SHMAT(3P)

💖 SHMAT(3P) — the glow-up 💖

okay bestie, welcome to the POSIX Programmer's Manual entry for shmat, aka the XSI shared memory attach operation. this is the moment your process finally links up with a shared memory segment. it's giving ✨*connection*✨. 🫶

---

🌸 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 of Linux behavior), or the interface may not be implemented on Linux.

bestie i need you to clock this: the Linux version might be different, and there's a real chance Linux doesn't even have it. read the Linux man page, mkay? 🙈

---

✨ NAME ✨

shmat — XSI shared memory attach operation

like. that's it. that's the whole name. iconic. 💅

---

🦋 SYNOPSIS 🦋

#include <sys/shm.h>

void *shmat(int shmid, const void *shmaddr, int shmflg);

---

💜 DESCRIPTION 💜

The shmat() function operates on XSI shared memory (see the Base Definitions volume of POSIX.1‐2017, Section 3.346, Shared Memory Object). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime.

so, translation bestie: shmat() takes the shared memory segment that belongs to the shared memory identifier you passed in as shmid, and it attaches that segment to the address space of the calling process. it's literally moving in. 🏠✨

where does it attach? depends on your vibes (and your args). the segment gets attached at the address picked by one of these criteria:

• If shmaddr is a null pointer, the segment is attached at the first available address as selected by the system. (system said "i gotchu, i'll pick" 🎀)
• If shmaddr is not a null pointer and (shmflg &SHM_RND) is non-zero, the segment is attached at the address given by (shmaddr -((uintptr_t)shmaddr %SHMLBA)). The character '%' is the C-language remainder operator.
• If shmaddr is not a null pointer and (shmflg &SHM_RND) is 0, the segment is attached at the address given by shmaddr.
• The segment is attached for reading if (shmflg &SHM_RDONLY) is non-zero and the calling process has read permission; otherwise, if it is 0 and the calling process has read and write permission, the segment is attached for reading and writing.

so yeah. read-only mode if you ask for it and you're allowed to read. full read+write mode if you didn't ask for read-only and you're allowed to do both. permissions check is real, bestie. 🔥

---

👑 RETURN VALUE 👑

Upon successful completion, shmat() shall increment the value of shm_nattch in the data structure associated with the shared memory ID of the attached shared memory segment and return the segment's start address. Also, the shm_atime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description.

so on the slay path: shm_nattch goes up 📈, you get back the segment's start address, and shm_atime gets stamped with the current time. main character timestamps only. ⭐

Otherwise, the shared memory segment shall not be attached, shmat() shall return (void *)-1, and errno shall be set to indicate the error.

💀 and if it fails? nothing attaches. you get (void *)-1 back and errno tells you what went wrong. not the vibe. 😭

---

🌈 ERRORS 🌈

The shmat() function shall fail if:

• EACCES — Operation permission is denied to the calling process; see Section 2.7, XSI Interprocess Communication. (gatekept, and not in the cute way 🙈)
• EINVAL — The value of shmid is not a valid shared memory identifier, the shmaddr is not a null pointer, and the value of (shmaddr -((uintptr_t)shmaddr %SHMLBA)) is an illegal address for attaching shared memory; or the shmaddr is not a null pointer, (shmflg &SHM_RND) is 0, and the value of shmaddr is an illegal address for attaching shared memory.
• EMFILE — The number of shared memory segments attached to the calling process would exceed the system-imposed limit. (too many besties attached to you, fr fr 😭)
• ENOMEM — The available data space is not large enough to accommodate the shared memory segment.

The following sections are informative.

---

🎀 EXAMPLES 🎀

None.

bestie there are literally no examples. i can't make them up, that would be lying, and we don't do that here. 🫶

---

🧚 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.

basically: if you're building IPC stuff, keep your modules flexible so you can swap over to the POSIX Realtime Extension interfaces later without a whole breakdown. future-proofing is self care. 💅✨

---

⭐ RATIONALE ⭐

None.

---

🌸 FUTURE DIRECTIONS 🌸

None.

---

🦋 SEE ALSO 🦋

• Section 2.7, XSI Interprocess Communication
• Section 2.8, Realtime
• exec
• exit()
• fork()
• shmctl()
• 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 .

---

okay that's the whole page, bestie, no cap. this one came from IEEE/The Open Group, 2017 — shoutout to the 2017 girlies for this absolute banger of a spec. now go attach that segment, you got this. 👑💜