mq_getattr(3)

🎀 NAME 🎀

okay so bestie, welcome to mq_getattr(3) — this is the Library Functions Manual page for mq_getattr, mq_setattr, and they're here to get/set message queue attributes. short n sweet, but so much tea inside 💅

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🏛️ LIBRARY 🏛️

Real-time library, aka librt, and you link it with -lrt. slay.

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✨ 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);

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🦋 DESCRIPTION 🦋

So like, mq_getattr() retrieves attributes, and mq_setattr() modifies them. Which attributes? The ones of the message queue that the message queue descriptor mqdes points at. that's the whole vibe 🫶

mq_getattr() returns an mq_attr structure in the buffer that attr points to. And 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 */
};

okay let's break down each field because they're all main characters fr fr:

• mq_flags — holds flags tied to the open message queue description. It gets initialized when the queue is created by mq_open(3). And the only flag that can show up in this field is O_NONBLOCK. one (1) flag. that's it. 💀
• mq_maxmsg and mq_msgsize — these get set when the message queue is created by mq_open(3). mq_maxmsg is the upper limit on how many messages can be placed on the queue using mq_send(3). mq_msgsize is the upper limit on the size of messages that can be placed on the queue. Both of these fields must have a value greater than zero, no debating. and there are two /proc files that put ceilings on the values for these fields — the details live in mq_overview(7).
• mq_curmsgs — returns the number of messages currently held in the queue. the receipt, basically. 🧾

Now for mq_setattr(): it sets message queue attributes using the info supplied in the mq_attr structure pointed to by newattr. Here's the catch bestie — the only attribute you can modify is the setting of the O_NONBLOCK flag in mq_flags. The other fields in newattr are ignored. literally just looked at and then vibed past. 🙈

If the oldattr field is not NULL, then the buffer it points to gets used to return an mq_attr structure containing the same information that mq_getattr() returns. cute lil two-for-one.

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🔥 RETURN VALUE 🔥

On success, mq_getattr() and mq_setattr() return 0. On error, -1 is returned, and errno is set to indicate the error. classic.

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💀 ERRORS 💀

• EBADF — the message queue descriptor specified in mqdes is invalid. not it.
• EINVAL — newattr->mq_flags contained set bits other than O_NONBLOCK. you tried to sneak extra flags in there and it Did Not Work.

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🌸 ATTRIBUTES 🌸

For an explanation of the terms used in this section, see attributes(7).

• Interface: mq_getattr(), mq_setattr()
• Attribute: Thread safety
• Value: MT-Safe

she's thread safe, she's unbothered, she's moisturized ✨

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🌈 VERSIONS 🌈

On Linux, mq_getattr() and mq_setattr() are library functions layered on top of the mq_getsetattr(2) system call. so like, they're the pretty face and the syscall is doing the actual work underneath. iconic team effort.

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👑 STANDARDS 👑

POSIX.1-2008.

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📜 HISTORY 📜

POSIX.1-2001. she's been around, she's got lore.

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🍓 EXAMPLES 🍓

The program below can be used to show the default mq_maxmsg and mq_msgsize values that get assigned to a message queue created with a call to mq_open(3) where the attr argument is NULL. (yes, NULL. we're letting the system decide.) and here's 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\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\n", attr.mq_maxmsg);
    printf("Maximum message size:             %ld\n", attr.mq_msgsize);

    if (mq_unlink(argv[1]) == -1)
        err(EXIT_FAILURE, "mq_unlink");

    exit(EXIT_SUCCESS);
}

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💖 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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and that's the whole page bestie!! this one's from Linux man-pages 6.20, dated 2026-02-08 — go forth and get/set those attributes, you absolute legend 💅✨🦋