malloc_usable_size(3)

💖 malloc_usable_size(3) 💖

okay so bestie, we're in the Library Functions Manual today and the star of the show is malloc_usable_size — yes that's the manual section 3 one, the library-call girlies. lets GO 🦋

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✨ NAME ✨

malloc_usable_size — obtain size of block of memory allocated from heap

📚 LIBRARY 📚

Standard C library (libc, -lc)

🎀 SYNOPSIS 🎀

#include <malloc.h>

size_t malloc_usable_size(void *_Nullable ptr);

🌸 DESCRIPTION 🌸

So like. This function is your little detective 🕵️♀️ bestie. You can use it for diagnostics or statistics about allocations that came from malloc(3) or one of its related functions.

That's it. That's the whole job. It's giving ✨informational✨, not ✨functional✨. iykyk.

🔥 RETURN VALUE 🔥

malloc_usable_size() returns a value no less than the size of the block of allocated memory pointed to by ptr.

And if ptr is NULL? You get 0 back. no drama, no crash, just a lil zero 🫶

💅 ATTRIBUTES 💅

(if you wanna know what any of these words even mean, go read attributes(7), she explains it all)

• Interface: malloc_usable_size()
  • Attribute: Thread safety
  • Value: MT-Safe

slay, she's MT-Safe, you can call her from multiple threads, she's not gonna start anything 💖

👑 STANDARDS 👑

GNU. (periodt.)

💀 CAVEATS 💀

okay bestie this is the part where you LISTEN TO ME because this is the important bit 🚨

• The value returned by malloc_usable_size() may be greater than the requested size of the allocation, because of various internal implementation details — and none of which the programmer should rely on. like. none. zero. not one. 🙈
• This function is intended to only be used for diagnostics and statistics.
• Writing to the excess memory without first calling realloc(3) to resize the allocation is not supported. NOT supported. do not do it. do not pass go. do not collect $200. 😭
• The returned value is only valid at the time of the call. it's a snapshot, a lil polaroid 📸, not a promise about the future.

fr fr, treat this number like a gossip girl told you — interesting, maybe useful for your stats, absolutely not something you build a house on. 🏠💀

🌈 SEE ALSO 🌈

• malloc(3)

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and that's the whole page bestie!! this one came from Linux man-pages 6.20, dated 2026-02-08, and it's the malloc_usable_size(3) page 💜 you ate that, go write some diagnostics 🌟