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mq_getattr(3) [minix man page]

MQ_GETATTR(3)						     Linux Programmer's Manual						     MQ_GETATTR(3)

mq_getattr, mq_setattr - get/set message queue attributes SYNOPSIS
#include <mqueue.h> int mq_getattr(mqd_t mqdes, struct mq_attr *attr); int mq_setattr(mqd_t mqdes, const struct mq_attr *newattr, struct mq_attr *oldattr); Link with -lrt. DESCRIPTION
mq_getattr() and mq_setattr() respectively retrieve and modify attributes of the message queue referred to by the message queue descriptor mqdes. 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 | +---------------------------+---------------+---------+ |mq_getattr(), mq_setattr() | Thread safety | MT-Safe | +---------------------------+---------------+---------+ CONFORMING TO
POSIX.1-2001, POSIX.1-2008. NOTES
On Linux, mq_getattr() and mq_setattr() are library functions layered on top of the mq_getsetattr(2) system call. EXAMPLE
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 <mqueue.h> #include <sys/stat.h> #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0) int main(int argc, char *argv[]) { mqd_t mqd; struct mq_attr attr; if (argc != 2) { fprintf(stderr, "Usage: %s mq-name ", argv[0]); exit(EXIT_FAILURE); } mqd = mq_open(argv[1], O_CREAT | O_EXCL, S_IRUSR | S_IWUSR, 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 ", attr.mq_maxmsg); printf("Maximum message size: %ld ", 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) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at Linux 2017-09-15 MQ_GETATTR(3)
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