NAME
msgsnd —
send a message to a message
queue
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/msg.h>
int
msgsnd(int
msqid, const void
*msgp, size_t
msgsz, int
msgflg);
DESCRIPTION
Themsgsnd()
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_NOWAITwas 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
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.