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Some bits on malloc(0) in C being allowed to return NULL

(utcc.utoronto.ca)
48 points ingve | 5 comments | 25 Jun 25 06:15 UTC | HN request time: 0.446s | source
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bobmcnamara ◴[26 Jun 25 17:01 UTC] No.44389188[source]▶
Ages ago I worked with a system where malloc(0) incremented a counter and returned -1.

free(-1) decremented the counter.

This way you could check for leaks :p

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sgerenser ◴[26 Jun 25 17:21 UTC] No.44389346[source]▶
I might be missing something, but how does this help in checking for leaks? I mean, I guess you could use it to check for leaks specifically of 0-sized allocations, but wouldn’t it be better just to return NULL and guarantee that 0-sized allocations never use any memory at all?
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1. bobmcnamara ◴[26 Jun 25 17:34 UTC] No.44389482[source]▶
At the end of main, if the count wasn't balanced, then you knew you had a mismatch between malloc()/free().

If malloc() had returned a real pointer, you'd have to free that too.

> wouldn’t it be better just to return NULL and guarantee that 0-sized allocations never use any memory at all?

Better: takes less memory Worse: blinds you to this portability issue.

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2. Someone ◴[26 Jun 25 18:15 UTC] No.44389856[source]▶
> At the end of main, if the count wasn't balanced, then you knew you had a mismatch between malloc()/free().

A mismatch between malloc(0) and free(-1).

You’d know nothing about calls to malloc with non-zero sizes.

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3. sgerenser ◴[26 Jun 25 19:56 UTC] No.44390689[source]▶
Yeah, exactly, that’s my point. How many programs have memory leaks limited to (or even just materially affected by) 0-sized allocations? I’d have to imagine its a very small minority.
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4. bobmcnamara ◴[26 Jun 25 23:37 UTC] No.44392512[source]▶
Those are identifiable by the end state of the heap not being empty.
5. bobmcnamara ◴[26 Jun 25 23:45 UTC] No.44392558{3}[source]▶
They're uncommon for sure. In the past they've been an issue for me on constrained systems where they can frag the heap as badly as any other long lived allocation.