#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
The
msgsnd ();
function sends a message to the message queue specified in
Fa msqid .
The
Fa msgp
argument
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(3)),
mtext
is an array of
Fa msgsz
bytes.
The argument
Fa msgsz
can range from 0 to a system-imposed maximum,
MSGMAX
If the number of bytes already on the message queue plus
Fa msgsz
is bigger than the maximum number of bytes on the message queue
( msg_qbytes
see
msgctl(3)),
or the number of messages on all queues system-wide is already equal to
the system limit,
Fa msgflg
determines the action of
msgsnd (.);
If
Fa msgflg
has
IPC_NOWAIT
mask set in it, the call will return immediately.
If
Fa 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
Er EINVAL .
The caller catches a signal.
The call returns with
errno
set to
Er 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
Rv -std msgsnd
ERRORS
The
msgsnd ();
function
will fail if:
Bq Er EINVAL
The
Fa msqid
argument
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.
The
Fa msgsz
argument
is greater than
msg_qbytes
The
Fa mtype
argument
is not greater than 0.
Bq Er EACCES
The calling process does not have write access to the message queue.
Bq Er EAGAIN
There was no space for this message either on the queue, or in the whole
system, and
IPC_NOWAIT
was set in
Fa msgflg .
Bq Er EFAULT
The
Fa msgp
argument
points to an invalid address.
Bq Er 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.
BUGS
Nx and
Fx do not define the
Er EIDRM
error value, which should be used
in the case of a removed message queue.
