msgsnd - Send a message to a message queue
#include <sys/msg.h>
int msgsnd(
int msqid,
const void *msgp,
size_t msgsz,
int msgflg );
Application developers may want to specify #include statements
for <sys/types.h> and <sys/ipc.h> before the one for
<sys/msg.h> if programs are being developed for multiple
platforms. The additional #include statements are not
required on Tru64 UNIX systems or by ISO or XSH standards,
but may be required on other vendors' systems that conform
to these standards.
Interfaces documented on this reference page conform to
industry standards as follows:
msgsnd(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
Specifies the ID of the message queue on which to place
the message. The ID is typically returned by a previous
msgget() function. Specifies a pointer to the msgbuf
structure that contains the message. Specifies the size
of the data array in the msgbuf structure. Specifies the
action to be taken by the kernel if it runs out of internal
buffer space.
The msgsnd() function sends a message to the queue associated
with the msqid parameter. When the kernel sends a
message, it allocates space for the message and copies the
data from user space. The kernel then allocates a structure
of type msg (message header), sets its fields, and
inserts the structure at the end of the message queue
associated with the message queue ID. The msg structure is
defined as follows:
struct msg {
struct msg *msg_next;
long msg_type;
long msg_ts;
caddr_t msg_addr; };
The msg_next field is a pointer to the next message in the
queue. The msg_type field is the message type supplied in
the user-specified msgbuf structure. The msg_ts field is
the size of the message text. The msg_addr field is the
address of the message text.
You pass the message that the kernel adds to the message
queue in the msgp parameter. This parameter points to a
msgbuf structure that you define as follows:
struct msgbuf {
long int mtype;
char mtext[]; };
The mtype field is a positive integer that indicates the
type of the message. A receiving process can use this message
type to select only those messages it wants to
receive from the queue. For more information about the
values you can specify in this field, see msgrcv(2).
The mtext field contains the text of the message. You
specify the size in bytes of the message in the msgsz
parameter. You can specify a value between 0 (zero) and,
by default, 8192 bytes. Note that the maximum number of
bytes allowed in a message string is configured at system
configuration time using the msgmax configuration file
keyword, so the limit might be different for your system.
The msgflg parameter specifies the action that the kernel
should take if either or both of the following are true:
The current number of bytes in the message queue is equal
to msg_qbytes (in the msqid_ds structure). The total number
of messages on all message queues is equal to the
limit defined by the msgtql configuration file keyword.
By default the limit is 40, but it might have been changed
for your system at system configuration time.
One of two kernel actions can be specified, as follows: If
IPC_NOWAIT is set, the kernel does not send the message
and returns to the calling process immediately. If
IPC_NOWAIT is not set, the kernel suspends the calling
process. The process remains suspended until one of the
following occurs: The blocking condition is removed. In
this case, the kernel sends the message. The specified
message queue ID is removed from the system. In this
case, the kernel sets errno to [EIDRM] and returns -1 to
the calling process. The calling process catches a signal.
In this case, the message is not sent and the process
resumes execution as directed by the signaction() function.
If the msgsnd() function completes successfully, the kernel
updates the msqid_ds structure associated with the
msgid parameter. Specifically, it: Increments msg_qnum by
1. [Tru64 UNIX] Increments msg_cbytes by the message
text size. Sets msg_lspid equal to the process ID of the
calling process. Sets msg_stime equal to the current
time.
Upon successful completion, the msgrcv() function returns
a value of 0 (zero). Otherwise, the function returns a
value of -1 and sets errno to indicate the error.
The msgsnd() function sets errno to the specified values
for the following conditions: The calling process does not
have permission for the operation. The IPC_NOWAIT flag is
set, and either the maximum number of message headers has
been allocated or the message queue is full. [Tru64
UNIX] Indicates that the requested address is in some way
invalid, for example, out of bounds. The message queue
identified by the msqid parameter has been removed from
the system. The operation was interrupted by a signal.
The msqid parameter is not a valid message queue ID, mtype
is less than 1, or msgsz is less than 0 (zero) or greater
than the limit defined by the msgmax configuration file
keyword. By default the limit is 8192 bytes, but it might
have been changed for your system at system configuration
time.
Functions: msgctl(2), msgget(2), msgrcv(2), sigaction(2)
Data Structures: msqid_ds(4)
Standards: standards(5)
msgsnd(2)
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