MPROTECT(3POSIX)

💖 MPROTECT(3POSIX) — POSIX Programmer's Manual, bestie edition 💖

okay so first things first, this page is literally living inside the POSIX Programmer's Manual. The Linux version of this interface might be a whole different girl (go check the Linux manual page for the deets on Linux behavior), or the interface might not even be implemented on Linux at all. 🙈

---

🌸 PROLOG 🌸

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — so consult the corresponding Linux manual page for details on Linux behavior — or honestly the interface may just not be implemented on Linux. No cap.

---

✨ NAME ✨

mprotect — set protection of memory mapping. she's here to guard your pages, periodt. 💅

---

🎀 SYNOPSIS 🎀

#include <sys/mman.h>

int mprotect(void *addr, size_t len, int prot);

---

🦋 DESCRIPTION 🦋

so the mprotect() function shall change the access protections to be exactly what prot says, for those whole pages containing any part of the address space of the process — starting at address addr and continuing for len bytes. Whole pages. Every time. The vibe check happens at page granularity, iykyk. ✨

The parameter prot decides whether read, write, execute, or some combination of accesses are permitted to the data being mapped. And prot should be either PROT_NONE or the bitwise-inclusive OR of one or more of PROT_READ, PROT_WRITE, and PROT_EXEC. That's the whole menu, bestie. 🍓

If an implementation cannot support the combination of access types specified by prot, the call to mprotect() shall fail. It's giving "we don't do that combo here."

An implementation may permit accesses other than those specified by prot — however, no implementation shall permit a write to succeed where PROT_WRITE has not been set, and no implementation shall permit any access where PROT_NONE alone has been set. Like, that's the floor. That's the line. Do not cross it. 💀

Implementations shall support at least these values of prot:

• PROT_NONE
• PROT_READ
• PROT_WRITE
• the bitwise-inclusive OR of PROT_READ and PROT_WRITE

If PROT_WRITE is specified, the application shall ensure that it has opened the mapped objects in the specified address range with write permission — unless MAP_PRIVATE was specified in the original mapping — and that's true regardless of whether the file descriptors used to map the objects have since been closed. Closed fds do NOT get you out of this, bestie. 🫶

The implementation may require that addr be a multiple of the page size as returned by sysconf(). (Spoiler: it probably will. See APPLICATION USAGE below, she's messy.)

⚠️ The behavior of this function is unspecified if the mapping was not established by a call to mmap(). Do not try it. Do not be that girl.

And listen, this one's important: when mprotect() fails for reasons other than [EINVAL], the protections on some of the pages in the range [addr,addr+len) may have been changed. So like, it's a partial situation. It's a "some of y'all got updated, some of y'all didn't" situation. 😭

---

🔥 RETURN VALUE 🔥

Upon successful completion, mprotect() shall return 0. Otherwise, it shall return -1 and set errno to indicate the error. Classic. Iconic. On brand. 👑

---

💜 ERRORS 💜

The mprotect() function shall fail if:

• EACCES — The prot argument specifies a protection that violates the access permission the process has to the underlying memory object. Rude, but fair.
• EAGAIN — The prot argument specifies PROT_WRITE over a MAP_PRIVATE mapping, and there are insufficient memory resources to reserve for locking the private page. 💀
• ENOMEM — Addresses in the range [addr,addr+len) are invalid for the address space of a process, or specify one or more pages which are not mapped. Nothing there, girl.
• ENOMEM — The prot argument specifies PROT_WRITE on a MAP_PRIVATE mapping, and it would require more space than the system is able to supply for locking the private pages, if required. (Yes, ENOMEM shows up twice. She's versatile.)
• ENOTSUP — The implementation does not support the combination of accesses requested in the prot argument. It's giving "not supported, sorry bestie." 🧚

The mprotect() function may fail if:

• EINVAL — The addr argument is not a multiple of the page size as returned by sysconf(). 😭

The following sections are informative.

---

🌈 EXAMPLES 🌈

None. Truly nothing. Empty. The void. 🙈

---

🎀 APPLICATION USAGE 🎀

Most implementations require that addr is a multiple of the page size as returned by sysconf(). So like, align it. Please. For me. 🫶

---

⭐ RATIONALE ⭐

None. We said what we said. 💅

---

🌸 FUTURE DIRECTIONS 🌸

None. The future is unknowable and also, apparently, identical to the present. ✨

---

💖 SEE ALSO 💖

• mmap()
• sysconf()

The Base Definitions volume of POSIX.1‐2017, <sys_mman.h> — go read her, she's got the constants.

---

👑 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 — brought to you by IEEE/The Open Group, 2017 edition, signing off with a lil ✨ mwah ✨ 💖