SHMCTL(3POSIX)

💖 SHMCTL(3POSIX) — the XSI shared memory control girlies 💖

okay bestie, so this is part of the POSIX Programmer's Manual (yes, the POSIX Programmer's Manual, we're being fancy today 💅). heads up tho: the Linux implementation of this interface may differ from what's written here — so if you're on Linux, go read the corresponding Linux manual page for the actual Linux behavior. or like… the interface might not even be implemented on Linux at all. 🙈 so keep that in mind before you come for me.

---

✨ PROLOG ✨

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details on Linux behavior), or the interface may not be implemented on Linux.

no cap, always double-check your platform. the vibes are portable, the behavior might not be. 🫶

---

🌸 NAME 🌸

shmctl — XSI shared memory control operations

---

🎀 SYNOPSIS 🎀

#include <sys/shm.h>

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

---

🦋 DESCRIPTION 🦋

So the shmctl() function operates on XSI shared memory — that's the thing defined in the Base Definitions volume of POSIX.1‐2017, Section 3.346, Shared Memory Object, iykyk. ✨

Real talk: it is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime. so like, don't assume. 💀

shmctl() gives you a whole menu of shared memory control operations, and which one you get depends on cmd. Here's the lineup:

• IPC_STAT — takes the current value of each member of the shmid_ds data structure associated with shmid and drops it into the structure pointed to by buf. the contents of that structure are defined in <sys/shm.h>. 👑
• IPC_SET — sets the value of the following members of the shmid_ds data structure associated with shmid to the corresponding value found in the structure pointed to by buf:

shm_perm.uid
shm_perm.gid
shm_perm.mode    Low-order nine bits.

also, the shm_ctime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description.

and listen — IPC_SET can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges, or to the value of shm_perm.cuid or shm_perm.uid in the shmid_ds data structure associated with shmid. no permission, no party. 🚫

• IPC_RMID — removes the shared memory identifier specified by shmid from the system, and destroys the shared memory segment and shmid_ds data structure associated with it. it's giving demolition day 💥. IPC_RMID can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges, or to the value of shm_perm.cuid or shm_perm.uid in the shmid_ds data structure associated with shmid.

---

⭐ RETURN VALUE ⭐

Upon successful completion, shmctl() shall return 0. otherwise, it shall return -1 and set errno to indicate the error. classic. 🎯

---

💀 ERRORS 💀

The shmctl() function shall fail if:

• EACCES — the argument cmd is equal to IPC_STAT and the calling process does not have read permission; see Section 2.7, XSI Interprocess Communication.
• EINVAL — the value of shmid is not a valid shared memory identifier, or the value of cmd is not a valid command.
• EPERM — the argument cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of a process with appropriate privileges, and it is not equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid.

The shmctl() function may fail if:

• EOVERFLOW — the cmd argument is IPC_STAT and the gid or uid value is too large to be stored in the structure pointed to by the buf argument.

The following sections are informative. (so like, cute bonus content, not the law 📜)

---

🌈 EXAMPLES 🌈

None. 😭 iconic of them honestly, no examples, we just vibe.

---

💜 APPLICATION USAGE 💜

The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use the alternative interfaces.

translation: future-proof your code bestie, keep that IPC layer swappable 🧚

---

🍓 RATIONALE 🍓

None. nothing to justify, it just is. 💅

---

🧚 FUTURE DIRECTIONS 🧚

None. the future is… stable. slay.

---

👑 SEE ALSO 👑

• Section 2.7, XSI Interprocess Communication
• Section 2.8, Realtime
• shmat()
• shmdt()
• shmget()
• shm_open()
• shm_unlink()

The Base Definitions volume of POSIX.1‐2017, Section 3.346, Shared Memory Object, <sys_shm.h>

---

📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

---

and that's the whole page, bestie!! this one's brought to you by IEEE/The Open Group, dated 2017, and it lives at SHMCTL(3POSIX) — go forth and control your shared memory responsibly 🫶✨💖