Manpage of MQ_OPEN
MQ_OPENSection: Linux Programmer's Manual (3)
NAMEmq_open - open a message queue
#include <fcntl.h> /* For O_* constants */ #include <sys/stat.h> /* For mode constants */ #include <mqueue.h>mqd_t mq_open(const char *name, int oflag);mqd_t mq_open(const char *name, int oflag, mode_t mode, struct mq_attr *attr);
DESCRIPTIONmq_open() 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 mq_overview(7).
The oflagargument specifies flags that control the operation of the call. (Definitions of the flags values can be obtained by including <fcntl.h>.) Exactly one of the following must be specified in oflag:
- Open the queue to receive messages only.
- Open the queue to send messages only.
- Open the queue to both send and receive messages.
Zero or more of the following flags can additionally be ORed in oflag:
- O_CLOEXEC (since Linux 2.6.26)
- Set the close-on-exec flag for the message queue descriptor. See open(2) for a discussion of why this flag is useful.
- 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.
- If O_CREATwas specified in oflag, and a queue with the given namealready exists, then fail with the error EEXIST.
- Open the queue in nonblocking mode. In circumstances where mq_receive(3) and mq_send(3) would normally block, these functions instead fail with the error EAGAIN.
If O_CREATis specified in oflag, then two additional arguments must be supplied. The modeargument specifies the permissions to be placed on the new queue, as for open(2). (Symbolic definitions for the permissions bits can be obtained by including <sys/stat.h>.) The permissions settings are masked against the process umask.
The mq_maxmsgand mq_msgsizefields of the struct mq_attrpointed to by the attrargument specify attributes for the queue. See mq_getattr(3) for details. (Note that the mq_flagsfield in that structure is ignored by mq_open(); the O_NONBLOCKflag is instead set via oflag.) If attris NULL, then the queue is created with implementation-defined default attributes. Since Linux 3.5, two /procfiles can be used to control these defaults; see mq_overview(7) for details.
RETURN VALUEOn success, mq_open() returns a message queue descriptor for use by other message queue functions. On error, mq_open() returns (mqd_t) -1,with errnoset to indicate the error.
- The queue exists, but the caller does not have permission to open it in the specified mode.
- namecontained more than one slash.
- Both O_CREATand O_EXCLwere specified in oflag, but a queue with this namealready exists.
- namedoesn't follow the format in mq_overview(7).
- O_CREATwas specified in oflag, and attrwas not NULL, but attr->mq_maxmsgor attr->mq_msqsizewas invalid. Both of these fields must be greater than zero. In a process that is unprivileged (does not have the CAP_SYS_RESOURCEcapability), attr->mq_maxmsgmust be less than or equal to the msg_maxlimit, and attr->mq_msgsizemust be less than or equal to the msgsize_maxlimit. In addition, even in a privileged process, attr->mq_maxmsgcannot exceed the HARD_MAXlimit. (See mq_overview(7) for details of these limits.)
- The per-process limit on the number of open file and message queue descriptors has been reached (see the description of RLIMIT_NOFILEin getrlimit(2)).
- namewas too long.
- The system-wide limit on the total number of open files and message queues has been reached.
- The O_CREATflag was not specified in oflag, and no queue with this nameexists.
- namewas just "/" followed by no other characters.
- Insufficient memory.
- Insufficient space for the creation of a new message queue. This probably occurred because the queues_maxlimit was encountered; see mq_overview(7).
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
CONFORMING TOPOSIX.1-2001, POSIX.1-2008.
C library/kernel differencesThe mq_open() library function is implemented on top of a system call of the same name. The library function performs the check that the namestarts with a slash (/), giving the EINVALerror if it does not. The kernel system call expects nameto contain no preceding slash, so the C library function passes namewithout the preceding slash (i.e., name+1) to the system call.
BUGSIn kernels before 2.6.14, the process umask was not applied to the permissions specified in mode.
SEE ALSOmq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)
- RETURN VALUE
- CONFORMING TO
- SEE ALSO
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