NAME
mqopen - open a message queue SYNOPSIS
#include
/* For O* constants */ #include
/* For mode constants */ #include
mqdt mqopen(const char *name, int oflag); mqdt mqopen(const char *name, int oflag, modet mode, struct mqattr *attr); Link with -lrt. DESCRIPTION mqopen() creates a new POSIX message queue or opens an existing queue. The queue is identified by name. For details of the construction of name, see mqoverview(7). The oflag argument specifies flags that control the operation of the call. (Definitions of the flags values can be obtained by including
.) Exactly one of the following must be specified in oflag: ORDONLY Open the queue to receive messages only. OWRONLY Open the queue to send messages only. ORDWR Open the queue to both send and receive messages. Zero or more of the following flags can additionally be ORed in oflag: ONONBLOCK Open the queue in nonblocking mode. In circumstances where mqreceive(3) and mqsend(3) would normally block, these func‐ tions instead fail with the error EAGAIN. OCREAT 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. OEXCL If OCREAT was specified in oflag, and a queue with the given name already exists, then fail with the error EEXIST. If OCREAT is specified in oflag, then two additional arguments must be supplied. The mode argument specifies the permissions to be placed on the new queue, as for open(2). (Symbolic definitions for the permis‐ sions bits can be obtained by including .) The permissions settings are masked against the process umask. The attr argument spec‐ ifies attributes for the queue. See mqgetattr(3) for details. If attr is NULL, then the queue is created with implementation-defined default attributes. RETURN VALUE On success, mqopen() returns a message queue descriptor for use by
other message queue functions. On error, mqopen() returns (mqdt) -1, with errno set to indicate the error. ERRORS EACCES The queue exists, but the caller does not have permission to open it in the specified mode. EACCES name contained more than one slash. EEXIST Both OCREAT and OEXCL were specified in oflag, but a queue with this name already exists. EINVAL OCREAT was specified in oflag, and attr was not NULL, but
attr->mqmaxmsg or attr->mqmsqsize was invalid. Both of these fields must be greater than zero. In a process that is unprivi‐ leged (does not have the CAPSYSRESOURCE capability),
attr->mqmaxmsg must be less than or equal to the msgmax limit,
and attr->mqmsgsize must be less than or equal to the msg‐ sizemax limit. In addition, even in a privileged process,
attr->mqmaxmsg cannot exceed the HARDMAX limit. (See mqover‐ view(7) for details of these limits.) EMFILE The process already has the maximum number of files and message queues open. ENAMETOOLONG name was too long. ENFILE The system limit on the total number of open files and message queues has been reached. ENOENT The OCREAT flag was not specified in oflag, and no queue with this name exists. ENOENT name was just "/" followed by no other characters. ENOMEM Insufficient memory. ENOSPC Insufficient space for the creation of a new message queue. This probably occurred because the queuesmax limit was encoun‐ tered; see mqoverview(7). ATTRIBUTES For an explanation of the terms used in this section, see attributes(7). ┌──────────┬───────────────┬─────────┐ │Interface │ Attribute │ Value │ ├──────────┼───────────────┼─────────┤
│mqopen() │ Thread safety │ MT-Safe │ └──────────┴───────────────┴─────────┘ CONFORMING TO
POSIX.1-2001. BUGS In kernels before 2.6.14, the process umask was not applied to the per‐ missions specified in mode. SEE ALSO mqclose(3), mqgetattr(3), mqnotify(3), mqreceive(3), mqsend(3), mqunlink(3), mqoverview(7) COLOPHON
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Linux 2009-02-20 MQOPEN(3)