msgsnd(3)

NAME

msgsnd - send a message to a message queue

LIBRARY

Standard C Library (libc, -lc)

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

The msgsnd() function sends a message to the message queue

specified in
msqid. The 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 se

lecting messages (see msgrcv(3)), 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(3)), or the number of messages on all queues system

wide is
already equal to the system limit, msgflg determines the ac

tion 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:

+o The condition which caused the call to block does no

longer exist.

The message will be sent.

+o The message queue is removed, in which case -1 will be

returned, and

errno is set to EINVAL.

+o 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:

+o msg_cbytes is incremented by the size of the message.

+o msg_qnum is incremented by 1.

+o msg_lspid is set to the pid of the calling process.

+o 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.

ERRORS

The msgsnd() function will fail if:

[EINVAL] The msqid argument is not a valid message

queue iden

tifier

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 less than 0, or

greater than
msg_qbytes.

The mtype argument 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 ei

ther on the

queue, or in the whole system, and

IPC_NOWAIT was set
in msgflg.

[EFAULT] The msgp argument 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.

BUGS

NetBSD and FreeBSD do not define the EIDRM error value,

which should be
used in the case of a removed message queue.

BSD November 24, 1997

BSD