SEMGET(2)

✨ semget(2) — System Calls Manual energy ✨

okay bestie, pull up a chair because we're about to talk about semaphores and honestly? it's giving iconic. this is semget(2), from the System Calls Manual, and she's here to hand you a semaphore id like it's a friendship bracelet. 💖

---

🎀 NAME 🎀

semget — obtain a semaphore id

---

📚 LIBRARY 📚

Standard C Library (libc, -lc) — yeah she lives in libc, no cap.

---

🦋 SYNOPSIS 🦋

#include <sys/sem.h>

int
semget(key_t key, int nsems, int flag);

---

💅 DESCRIPTION 💅

so basically, based on the values of key and flag, semget() returns the identifier of a newly created or previously existing set of semaphores. periodt.

think of key like a filename, fr fr — it's the handle that names an IPC object. it's literally the name tag at the party so everyone knows which semaphore set they're talking about. 🪪

there are three ways to specify a key, and yes you have to pick one:

• IPC_PRIVATE may be specified, in which case a new IPC object will be created. brand new, no notes, main character debut. ✨
• An integer constant may be specified. If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in flag, a new one will be created. (yes, the IPC_CREAT bit is doing all the heavy lifting here, she's the moment.) 💅
• The ftok(3) function may be used to generate a key from a pathname. cute lil pathname-to-key pipeline, obsessed. 🧚

the mode of a newly created IPC object is determined by ORing these constants into the flag argument:

• 0400 — Read access for user. 👑
• 0200 — Alter access for user. ✏️
• 0040 — Read access for group. 🫶
• 0020 — Alter access for group. 🖊️
• 0004 — Read access for other. 🌸
• 0002 — Alter access for other. 🌈

and if a new set of semaphores is being created, nsems is used to indicate the number of semaphores the set should contain. otherwise, nsems may be specified as 0. easy peasy, lowkey. 💜

---

🌟 RETURN VALUES 🌟

The semget() system call returns the id of a semaphore set if successful; otherwise, -1 is returned and errno is set to indicate the error. so if you see -1, that's not a vibe check passing, bestie. 😭

---

💀 ERRORS 💀

The semget() system call will fail if:

• [EACCES] — Access permission failure. (the door said no. 🙈)
• [EEXIST] — IPC_CREAT and IPC_EXCL were specified, and a semaphore set corresponding to key already exists. (she's already here, babes, you can't double-book.) 👀
• [EINVAL] — The number of semaphores requested exceeds the system imposed maximum per set. (too greedy, the system said enough.) 🛑
• [EINVAL] — A semaphore set corresponding to key already exists and contains fewer semaphores than nsems. (you asked for more than she has. awkward.) 😬
• [EINVAL] — A semaphore set corresponding to key does not exist and nsems is 0 or negative. (nothing there, and you brought no semaphores. tragic.) 💀
• [ENOSPC] — Insufficiently many semaphores are available. (the semaphore shelf is empty, we're out of stock.) 🍓
• [ENOSPC] — The kernel could not allocate a struct semid_ds. (kernel said "no thoughts, head empty, can't allocate.") 😭
• [ENOENT] — No semaphore set was found corresponding to key, and IPC_CREAT was not specified. (you didn't ask her to create one, so she didn't. respect the rules.) 🙅

---

🔥 SEE ALSO 🔥

semctl(2), semop(2), ftok(3) — go read these next, iykyk. the whole semaphore friend group is here and they all ate. 👑

---

fr fr bestie, if you're gonna create IPC objects, be careful with those mode bits. permissions are not a joke, even when they're giving cute. 💖

---

and that's the page!! shoutout to Debian for shipping this, dated March 4, 2018 — a classic, honestly, she ate and left no crumbs. now go forth and semget responsibly, my love. 🫶✨