Visit Our UNIX and Linux User Community

Linux and UNIX Man Pages

Test Your Knowledge in Computers #706
Difficulty: Medium
Shared information bias is known as the tendency for group members to spend more time and energy discussing information that all members are already familiar with (i.e., shared information), and less time and energy discussing information that only some members are aware of.
True or False?
Linux & Unix Commands - Search Man Pages

mq_setattr(3) [bsd 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)

Featured Tech Videos