SHMCTL(2)

✨ SHMCTL(2) ✨

okay bestie, sit DOWN because we're talking about shmctl — that's the shared memory control girlie from the System Calls Manual 💅 she's part of the Standard C Library (libc, -lc) and she is literally the queen of telling your shared memory segments what to do.

🌸 SYNOPSIS 🌸

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

int
shmctl(int shmid, int cmd, struct shmid_ds *buf);

💖 DESCRIPTION 💖

So basically she performs whatever action you specify in cmd, and she does it on the shared memory segment that's identified by shmid. That's it. That's the whole vibe check. 🦋

Here's the tea on each cmd:

• IPC_STAT — she fetches the segment's struct shmid_ds and stores it in the memory that buf points to. Grabby hands, but make it legal ✨
• IPC_SET — she changes the shm_perm.uid, shm_perm.gid, and shm_perm.mode members of the segment's struct shmid_ds so they match the ones in the struct that buf points to. ⚠️ but like, permission check: the calling process's effective uid has to match either shm_perm.uid or shm_perm.cuid, OR it needs superuser privileges. No shortcuts bestie.
• IPC_RMID — she removes the segment from the system. BUT — and this is the important part — the removal does not take effect until every single process that has attached the segment has exited. She's patient like that. 🧚 For the operation to succeed, the calling process's effective uid must match shm_perm.uid or shm_perm.cuid, or the process must have superuser privileges.

lowkey a big deal: if the kern.ipc.shm_allow_removed sysctl(3) variable is set to 0, then once the IPC_RMID operation has taken place, no further processes will be allowed to attach the segment. That's it. Doors closed. Not me being dramatic, that's just what it says 😭

The shmid_ds structure is defined as follows — copy it exactly, don't freestyle it:

struct shmid_ds {
    struct ipc_perm shm_perm;   /* operation permission structure */
    size_t          shm_segsz;  /* size of segment in bytes */
    pid_t           shm_lpid;   /* process ID of last shared memory op */
    pid_t           shm_cpid;   /* process ID of creator */
    int             shm_nattch; /* number of current attaches */
    time_t          shm_atime;  /* time of last shmat() */
    time_t          shm_dtime;  /* time of last shmdt() */
    time_t          shm_ctime;  /* time of last change by shmctl() */
};

🎀 RETURN VALUES 🎀

The shmctl() function returns the value 0 if successful. If not? She returns the value -1 and sets the global variable errno to indicate the error. The duality of woman 💀

🔥 ERRORS 🔥

The shmctl() system call will fail if:

• [EINVAL] — invalid operation, or no shared memory segment was found corresponding to shmid. She looked. She found nothing. 🙈
• [EPERM] — the calling process's effective uid does not match the uid of the shared memory segment's owner or creator. Not your segment, not your rules, bestie.
• [EACCES] — permission denied due to a mismatch between the operation and the mode of the shared memory segment. The vibes were off, so the access was denied.

👑 SEE ALSO 👑

shmat(2), shmdt(2), shmget(2), ftok(3)

and that's the whole page, no cap 🫶 this Debian-flavoured icon dropped on October 10, 2015 — yeah she's an elder but she ate and left no crumbs ✨💜