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mq_open(3) Library Functions Manual mq_open(3) NAME mq_open - Establishes the connection between a message queue and a mes- sage queue descriptor (P1003.1b) SYNOPSIS #include <mqueue.h> mqd_t mq_open ( const char *name, int oflag, ...); PARAMETERS *name A pointer to the name of a string naming the message queue. The string can be a pathname. oflag Specifies the request for receive or send access to the message queue, as well as the blocking attributes of the resulting open descriptor for the calling process. This value of this argument is the bitwise inclusive OR of the values from the following list of flags. An application must specify exactly one of the first three flags: O_RDONLY Open the message queue for receiving messages. This en- ables calls to the mq_receive function but not calls to the mq_send function for the calling process. Note that each process has its own read/write access to a queue. O_WRONLY Open the queue for sending messages. This enables calls to the mq_send function but not calls to the mq_receive function for the calling process. O_RDWR Open the queue for receiving and sending messages for the calling process. Any combination of the following flags is valid: O_CREAT Create a message queue. Use of this flag requires spec- ification of two additional arguments: mode, which is of type mode_t, and attr, which points to a mq_attr structure. If name has already been used to create a message queue that is still open, this flag has no ef- fect, except as noted under O_EXCL. Otherwise, a mes- sage queue is created. The ID of the message queue be- comes the effective user ID, and the group ID of the message queue becomes the effective group ID of the process. The mode argument sets the file permission bits. If the attr argument is NULL, the function cre- ates the message queue with default attributes. If the attr argument is not NULL, and the calling process has appropriate privilege or permission, the message queue attributes are set as if the mq_setattr function were called with the attribute argument attr. If the attr argument is not NULL, but the calling process does not have the appropriate privilege or permission the mq_open argument fails and returns an error.) O_EXCL Prevent the opening of a message queue if O_CREAT is set and the message queue already exists. Use this flag only in combination with O_CREAT. O_NONBLOCK Allows calls to the mq_send and mq_receive functions to continue execution while waiting for resources or mes- sages. When this flag is set and resources are not available, the mq_send and mq_receive functions fail and errno is set to EAGAIN. This flag is a per-open de- scriptor flag, and applies to the calling process only. Each process has its own blocking attributes for a queue. DESCRIPTION Establishes the connection between a message queue and a message queue descriptor. This function creates a new open message queue description that refers to a specified message queue and a message queue descriptor that refers to that open message queue description. The message queue descriptor is then available for use by other functions for reference to the message queue from the calling process. RETURN VALUES On successful completion this function returns a message queue descrip- tor. Otherwise, the function returns -1 and sets errno to indicate the er- ror. ERRORS The mq_open function fails under the following conditions: [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 and O_EXCL are set, and the named message queue already exists. [EFAULT] Internal virtual memory error. [EINTR] The mq_open operation was interrupted by a signal. [EINVAL] The mq_open operation is not supported for the given name. O_CREAT was specified in oflag, the value of attr is not NULL, 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 string exceeds {PATH_MAX}, or a pathname component is longer than {NAME_MAX} while {_POSIX_NO_TRUNC} is in effect. [ENFILE] Too many message queues are currently open in the sys- tem. [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. RELATED INFORMATION Functions: mq_close(3), mq_getattr(3), mq_receive(3), mq_send(3), mq_setattr(3), mq_unlink(3) delim off mq_open(3)