Originální popis anglicky:
mq_open - open a message queue (
REALTIME)
Návod, kniha: POSIX Programmer's Manual
#include <mqueue.h>
mqd_t mq_open(const char *
name, int
oflag , ...);
The
mq_open() function shall establish the connection between a process
and a message queue with a message queue descriptor. It shall create an open
message queue description that refers to the message queue, and a message
queue descriptor that refers to that open message queue description. The
message queue descriptor is used by other functions to refer to that message
queue. The
name argument points to a string naming a message queue. It
is unspecified whether the name appears in the file system and is visible to
other functions that take pathnames as arguments. The
name argument
shall conform to the construction rules for a pathname. If
name begins
with the slash character, then processes calling
mq_open() with the
same value of
name shall refer to the same message queue object, as
long as that name has not been removed. If
name does not begin with the
slash character, the effect is implementation-defined. The interpretation of
slash characters other than the leading slash character in
name is
implementation-defined. If the
name argument is not the name of an
existing message queue and creation is not requested,
mq_open() shall
fail and return an error.
A message queue descriptor may be implemented using a file descriptor, in which
case applications can open up to at least {OPEN_MAX} file and message queues.
The
oflag argument requests the desired receive and/or send access to the
message queue. The requested access permission to receive messages or send
messages shall be granted if the calling process would be granted read or
write access, respectively, to an equivalently protected file.
The value of
oflag is the bitwise-inclusive OR of values from the
following list. Applications shall specify exactly one of the first three
values (access modes) below in the value of
oflag:
- O_RDONLY
- Open the message queue for receiving messages. The process
can use the returned message queue descriptor with mq_receive(),
but not mq_send(). A message queue may be open multiple times in
the same or different processes for receiving messages.
- O_WRONLY
- Open the queue for sending messages. The process can use
the returned message queue descriptor with mq_send() but not
mq_receive(). A message queue may be open multiple times in the
same or different processes for sending messages.
- O_RDWR
- Open the queue for both receiving and sending messages. The
process can use any of the functions allowed for O_RDONLY and O_WRONLY. A
message queue may be open multiple times in the same or different
processes for sending messages.
Any combination of the remaining flags may be specified in the value of
oflag:
- O_CREAT
- Create a message queue. It requires two additional
arguments: mode, which shall be of type mode_t, and
attr, which shall be a pointer to an mq_attr structure. If
the pathname name has already been used to create a message queue
that still exists, then this flag shall have no effect, except as noted
under O_EXCL. Otherwise, a message queue shall be created without any
messages in it. The user ID of the message queue shall be set to the
effective user ID of the process, and the group ID of the message queue
shall be set to the effective group ID of the process. The file permission
bits shall be set to the value of mode. When bits in mode
other than file permission bits are set, the effect is
implementation-defined. If attr is NULL, the message queue shall be
created with implementation-defined default message queue attributes. If
attr is non-NULL and the calling process has the appropriate
privilege on name, the message queue mq_maxmsg and
mq_msgsize attributes shall be set to the values of the
corresponding members in the mq_attr structure referred to by
attr. If attr is non-NULL, but the calling process does not
have the appropriate privilege on name, the mq_open()
function shall fail and return an error without creating the message
queue.
- O_EXCL
- If O_EXCL and O_CREAT are set, mq_open() shall fail
if the message queue name exists. The check for the existence of
the message queue and the creation of the message queue if it does not
exist shall be atomic with respect to other threads executing
mq_open() naming the same name with O_EXCL and O_CREAT set.
If O_EXCL is set and O_CREAT is not set, the result is undefined.
- O_NONBLOCK
- Determines whether an mq_send() or
mq_receive() waits for resources or messages that are not currently
available, or fails with errno set to [EAGAIN]; see
mq_send() and mq_receive() for details.
The
mq_open() function does not add or remove messages from the queue.
Upon successful completion, the function shall return a message queue
descriptor; otherwise, the function shall return (
mqd_t)-1 and set
errno to indicate the error.
The
mq_open() function shall fail if:
- EACCES
- The message queue exists and the permissions specified by
oflag are denied, or the message queue does not exist and
permission to create the message queue is denied.
- EEXIST
- O_CREAT and O_EXCL are set and the named message queue
already exists.
- EINTR
- The mq_open() function was interrupted by a
signal.
- EINVAL
- The mq_open() function is not supported for the
given name.
- EINVAL
- O_CREAT was specified in oflag, the value of
attr is not NULL, and either mq_maxmsg or mq_msgsize
was less than or equal to zero.
- EMFILE
- Too many message queue descriptors or file descriptors are
currently in use by this process.
- ENAMETOOLONG
- The length of the name argument exceeds {PATH_MAX}
or a pathname component is longer than {NAME_MAX}.
- ENFILE
- Too many message queues are currently open in the
system.
- ENOENT
- O_CREAT is not set and the named message queue does not
exist.
- ENOSPC
- There is insufficient space for the creation of the new
message queue.
The following sections are informative.
None.
None.
None.
None.
mq_close() ,
mq_getattr() ,
mq_receive() ,
mq_send()
,
mq_setattr() ,
mq_timedreceive() ,
mq_timedsend() ,
mq_unlink() ,
msgctl() ,
msgget() ,
msgrcv() ,
msgsnd() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<mqueue.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
