1MSGOP(2)                   Linux Programmer's Manual                  MSGOP(2)
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NAME

6       msgrcv, msgsnd - message operations
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SYNOPSIS

9       #include <sys/types.h>
10       #include <sys/ipc.h>
11       #include <sys/msg.h>
12
13       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
14
15       ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
16                      int msgflg);
17

DESCRIPTION

19       The  msgsnd() and msgrcv() system calls are used, respectively, to send
20       messages to, and receive messages from, a message queue.   The  calling
21       process  must  have  write  permission on the message queue in order to
22       send a message, and read permission to receive a message.
23
24       The msgp argument is a pointer to caller-defined structure of the  fol‐
25       lowing general form:
26
27           struct msgbuf {
28               long mtype;       /* message type, must be > 0 */
29               char mtext[1];    /* message data */
30           };
31
32       The  mtext  field is an array (or other structure) whose size is speci‐
33       fied by msgsz, a nonnegative integer value.  Messages  of  zero  length
34       (i.e.,  no  mtext  field)  are  permitted.  The mtype field must have a
35       strictly positive integer value.  This value can be used by the receiv‐
36       ing  process  for  message  selection  (see the description of msgrcv()
37       below).
38
39   msgsnd()
40       The msgsnd() system call appends a copy of the message  pointed  to  by
41       msgp to the message queue whose identifier is specified by msqid.
42
43       If  sufficient space is available in the queue, msgsnd() succeeds imme‐
44       diately.  (The queue capacity is defined by the msg_qbytes field in the
45       associated data structure for the message queue.  During queue creation
46       this field is initialized to MSGMNB bytes, but this limit can be  modi‐
47       fied  using  msgctl(2).)   If  insufficient  space  is available in the
48       queue, then the default behavior of msgsnd() is to  block  until  space
49       becomes available.  If IPC_NOWAIT is specified in msgflg, then the call
50       instead fails with the error EAGAIN.
51
52       A blocked msgsnd() call may also fail if:
53
54       * the queue is removed, in which case the system call fails with  errno
55         set to EIDRM; or
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57       * a  signal  is  caught, in which case the system call fails with errno
58         set  to  EINTR;see  signal(7).   (msgsnd()  is  never   automatically
59         restarted  after being interrupted by a signal handler, regardless of
60         the setting of the SA_RESTART flag when establishing  a  signal  han‐
61         dler.)
62
63       Upon  successful completion the message queue data structure is updated
64       as follows:
65
66              msg_lspid is set to the process ID of the calling process.
67
68              msg_qnum is incremented by 1.
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70              msg_stime is set to the current time.
71
72   msgrcv()
73       The msgrcv() system call removes a message from the queue specified  by
74       msqid and places it in the buffer pointed to by msgp.
75
76       The  argument  msgsz specifies the maximum size in bytes for the member
77       mtext of the structure pointed to by the msgp argument.  If the message
78       text  has  length  greater  than  msgsz,  then  the behavior depends on
79       whether MSG_NOERROR is specified in msgflg.  If MSG_NOERROR  is  speci‐
80       fied,  then  the message text will be truncated (and the truncated part
81       will be lost); if MSG_NOERROR is not specified, then the message  isn't
82       removed  from  the  queue  and  the system call fails returning -1 with
83       errno set to E2BIG.
84
85       The argument msgtyp specifies the type of message requested as follows:
86
87       * If msgtyp is 0, then the first message in the queue is read.
88
89       * If msgtyp is greater than 0, then the first message in the  queue  of
90         type  msgtyp  is  read, unless MSG_EXCEPT was specified in msgflg, in
91         which case the first message in the queue of type not equal to msgtyp
92         will be read.
93
94       * If  msgtyp  is  less than 0, then the first message in the queue with
95         the lowest type less than or equal to the absolute  value  of  msgtyp
96         will be read.
97
98       The msgflg argument is a bit mask constructed by ORing together zero or
99       more of the following flags:
100
101       IPC_NOWAIT
102              Return immediately if no message of the requested type is in the
103              queue.  The system call fails with errno set to ENOMSG.
104
105       MSG_EXCEPT
106              Used with msgtyp greater than 0 to read the first message in the
107              queue with message type that differs from msgtyp.
108
109       MSG_NOERROR
110              To truncate the message text if longer than msgsz bytes.
111
112       If no message of the requested type is available and  IPC_NOWAIT  isn't
113       specified  in  msgflg,  the calling process is blocked until one of the
114       following conditions occurs:
115
116       * A message of the desired type is placed in the queue.
117
118       * The message queue is removed from the system.  In this case the  sys‐
119         tem call fails with errno set to EIDRM.
120
121       * The  calling  process catches a signal.  In this case the system call
122         fails with errno set to  EINTR.   (msgrcv()  is  never  automatically
123         restarted  after being interrupted by a signal handler, regardless of
124         the setting of the SA_RESTART flag when establishing  a  signal  han‐
125         dler.)
126
127       Upon  successful completion the message queue data structure is updated
128       as follows:
129
130              msg_lrpid is set to the process ID of the calling process.
131
132              msg_qnum is decremented by 1.
133
134              msg_rtime is set to the current time.
135

