'\" t .\" Copyright (C) 2006 Michael Kerrisk .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .TH mq_open 3 2024-05-02 "Linux man-pages 6.9.1" .SH NAME mq_open \- open a message queue .SH LIBRARY Real-time library .RI ( librt ", " \-lrt ) .SH SYNOPSIS .nf .BR "#include " " /* For O_* constants */" .BR "#include " " /* For mode constants */" .B #include .P .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 .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). .P 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. .P Zero or more of the following flags can additionally be .IR OR ed in .IR oflag : .TP .BR O_CLOEXEC " (since Linux 2.6.26)" .\" commit 269f21344b23e552c21c9e2d7ca258479dcd7a0a Set the close-on-exec flag for the message queue descriptor. See .BR open (2) for a discussion of why this flag is useful. .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 filesystem 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 . .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 . .P 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. .P The fields of the .I struct mq_attr pointed to .I attr specify the maximum number of messages and the maximum size of messages that the queue will allow. This structure is defined as follows: .P .in +4n .EX struct mq_attr { long mq_flags; /* Flags (ignored for mq_open()) */ long mq_maxmsg; /* Max. # of messages on queue */ long mq_msgsize; /* Max. message size (bytes) */ long mq_curmsgs; /* # of messages currently in queue (ignored for mq_open()) */ }; .EE .in .P Only the .I mq_maxmsg and .I mq_msgsize fields are employed when calling .BR mq_open (); the values in the remaining fields are ignored. .P If .I attr is NULL, then the queue is created with implementation-defined default attributes. Since Linux 3.5, two .I /proc files can be used to control these defaults; see .BR mq_overview (7) for details. .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 .\" glibc checks whether the name starts with a "/" and if not, .\" gives this error .I name doesn't follow the format in .BR mq_overview (7). .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 per-process limit on the number of open file and message queue descriptors has been reached (see the description of .B RLIMIT_NOFILE in .BR getrlimit (2)). .TP .B ENAMETOOLONG .I name was too long. .TP .B ENFILE The system-wide 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 ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .na .nh .BR mq_open () T} Thread safety MT-Safe .TE .SH VERSIONS .SS C library/kernel differences The .BR mq_open () library function is implemented on top of a system call of the same name. The library function performs the check that the .I name starts with a slash (/), giving the .B EINVAL error if it does not. The kernel system call expects .I name to contain no preceding slash, so the C library function passes .I name without the preceding slash (i.e., .IR name+1 ) to the system call. .SH STANDARDS POSIX.1-2008. .SH HISTORY POSIX.1-2001. .SH BUGS Before Linux 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)