Name
mq_getattr, mq_setattr - get/set message queue attributesLibrary
Real-time library ( librt ", " -lrt )Synopsis
#include <mqueue.h> int mq_getattr(mqd_t " mqdes ", struct mq_attr * attr );
int mq_setattr(mqd_t " mqdes ", const struct mq_attr *restrict newattr ,
struct mq_attr *restrict oldattr );
Description
mq_getattr() and mq_setattr() respectively retrieve and modify attributes of the message queue referred to by the message queue descriptormqdes
. mq_getattr() returns an mq_attr structure in the buffer pointed by attr
. This structure is defined as:
struct mq_attr { long mq_flags; /* Flags: 0 or O_NONBLOCK */ long mq_maxmsg; /* Max. # of messages on queue */ long mq_msgsize; /* Max. message size (bytes) */ long mq_curmsgs; /* # of messages currently in queue */ };
The mq_flags field contains flags associated with the open message queue description. This field is initialized when the queue is created by mq_open(3). The only flag that can appear in this field is O_NONBLOCK .
The mq_maxmsg and mq_msgsize fields are set when the message queue is created by mq_open(3). The mq_maxmsg field is an upper limit on the number of messages that may be placed on the queue using mq_send(3). The mq_msgsize field is an upper limit on the size of messages that may be placed on the queue. Both of these fields must have a value greater than zero. Two /proc files that place ceilings on the values for these fields are described in mq_overview(7).
The mq_curmsgs field returns the number of messages currently held in the queue.
mq_setattr() sets message queue attributes using information supplied in the mq_attr structure pointed to by newattr
. The only attribute that can be modified is the setting of the O_NONBLOCK flag in mq_flags
. The other fields in newattr are ignored. If the oldattr field is not NULL, then the buffer that it points to is used to return an mq_attr structure that contains the same information that is returned by mq_getattr()
Return Value
On success mq_getattr() and mq_setattr() return 0; on error, -1 is returned, with errno set to indicate the error.Errors
EBADF The message queue descriptor specified in mqdes is invalid.
EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK .
Attributes
For an explanation of the terms used in this section, see attributes(7).Interface | Attribute | Value |
T} | Thread safety | MT-Safe |
Versions
On Linux, mq_getattr() and mq_setattr() are library functions layered on top of the mq_getsetattr(2) system call.Standards
POSIX.1-2008.History
POSIX.1-2001.Examples
The program below can be used to show the default mq_maxmsg and mq_msgsize values that are assigned to a message queue that is created with a call to mq_open(3) in which the attr argument is NULL. Here is an example run of the program:$ ./a.out /testq
Maximum # of messages on queue: 10
Maximum message size: 8192
Since Linux 3.5, the following /proc files (described in mq_overview(7)) can be used to control the defaults:
$ uname -sr Linux 3.8.0 $ cat /proc/sys/fs/mqueue/msg_default 10 $ cat /proc/sys/fs/mqueue/msgsize_default 8192
Program Source
#include <fcntl.h> #include <mqueue.h> #include <stdio.h> #include <stdlib.h> #include <sys/stat.h> #include <unistd.h> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e } while (0) int main(int argc, char *argv[]) { mqd_t mqd; struct mq_attr attr; if (argc != 2) { fprintf(stderr, "Usage: %s mq-name\en", argv[0]); exit(EXIT_FAILURE); } mqd = mq_open(argv[1], O_CREAT | O_EXCL, 0600, NULL); if (mqd == (mqd_t) -1) errExit("mq_open"); if (mq_getattr(mqd, &attr) == -1) errExit("mq_getattr"); printf("Maximum # of messages on queue: %ld\en", attr.mq_maxmsg); printf("Maximum message size: %ld\en", attr.mq_msgsize); if (mq_unlink(argv[1]) == -1) errExit("mq_unlink"); exit(EXIT_SUCCESS); }
See Also
- mq_close(3),
- mq_notify(3),
- mq_open(3),
- mq_receive(3),
- mq_send(3),
- mq_unlink(3),
- mq_overview(7)