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)
