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| .TH MQ_OPEN 3 2017-09-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| mq_open \- open a message queue |
| .SH SYNOPSIS |
| .nf |
| .BR "#include <fcntl.h>" " /* For O_* constants */" |
| .BR "#include <sys/stat.h>" " /* For mode constants */" |
| .B #include <mqueue.h> |
| .PP |
| .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 |
| .PP |
| 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). |
| .PP |
| The |
| .I oflag |
| argument specifies flags that control the operation of the call. |
| (Definitions of the flags values can be obtained by including |
| .IR <fcntl.h> .) |
| 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 |
| .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 . |
| .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 <sys/stat.h> .) |
| The permissions settings are masked against the process umask. |
| .PP |
| The fields of the |
| .IR "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: |
| .PP |
| .PP |
| .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 |
| .PP |
| 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. |
| .PP |
| 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 |
| .BR 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; |
| lb lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR mq_open () |
| T} Thread safety MT-Safe |
| .TE |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| .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 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) |