MQ_OPEN(2) | System Calls Manual | MQ_OPEN(2) |
NAME¶
mq_open
—
LIBRARY¶
POSIX Real-time Library (librt, -lrt)SYNOPSIS¶
#include <mqueue.h>
mqd_t
mq_open
(const
char *name, int
oflag, ...);
DESCRIPTION¶
Themq_open
() system call establishes the connection
between a process and a message queue with a message queue descriptor. It
creates 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. The name argument should
conform to the construction rules for a pathname. The
name should begin with a slash character. Processes
calling mq_open
() with the same value of
name refers to the same message queue object, as long as
that name has not been removed. If the name argument is
not the name of an existing message queue and creation is not requested,
mq_open
() will fail and return an error.
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 would 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 should 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 notmq_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 notmq_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
andO_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 is of type mode_t,
and attr, which is 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 has no effect, except as noted under
O_EXCL
. Otherwise, a message queue will be created without any messages in it. The user ID of the message queue will be set to the effective user ID of the process, and the group ID of the message queue will be set to the effective group ID of the process. The permission bits of the message queue will be set to the value of the mode argument, except those set in the file mode creation mask of the process. When bits in mode other than the file permission bits are specified, the effect is unspecified. If attr isNULL
, the message queue is 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 will 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, themq_open
() function will fail and return an error without creating the message queue. O_EXCL
- If
O_EXCL
andO_CREAT
are set,mq_open
() will fail if the message queue name exists. O_NONBLOCK
- Determines whether an
mq_send
() ormq_receive
() waits for resources or messages that are not currently available, or fails with errno set toEAGAIN
; see mq_send(2) and mq_receive(2) for details.
The mq_open
() system call does not add or
remove messages from the queue.
NOTES¶
FreeBSD implements message queue based on file descriptor. The descriptor is inherited by child after fork(2). The descriptor is closed in a new image after exec(3). The select(2) and kevent(2) system calls are supported for message queue descriptor.Please see the mqueuefs(5) man page for instructions on loading the module or compiling the service into the kernel.
RETURN VALUES¶
Upon successful completion, the function returns a message queue descriptor; otherwise, the function returns (mqd_t)-1 and sets the global variable errno to indicate the error.ERRORS¶
Themq_open
() system call will 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
andO_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 notNULL
, 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.
SEE ALSO¶
mq_close(2), mq_getattr(2), mq_receive(2), mq_send(2), mq_setattr(2), mq_timedreceive(3), mq_timedsend(3), mq_unlink(3), mqueuefs(5)STANDARDS¶
Themq_open
() system call conforms to
IEEE Std 1003.1-2004 (“POSIX.1”).
HISTORY¶
Support for POSIX message queues first appeared in FreeBSD 7.0.BUGS¶
This implementation places strict requirements on the value of name: it must begin with a slash (‘/
’) and contain no other slash
characters.
COPYRIGHT¶
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2004 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.September 15, 2014 | Linux 4.9.0-9-amd64 |