NAME [Toc] [Back]
mq_open - create/open a message queue
SYNOPSIS [Toc] [Back]
mqd_t mq_open(const char *name,
int oflag, ...
* [mode_t mode, struct mq_attr]
Remarks [Toc] [Back]
The ANSI C ", ..." construct specifies a variable length argument list
whose optional members are given in the associated comment (/* */).
DESCRIPTION [Toc] [Back]
The mq_open() system call establishes a connection between a process
and a message queue. It returns a new message queue descriptor which
is used by other message queue system calls to refer to that queue.
The name argument points to the message queue name, and must conform
to the rules listed in Message Queue Naming Convention.
The oflag argument is the bitwise inclusive OR of the flags listed in
Read-Write Flags, and General Flags below.
The new message queue descriptor returned, remains open across the
fork() system call and is inherited by the child process.
Read-Write Flags [Toc] [Back]
The value of oflag must be composed by taking the inclusive OR of
exactly one of the following flags:
O_RDONLY Open for receiving only.
O_WRONLY Open for sending only.
O_RDWR Open for sending and receiving.
General Flags [Toc] [Back]
Any combination of the following flags may also be used in setting the
value of oflag.
O_CREAT This flag must be used to create a message queue,
and it uses two additional arguments: mode which
is of type mode_t, and attr which is a pointer to
a mq_attr structure. If a message queue with name,
name, exists, this flag has no effect, except as
noted under O_EXCL. Otherwise a new message queue
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is created. The user ID of the queue will be set
to the effective user ID of the process, and the
group ID of the queue will be set to the effective
group ID of the process. The "file permission
bits" will be set to the value of mode. If attr
is NULL, the message queue is created with default
attributes - MQ_MAXMSG and MQ_MSGSIZE (defined in
sys/mqueue.h) If attr is non-NULL and the message
queue mq_maxmsg and mq_msgsize attributes are set
to the values of the corresponding members in the
mq_attr structure referred to by attr.
O_EXCL If O_EXCL and O_CREAT are set in oflag and the
named message queue exists, mq_open() will fail.
The O_CREAT flag is ignored if O_CREAT is not set
O_NONBLOCK This flag is used to specify the blocking status
of the message queue descriptor and determines
whether a mq_send() or a mq_receive() will wait
for resources or messages respectively, that are
not currently available, or fail with errno set to
Message Queue Naming Convention [Toc] [Back]
A valid message queue name string, must conform to pathname
construction rules. In addition it must also meet the following
a. Begin with a slash character.
b. Contain no pathname component consisting of a dot or dotdot;
e.g., /./tmp and /../tmp.
c. Contain no illegal characters.
d. Contain no pathname components longer that _POSIX_NAME_MAX.
e. Entire name should not be longer that _POSIX_PATH_MAX.
To use this function, link in the realtime library by specifying -lrt
on the compiler or linker command line.
RETURN VALUE [Toc] [Back]
mq_open() returns the following values:
n Successful completion. n is a message queue descriptor for
the opened message queue and is greater than or equal to 0.
-1 Failure. errno is set to indicate the error.
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ERRORS [Toc] [Back]
If mq_open() fails, errno is set to one of the following values:
[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
queue is denied.
[EEXIST] The O_CREAT and O_EXCL flags are set in oflag and
the named message queue exists.
[EINTR] mq_open() was interrupted by a signal.
[EINVAL] The argument name, does not conform to the Message
Queue Naming Convention.
O_CREAT has been specified in oflag, the value of
attr is not NULL, and either mq_maxmsg or
mq_msgsize is less than or equal to zero.
[EMFILE] Too many message queue descriptors are currently
in use by this process.
[ENAMETOOLONG] The length of the name string exceeds PATH_MAX
bytes, or the length of a (pathname) component of
the name string exceeds NAME_MAX bytes while
_POSIX_NO_TRUNC is in effect.
[ENFILE] Too many message queues are currently open in the
[ENOENT] The O_CREAT flag is not set in oflag and the named
message queue does not exist.
[ENOSPC] There are insufficient resources for the creation
of the new message queue.
[ENOSYS] mq_open() is not supported by the implementation.
SEE ALSO [Toc] [Back]
mq_close(2), mq_unlink(2), mq_send(2), mq_receive(2), mq_setattr(2),
STANDARDS CONFORMANCE [Toc] [Back]
mq_open(): POSIX 1003.1b
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