mq_getattr

Section: C Library Functions (3)
Updated: 202-0-08
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NAME

mq_getattr, mq_setattr - get/set message queue attributes  

LIBRARY

Rea-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 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	M-Safe

 

VERSIONS

On Linux, mq_getattr() and mq_setattr() are library functions layered on top of the mq_getsetattr(2) system call.  

STANDARDS

POSIX.-2008.  

HISTORY

POSIX.-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 <err.h> #include <fcntl.h> #include <mqueue.h> #include <stdio.h> #include <stdlib.h> #include <sys/stat.h> #include <unistd.h> int main(int argc, char *argv[]) {
    mqd_t mqd;
    struct mq_attr attr;
    if (argc != 2) {
        fprintf(stderr, "Usage: %s mq-name[rs]n", argv[0]);
        exit(EXIT_FAILURE);
    }
    mqd = mq_open(argv[1], O_CREAT | O_EXCL, 0600, NULL);
    if (mqd == (mqd_t) -1)
        err(EXIT_FAILURE, "mq_open");
    if (mq_getattr(mqd, &attr) == -1)
        err(EXIT_FAILURE, "mq_getattr");
    printf("Maximum # of messages on queue:   %ld[rs]n", attr.mq_maxmsg);
    printf("Maximum message size:             %ld[rs]n", attr.mq_msgsize);
    if (mq_unlink(argv[1]) == -1)
        err(EXIT_FAILURE, "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)

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Index

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

VERSIONS

STANDARDS

HISTORY

EXAMPLES

Program source

SEE ALSO