NAME
msgsnd —
send a message to a message
queue
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/msg.h>
int
msgsnd(
int
msqid,
const void
*msgp,
size_t msgsz,
int msgflg);
DESCRIPTION
The
msgsnd() function sends a message from the message queue
specified in
msqid. The
msgp
argument is a pointer to a user-defined structure containing the message. This
structure must contain a first field of type
long that
will indicate the user-defined type of the message. The remaining fields will
contain the contents of the message. The following is an example of what this
user-defined structure might look like:
struct mymsg {
long mtype; /* message type */
char mtext[1]; /* body of message */
};
The
mtype field is an integer greater than 0 that can be
used for selecting messages (see
msgrcv(2)). The
mtext field is an array of bytes of length
msgsz, with size up to the system limit
MSGMAX.
If the number of bytes already on the message queue plus
msgsz is greater than the maximum number of bytes in the
message queue (
msg_qbytes, see
msgctl(2)), or if 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 no longer
exists. The message was sent.
- The message queue is removed, in which case -1 will be
returned and errno 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_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
Upon successful completion, 0 is returned. Otherwise, -1 is returned and
errno is set to indicate the error.
ERRORS
msgsnd() will fail if:
-
-
- [
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_NOWAIT was set in
msgflg.
-
-
- [
EFAULT]
- msgp points to an invalid
address.
-
-
- [
EINTR]
- The system call was interrupted by the delivery of a
signal.
-
-
- [
EINVAL]
- The msqid argument is not a valid
message queue identifier, or the value of mtype is
less than 1.
The message queue was removed while msgsnd() was waiting
for a resource to become available in order to deliver the message.
The msgsz argument is greater than
msg_qbytes or
SSIZE_MAX.
SEE ALSO
msgctl(2),
msgget(2),
msgrcv(2)
STANDARDS
The
msgsnd system call conforms to
X/Open
System Interfaces and Headers Issue 5 (“XSH5”) and
IEEE Std 1003.1-2001 (“POSIX.1”).
HISTORY
Message queues appeared in the first release of
AT&T
System V UNIX.