RETURN VALUE

137       On failure both functions return -1 with errno  indicating  the  error,
138       otherwise  msgsnd()  returns 0 and msgrcv() returns the number of bytes
139       actually copied into the mtext array.
140

ERRORS

142       When msgsnd() fails, errno will be set to one among the following  val‐
143       ues:
144
145       EACCES The  calling  process does not have write permission on the mes‐
146              sage queue, and does not have the CAP_IPC_OWNER capability.
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148       EAGAIN The message can't be sent due to the msg_qbytes  limit  for  the
149              queue and IPC_NOWAIT was specified in msgflg.
150
151       EFAULT The address pointed to by msgp isn't accessible.
152
153       EIDRM  The message queue was removed.
154
155       EINTR  Sleeping on a full message queue condition, the process caught a
156              signal.
157
158       EINVAL Invalid msqid value, or  nonpositive  mtype  value,  or  invalid
159              msgsz  value  (less than 0 or greater than the system value MSG‐
160              MAX).
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162       ENOMEM The system does not have enough memory to make  a  copy  of  the
163              message pointed to by msgp.
164
165       When  msgrcv() fails, errno will be set to one among the following val‐
166       ues:
167
168       E2BIG  The message text length is greater than  msgsz  and  MSG_NOERROR
169              isn't specified in msgflg.
170
171       EACCES The calling process does not have read permission on the message
172              queue, and does not have the CAP_IPC_OWNER capability.
173
174       EAGAIN No message was available in the queue and IPC_NOWAIT was  speci‐
175              fied in msgflg.
176
177       EFAULT The address pointed to by msgp isn't accessible.
178
179       EIDRM  While the process was sleeping to receive a message, the message
180              queue was removed.
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182       EINTR  While the process was sleeping to receive a message, the process
183              caught a signal; see signal(7).
184
185       EINVAL msgqid was invalid, or msgsz was less than 0.
186
187       ENOMSG IPC_NOWAIT  was  specified  in  msgflg  and  no  message  of the
188              requested type existed on the message queue.
189

CONFORMING TO

191       SVr4, POSIX.1-2001.
192

NOTES

194       The msgp argument is declared as struct msgbuf  *  with  libc4,  libc5,
195       glibc  2.0,  glibc  2.1.   It  is declared as void * with glibc 2.2 and
196       later, as required by SUSv2 and SUSv3.
197
198       The following limits on message queue  resources  affect  the  msgsnd()
199       call:
200
201       MSGMAX Maximum  size  for  a  message  text: 8192 bytes (on Linux, this
202              limit can be read and modified via /proc/sys/kernel/msgmax).
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204       MSGMNB Default maximum size in bytes of a message  queue:  16384  bytes
205              (on   Linux,   this   limit   can   be  read  and  modified  via
206              /proc/sys/kernel/msgmnb).  The superuser can increase  the  size
207              of a message queue beyond MSGMNB by a msgctl(2) system call.
208
209       The  implementation has no intrinsic limits for the system wide maximum
210       number of message headers (MSGTQL) and for the system wide maximum size
211       in bytes of the message pool (MSGPOOL).
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SEE ALSO

214       msgctl(2), msgget(2), capabilities(7), mq_overview(7), svipc(7)
215

COLOPHON

217       This  page  is  part of release 3.25 of the Linux man-pages project.  A
218       description of the project, and information about reporting  bugs,  can
219       be found at http://www.kernel.org/doc/man-pages/.
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223Linux                             2008-04-23                          MSGOP(2)
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