mq_setattr

<html> <head> <title>mq_setattr</title> <META NAME="KEYWORDS" CONTENT="mq_setattr"> </head> <body BGCOLOR="#ffffff" LINK="#0000ff" VLINK="#0000ff" ALINK="#0000ff" TEXT="#000000"> <center> <h1>mq_setattr</h1></center> <PRE> <A HREF="/man3/MQ_GETATTR">MQ_GETATTR(3)</A> Linux Programmer's Manual <A HREF="/man3/MQ_GETATTR">MQ_GETATTR(3)</A> NAME 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 at- tributes of the message queue referred to by the message queue descrip- tor 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 <A HREF="/man3/mq_open">mq_open(3)</A>. The only flag that can appear in this field is O_NON- BLOCK. The mq_maxmsg and mq_msgsize fields are set when the message queue is created by <A HREF="/man3/mq_open">mq_open(3)</A>. The mq_maxmsg field is an upper limit on the number of messages that may be placed on the queue using <A HREF="/man3/mq_send">mq_send(3)</A>. 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 <A HREF="/man7/mq_overview">mq_overview(7)</A>. 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 re- turned, 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 at- <A HREF="/man7/tributes">tributes(7)</A>. +---------------------------+---------------+---------+ |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 <A HREF="/man2/mq_getsetattr">mq_getsetattr(2)</A> system call. EXAMPLE The program below can be used to show the default mq_maxmsg and mq_msg- size values that are assigned to a message queue that is created with a call to <A HREF="/man3/mq_open">mq_open(3)</A> in which the attr argument is NULL. Here is an ex- ample 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_over- <A HREF="/man7/view">view(7)</A>) 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\n", 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\n", attr.mq_maxmsg); printf("Maximum message size: %ld\n", attr.mq_msgsize); if (mq_unlink(argv[1]) == -1) errExit("mq_unlink"); exit(EXIT_SUCCESS); } SEE ALSO <A HREF="/man3/mq_close">mq_close(3)</A>, <A HREF="/man3/mq_notify">mq_notify(3)</A>, <A HREF="/man3/mq_open">mq_open(3)</A>, <A HREF="/man3/mq_receive">mq_receive(3)</A>, <A HREF="/man3/mq_send">mq_send(3)</A>, <A HREF="/man3/mq_unlink">mq_unlink(3)</A>, <A HREF="/man7/mq_overview">mq_overview(7)</A> COLOPHON This page is part of release 5.05 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 https://www.kernel.org/doc/man-pages/. Linux 2019-03-06 <A HREF="/man3/MQ_GETATTR">MQ_GETATTR(3)</A></PRE> <center> <h6>Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2025 <a href="http://www.he.net">Hurricane Electric</a>. All Rights Reserved.</h6></center> </body> </html>

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mq_setattr