'\" t .\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" Copyright (C) 2006 Michael Kerrisk .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .TH MQ_OPEN 3 2009-02-20 "Linux" "Linux Programmer's Manual" .SH NAME mq_open \- open a message queue .SH SYNOPSIS .nf .BR "#include " " /* For O_* constants */" .BR "#include " " /* For mode constants */" .B #include .sp .BI "mqd_t mq_open(const char *" name ", int " oflag ); .BI "mqd_t mq_open(const char *" name ", int " oflag ", mode_t " mode , .BI " struct mq_attr *" attr ); .fi .sp Link with \fI\-lrt\fP. .SH DESCRIPTION .BR mq_open () creates a new POSIX message queue or opens an existing queue. The queue is identified by .IR name . For details of the construction of .IR name , see .BR mq_overview (7). The .I oflag argument specifies flags that control the operation of the call. (Definitions of the flags values can be obtained by including .IR .) Exactly one of the following must be specified in .IR oflag : .TP .B O_RDONLY Open the queue to receive messages only. .TP .B O_WRONLY Open the queue to send messages only. .TP .B O_RDWR Open the queue to both send and receive messages. .PP Zero or more of the following flags can additionally be .IR OR ed in .IR oflag : .TP .B O_NONBLOCK Open the queue in nonblocking mode. In circumstances where .BR mq_receive (3) and .BR mq_send (3) would normally block, these functions instead fail with the error .BR EAGAIN . .TP .B O_CREAT Create the message queue if it does not exist. The owner (user ID) of the message queue is set to the effective user ID of the calling process. The group ownership (group ID) is set to the effective group ID of the calling process. .\" In reality the file system IDs are used on Linux. .TP .B O_EXCL If .B O_CREAT was specified in .IR oflag , and a queue with the given .I name already exists, then fail with the error .BR EEXIST . .PP If .B O_CREAT is specified in .IR oflag , then two additional arguments must be supplied. The .I mode argument specifies the permissions to be placed on the new queue, as for .BR open (2). (Symbolic definitions for the permissions bits can be obtained by including .IR .) The permissions settings are masked against the process umask. The .I attr argument specifies attributes for the queue. See .BR mq_getattr (3) for details. If .I attr is NULL, then the queue is created with implementation-defined default attributes. .SH RETURN VALUE On success, .BR mq_open () returns a message queue descriptor for use by other message queue functions. On error, .BR mq_open () returns .IR "(mqd_t)\ \-1", with .I errno set to indicate the error. .SH ERRORS .TP .B EACCES The queue exists, but the caller does not have permission to open it in the specified mode. .TP .B EACCES .I name contained more than one slash. .\" Note that this isn't consistent with the same case for sem_open() .TP .B EEXIST Both .B O_CREAT and .B O_EXCL were specified in .IR oflag , but a queue with this .I name already exists. .TP .B EINVAL .B O_CREAT was specified in .IR oflag , and .I attr was not NULL, but .I attr\->mq_maxmsg or .I attr\->mq_msqsize was invalid. Both of these fields must be greater than zero. In a process that is unprivileged (does not have the .B CAP_SYS_RESOURCE capability), .I attr\->mq_maxmsg must be less than or equal to the .I msg_max limit, and .I attr\->mq_msgsize must be less than or equal to the .I msgsize_max limit. In addition, even in a privileged process, .I attr\->mq_maxmsg cannot exceed the .B HARD_MAX limit. (See .BR mq_overview (7) for details of these limits.) .TP .B EMFILE The process already has the maximum number of files and message queues open. .TP .B ENAMETOOLONG .I name was too long. .TP .B ENFILE The system limit on the total number of open files and message queues has been reached. .TP .B ENOENT The .B O_CREAT flag was not specified in .IR oflag , and no queue with this .I name exists. .TP .B ENOENT .I name was just "/" followed by no other characters. .\" Note that this isn't consistent with the same case for sem_open() .TP .B ENOMEM Insufficient memory. .TP .B ENOSPC Insufficient space for the creation of a new message queue. This probably occurred because the .I queues_max limit was encountered; see .BR mq_overview (7). .SH CONFORMING TO POSIX.1-2001. .SH BUGS In kernels before 2.6.14, the process umask was not applied to the permissions specified in .IR mode . .SH "SEE ALSO" .BR mq_close (3), .BR mq_getattr (3), .BR mq_notify (3), .BR mq_receive (3), .BR mq_send (3), .BR mq_unlink (3), .BR mq_overview (7) .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.