msgsnd(2)
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msgsnd System Call
Send a message.
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SYNTAX
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd (id, user_msg_ptr, size, flag)
int id;
struct msgbuf *user_msg_ptr;
int size;
int flag;
PARAMETERS
id A message queue identifier.
user_msg_ptr The message buffer of the message to be sent.
size The size in bytes of the mtext portion of the
message buffer.
flag A set of flags modifying the action of msgsnd.
DESCRIPTION
Msgsnd sends a message to the queue associated with the message
queue identifier specified by <id>. <user_msg_ptr> points to the
users message buffer containing a message type used for message
selection, and the text of the message. <size> is the length of
the message. It can range from 0 to a configurable system-imposed
maximum.
<flag> specifies the action to be taken if either the number of
bytes already on the queue after this message is added would
exceed the maximum queue size, or the total number of messages on
all queues system-wide is equal to the system-imposed limit. If
either of these conditions hold, the following actions are taken:
* If the IPC_NOWAIT bit of <flag> is set, the message will not
be sent and the calling process will return immediately.
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msgsnd(2)
* If the IPC_NOWAIT bit of <flag> is clear, the calling
process will be suspended until:
* the condition responsible for the suspension no longer
exists, in which case, the operation is successful,
* <id> is removed from the system, in which case, msgsnd will
return with the error condition EIDRM, or
* the calling process receives a signal that is to be caught,
in which case, msgsnd will return with the error condition
EINTR.
If msgsnd fails, no message is sent and the message queue is
unchanged. Upon successful completion, the message queue
attributes are changed as follows:
* The number of messages on the queue (msg_qnum) is
incremented.
* The number of bytes on the queue (msg_cbytes) is increased
by the size of the mtext portion of the message being sent.
* The process id of the last process performing a msgsnd
operation (msg_lspid) is set to the calling process.
* The most recent time a msgsnd operation was performed
(msg_stime) is set to the current time.
ACCESS CONTROL
Write access to the message queue is required.
RETURN VALUE
0 Completed successfully.
-1 An error occurred. Errno is set to indicate the
error.
EXCEPTIONS
Errno may be set to one of the following error codes:
EINVAL <id> is not a valid message queue identifier.
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msgsnd(2)
EINVAL Message type is less than 1.
EINVAL <size> is less than zero or greater than the
system-imposed limit.
EACCES Write permission is denied to the calling process.
EAGAIN The message cannot be sent for some reason and the
IPC_NOWAIT bit of <flag> is set.
EFAULT <user_msg_ptr> is an illegal address.
EIDRM <id> was removed from the system while the calling
process was suspended by msgsnd.
EINTR The calling process received a signal that was set
to be caught while suspended by msgsnd.
SEE ALSO
The related system calls: msgctl, msgget, msgrcv, signal.
The related manual section: intro(2).
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