MSGSND(2) BSD Programmer's Manual MSGSND(2)NAMEmsgsnd - send a message to a message queue
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
msgsnd(int msqid, void *msgp, size_t msgsz, int msgflg);
DESCRIPTION
The msgsnd() function sends a message from the message queue specified in
msqid. msgp 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 */
The mtype field is an integer greater than 0 that can be used for select-
ing 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(2)), or the number of messages on all queues system-wide is al-
ready 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:
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 EIN-
TR.
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
Upon successful completion, 0 is returned. Otherwise, -1 is returned and
errno is set to indicate the error.
ERRORSmsgsnd() will fail if:
[EINVAL] msqid 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.
msgsz is less than 0, or greater than msg_qbytes.
[EACCESS] The calling process does not have write access to the mes-
sage 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 sig-
nal.
SEE ALSOmsgctl(2), msgrcv(2), msgget(2)4th Berkeley Distribution July 3, 1996 2
