операция отправки сообщения XSI (XSI message send operation)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
msgsnd — XSI message send operation
Синопсис (Synopsis)
#include <sys/msg.h>
int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
Описание (Description)
The msgsnd() function operates on XSI message queues (see the
Base Definitions volume of POSIX.1‐2017, Section 3.226, Message
Queue). It is unspecified whether this function interoperates
with the realtime interprocess communication facilities defined
in Section 2.8, Realtime.
The msgsnd() function shall send a message to the queue
associated with the message queue identifier specified by msqid.
The application shall ensure that the argument msgp points to a
user-defined buffer that contains first a field of type long
specifying the type of the message, and then a data portion that
holds 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 structure member mtype is a non-zero positive type long that
can be used by the receiving process for message selection.
The structure member mtext is any text of length msgsz bytes. The
argument msgsz can range from 0 to a system-imposed maximum.
The argument msgflg specifies the action to be taken if one or
more of the following is true:
* The number of bytes already on the queue is equal to
msg_qbytes; see <sys/msg.h>.
* The total number of messages on all queues system-wide is
equal to the system-imposed limit.
These actions are as follows:
* If (msgflg & IPC_NOWAIT) is non-zero, the message shall not
be sent and the calling thread shall return immediately.
* If (msgflg & IPC_NOWAIT) is 0, the calling thread shall
suspend execution until one of the following occurs:
-- The condition responsible for the suspension no longer
exists, in which case the message is sent.
-- The message queue identifier msqid is removed from the
system; when this occurs, errno shall be set to [EIDRM]
and -1 shall be returned.
-- The calling thread receives a signal that is to be
caught; in this case the message is not sent and the
calling thread resumes execution in the manner prescribed
in sigaction(3p).
Upon successful completion, the following actions are taken with
respect to the data structure associated with msqid; see
<sys/msg.h>:
* msg_qnum shall be incremented by 1.
* msg_lspid shall be set to the process ID of the calling
process.
* msg_stime shall be set to the current time, as described in
Section 2.7.1, IPC General Description.
Возвращаемое значение (Return value)
Upon successful completion, msgsnd() shall return 0; otherwise,
no message shall be sent, msgsnd() shall return -1, and errno
shall be set to indicate the error.
Ошибки (Error)
The msgsnd() function shall fail if:
EACCES Operation permission is denied to the calling process; see
Section 2.7, XSI Interprocess Communication.
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 msqid 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; or the
value of msgsz is greater than the system-imposed limit.
The following sections are informative.
Примеры (Examples)
Sending a Message
The following example sends a message to the queue identified by
the msqid argument (assuming that value has previously been set).
This call specifies that an error should be reported if no
message is available. The message size is calculated directly
using the sizeof operator.
#include <sys/msg.h>
...
int result;
int msqid;
struct message {
long type;
char text[20];
} msg;
msg.type = 1;
strcpy(msg.text, "This is message 1");
...
result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), IPC_NOWAIT);
Использование в приложениях (Application usage)
The POSIX Realtime Extension defines alternative interfaces for
interprocess communication (IPC). Application developers who need
to use IPC should design their applications so that modules using
the IPC routines described in Section 2.7, XSI Interprocess
Communication can be easily modified to use the alternative
interfaces.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
Section 2.7, XSI Interprocess Communication, Section 2.8,
Realtime, mq_close(3p), mq_getattr(3p), mq_notify(3p),
mq_open(3p), mq_receive(3p), mq_send(3p), mq_setattr(3p),
mq_unlink(3p), msgctl(3p), msgget(3p), msgrcv(3p), sigaction(3p)
The Base Definitions volume of POSIX.1‐2017, Section 3.226,
Message Queue, sys_msg.h(0p)
