MSGGET(2)

💬 MSGGET(2) — get message queue 💬

okay bestie, welcome to the msggot... no wait, msgget page 💖 it's a System Calls Manual entry, and it is exactly the kind of thing you need in your life if you're out here doing inter-process communication like a boss. let's get into it 🫶

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✨ NAME ✨

msgget — get a message queue, babes 💅

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📚 LIBRARY 📚

Standard C Library (libc, -lc) 🎀

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🌸 SYNOPSIS 🌸

#include <sys/msg.h>

int
msgget(key_t key, int msgflg);

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🦋 DESCRIPTION 🦋

so, the msgget() function returns the message queue identifier that's associated with key. and what even is a message queue identifier? it's a unique integer, and it's greater than zero, periodt 💯

here's the tea ☕ — a message queue gets created if EITHER:

• key is equal to IPC_PRIVATE, OR
• key doesn't have a message queue identifier associated with it and the IPC_CREAT bit is set in msgflg

not me pointing out that you literally need the IPC_CREAT bit if you want a fresh queue and the key isn't IPC_PRIVATE. that's just how it is, no cap.

if a brand new message queue gets created, the data structure that's associated with it — that's the msqid_ds structure, see msgctl(2) — gets initialized like this 👇

• msg_perm.cuid and msg_perm.uid are set to the effective uid of the calling process 👑
• msg_perm.gid and msg_perm.cgid are set to the effective gid of the calling process 👑
• msg_perm.mode is set to the lower 9 bits of msgflg, which are set by ORing these constants together:
  • 0400 — Read access for user 🌈
  • 0200 — Write access for user ✍️
  • 0040 — Read access for group 🌈
  • 0020 — Write access for group ✍️
  • 0004 — Read access for other 🌈
  • 0002 — Write access for other ✍️
• msg_cbytes, msg_qnum, msg_lspid, msg_lrpid, msg_rtime, and msg_stime are all set to 0 🫧 (yes, all of them, they start from scratch, we love a clean slate)
• msg_qbytes is set to the system wide maximum value for the number of bytes in a queue — that's MSGMNB 🧮
• msg_ctime is set to the current time ⏰

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🎀 RETURN VALUES 🎀

if everything goes slay, a positive message queue identifier comes back at you 💖

otherwise? you get -1 back, and the global variable errno gets set to indicate the error 😭 (we hate to see it, but at least it tells you why)

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💀 ERRORS 💀

• [EACCES] — a message queue is already associated with key and the caller has no permission to access it 🙈
• [EEXIST] — both IPC_CREAT and IPC_EXCL are set in msgflg, and a message queue is already associated with key 😤
• [ENOSPC] — a new message queue could not be created because the system limit for the number of message queues has been reached 💥
• [ENOENT] — IPC_CREAT was not set in msgflg and no message queue associated with key was found 😔

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👀 SEE ALSO 👀

msgctl(2), msgrcv(2), msgsnd(2) — go read those next, bestie, they're the whole friend group 🫶

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📜 HISTORY 📜

Message queues appeared in the first release of AT&T System V UNIX ⭐ like... iconic behavior honestly, they've been around forever and they still ate 👑

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and that's the whole page, my love 💖 this one's from the Debian fam, dated March 4, 2018 🎀 go make some queues, and don't forget to clean up after yourself, iykyk 💅✨