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

(utcc.utoronto.ca)
55 points ingve | 1 comments | 25 Jun 25 06:15 UTC | HN request time: 0.229s | 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. spacechild1 ◴[26 Jun 25 17:57 UTC] No.44389702[source]▶
> but wouldn’t it be better just to return NULL and guarantee that 0-sized allocations never use any memory at all?

This works if you are only interested in the overall memory balance. However, if you want to make sure that all malloc() calls are matched by a free() call, you need to distinguish between NULL and a successfull zero-sized allocation, otherwise you run into troubles when you call free on an "actual" NULL pointer (which the standard defines as a no-op).