MSGSND(2) System Calls MSGSND(2)

NAME

msgsnd - message send operation

SYNOPSIS

#include <sys/msg.h>

int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

DESCRIPTION

The msgsnd() function is used to send a message to the queue associated
with the message queue identifier specified by msqid.


The msgp argument points to a user-defined buffer that must contain first
a field of type long int that will specify the type of the message, and
then a data portion that will hold the data bytes of the message. The
structure below is an example of what this user-defined buffer might look
like:

struct mymsg {
long mtype; /* message type */
char mtext[1]; /* message text */
}


The mtype member is a non-zero positive type long int that can be used by
the receiving process for message selection.


The mtext member is any text of length msgsz bytes. The msgsz argument
can range from 0 to a system-imposed maximum.


The msgflg argument specifies the action to be taken if one or more of
the following are true:

o The number of bytes already on the queue is equal to
msg_qbytes. See Intro(2).

o The total number of messages on the queue would exceed the
maximum allowed by the system. See NOTES.


These actions are as follows:

o If (msgflg&IPC_NOWAIT) is non-zero, the message will not be
sent and the calling process will return immediately.

o If (msgflg&IPC_NOWAIT) is 0, the calling process will suspend
execution until one of the following occurs:

o The condition responsible for the suspension no longer
exists, in which case the message is sent.

o The message queue identifier msqid is removed from the
system (see msgctl(2)); when this occurs, errno is set
equal to EIDRM and -1 is returned.

o The calling process receives a signal that is to be
caught; in this case the message is not sent and the
calling process resumes execution in the manner prescribed
in sigaction(2).


Upon successful completion, the following actions are taken with respect
to the data structure associated with msqid (see Intro(2)):

o msg_qnum is incremented by 1.

o msg_lspid is set equal to the process ID of the calling
process.

o msg_stime is set equal to the current time.

RETURN VALUES

Upon successful completion, 0 is returned. Otherwise, -1 is returned, no
message is sent, and errno is set to indicate the error.

ERRORS

The msgsnd() function will fail if:

EACCES
Operation permission is denied to the calling process. See
Intro(2).


EAGAIN
The message cannot be sent for one of the reasons cited above
and (msgflg&IPC_NOWAIT) is non-zero.


EIDRM
The message queue identifier msgid is removed from the system.


EINTR
The msgsnd() function was interrupted by a signal.


EINVAL
The value of msqid is not a valid message queue identifier, or
the value of mtype is less than 1.

The value of msgsz is less than 0 or greater than the system-
imposed limit.


The msgsnd() function may fail if:

EFAULT
The msgp argument points to an illegal address.

USAGE

The value passed as the msgp argument should be converted to type void *.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+

SEE ALSO

Intro(2), msgctl(2), msgget(2), msgrcv(2), setrctl(2), sigaction(2),
attributes(7), standards(7), rctladm(8)

NOTES

The maximum number of messages allowed on a message queue is the minimum
enforced value of the process.max-msg-messages resource control of the
creating process at the time msgget(2) was used to allocate the queue.


See rctladm(8) and setrctl(2) for information about using resource
controls.

February 11, 2003 MSGSND(2)