NAME
msgsnd —
send a message to a message
queue
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
msgsnd(int
msqid, const void
*msgp, size_t
msgsz, int
msgflg);
DESCRIPTION
Themsgsnd()
system call sends a message to the message queue specified in
msqid. msgp points to a structure
containing the message. This structure should consist of the following
members:
long mtype; /* message type */
char mtext[1]; /* body of message */
mtype is an integer greater than 0 that can
be used for selecting messages (see
msgrcv(2)), mtext is an array of bytes, with a
size up to that of the system limit (MSGMAX).
If the number of bytes already on the message queue
plus msgsz is bigger than the maximum number of bytes
on the message queue (msg_qbytes, see
msgctl(2)), or the number of messages on all queues system-wide is
already equal to the system limit, msgflg determines
the action of
msgsnd().
If msgflg has IPC_NOWAIT mask
set in it, the call will return immediately. If msgflg
does not have IPC_NOWAIT set in it, the call will
block until:
- The condition which caused the call to block does no longer exist. The message will be sent.
- The message queue is removed, in which case -1 will be returned, and
errno is set to
EINVAL. - The caller catches a signal. The call returns with
errno set to
EINTR.
After a successful call, the data structure associated with the message queue is updated in the following way:
- msg_cbytes is incremented by the size of the message.
- msg_qnum is incremented by 1.
- msg_lspid is set to the pid of the calling process.
- msg_stime is set to the current time.
RETURN VALUES
The msgsnd() function returns the
value 0 if successful; otherwise the value -1 is returned and
the global variable errno is set to indicate the
error.
ENVIRONMENT
The XSI Interprocess Communication family of functions is also available as an implementation in userspace. To use it, the sysvipcd(8) daemon has to be running.
If the USR_SYSVIPC variable is set in a
process' environment, the process and its children will use the userspace
implementation.
ERRORS
msgsnd() will fail if:
- [
EINVAL] - msqid is not a valid message queue identifier
The message queue was removed while
msgsnd() was waiting for a resource to become available in order to deliver the message.msgsz is less than 0, or greater than msg_qbytes.
mtype is not greater than 0.
- [
EACCES] - The calling process does not have write access to the message queue.
- [
EAGAIN] - There was no space for this message either on the queue, or in the whole
system, and
IPC_NOWAITwas set in msgflg. - [
EFAULT] - msgp points to an invalid address.
- [
EINTR] - The system call was interrupted by the delivery of a signal.
HISTORY
Message queues appeared in the first release of AT&T Unix System V.
AUTHORS
The DragonFly specific userspace implementation (see ENVIRONMENT) was written by Larisa Grigore.
BUGS
NetBSD, DragonFly,
and FreeBSD do not define the
EIDRM error value, which should be used in the case
of a removed message queue.