of the  mes‐
51       sage  shall  be lost and no indication of the truncation shall be given
52       to the calling process.
53
54       If the value of msgsz is greater than {SSIZE_MAX}, the result is imple‐
55       mentation-defined.
56
57       The argument msgtyp specifies the type of message requested as follows:
58
59        * If msgtyp is 0, the first message on the queue shall be received.
60
61        * If  msgtyp is greater than 0, the first message of type msgtyp shall
62          be received.
63
64        * If msgtyp is less than 0, the first message of the lowest type  that
65          is  less  than  or  equal  to  the absolute value of msgtyp shall be
66          received.
67
68       The argument msgflg specifies the action to be taken if  a  message  of
69       the desired type is not on the queue. These are as follows:
70
71        * If  (msgflg  &  IPC_NOWAIT)  is  non-zero,  the calling thread shall
72          return immediately with a return  value  of  -1  and  errno  set  to
73          [ENOMSG].
74
75        * If (msgflg & IPC_NOWAIT) is 0, the calling thread shall suspend exe‐
76          cution until one of the following occurs:
77
78           * A message of the desired type is placed on the queue.
79
80           * The message queue identifier msqid is removed  from  the  system;
81             when  this  occurs,  errno  shall  be set equal to [EIDRM] and -1
82             shall be returned.
83
84           * The calling thread receives a signal that is  to  be  caught;  in
85             this  case  a  message  is  not  received  and the calling thread
86             resumes execution in the manner prescribed in sigaction().
87
88       Upon successful  completion,  the  following  actions  are  taken  with
89       respect to the data structure associated with msqid:
90
91        * msg_qnum shall be decremented by 1.
92
93        * msg_lrpid  shall  be  set  equal  to  the  process ID of the calling
94          process.
95
96        * msg_rtime shall be set equal to the current time.
97

RETURN VALUE

99       Upon successful completion, msgrcv() shall return a value equal to  the
100       number  of  bytes  actually placed into the buffer mtext. Otherwise, no
101       message shall be received, msgrcv() shall return (ssize_t)-1, and errno
102       shall be set to indicate the error.
103

ERRORS

105       The msgrcv() function shall fail if:
106
107       E2BIG  The value of mtext is greater than msgsz and (msgflg & MSG_NOER‐
108              ROR) is 0.
109
110       EACCES Operation permission is denied to the calling process;  see  XSI
111              Interprocess Communication .
112
113       EIDRM  The message queue identifier msqid is removed from the system.
114
115       EINTR  The msgrcv() function was interrupted by a signal.
116
117       EINVAL msqid is not a valid message queue identifier.
118
119       ENOMSG The  queue  does  not  contain a message of the desired type and
120              (msgflg & IPC_NOWAIT) is non-zero.
121
122
123       The following sections are informative.
124

EXAMPLES

126   Receiving a Message
127       The following example receives the first message on the queue (based on
128       the  value  of  the msgtyp argument, 0). The queue is identified by the
129       msqid argument (assuming that the value has previously been set).  This
130       call specifies that an error should be reported if no message is avail‐
131       able, but not if the message is too large. The message size  is  calcu‐
132       lated directly using the sizeof operator.
133
134
135              #include <sys/msg.h>
136              ...
137              int result;
138              int msqid;
139              struct message {
140                  long type;
141                  char text[20];
142              } msg;
143              long msgtyp = 0;
144              ...
145              result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
146                       msgtyp, MSG_NOERROR | IPC_NOWAIT);
147

APPLICATION USAGE

149       The  POSIX Realtime Extension defines alternative interfaces for inter‐
150       process communication (IPC). Application developers who need to use IPC
151       should design their applications so that modules using the IPC routines
152       described in XSI Interprocess Communication can be easily  modified  to
153       use the alternative interfaces.
154

RATIONALE

156       None.
157

FUTURE DIRECTIONS

159       None.
160

COPYRIGHT

168       Portions of this text are reprinted and reproduced in  electronic  form
169       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
170       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
171       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
172       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
173       event of any discrepancy between this version and the original IEEE and
174       The Open Group Standard, the original IEEE and The Open Group  Standard
175       is  the  referee document. The original Standard can be obtained online
176       at http://www.opengroup.org/unix/online.html .
177
178
179
180IEEE/The Open Group                  2003                           MSGRCV(3P)