July 9, 2009
freebsd-manpages
Manual pages for a GNU/kFreeBSD system
NAME
msgsnd - send a message to a message queue
LIBRARY
.Lb libc
SYNOPSIS
.In sys/types.h
.In sys/ipc.h
.In 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 selecting messages (see msgrcv(2)), mtext is an array of msgsz bytes. The argument msgsz can range from 0 to a system-imposed maximum,
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
.Rv -std msgsnd
ERRORS
The
msgsnd
function will fail if:
[EINVAL ] |
|
The 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 msgsz argument is 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 either 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
.Nx and
.Fx do not define the
EIDRM
error value, which should be used in the case of a removed message queue.