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Comments on Which platforms return a non-null pointer on malloc(0)

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Which platforms return a non-null pointer on malloc(0)

+4
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What is the portability of malloc(0);?

  • Which platforms return NULL without setting errno?
  • Which platforms return a non-null pointer?
  • Do any platforms have some other behavior?
c malloc
posted 3 months ago
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alx‭
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1 comment thread

realloc(ptr, 0) (1 comment)
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The behavior of malloc(0) is implementation-defined and varies between platforms. Some platforms will return a NULL pointer, others might return a non-null pointer, but such a pointer should not be used. There's no widely known behavior outside of these.

posted 2 months ago
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harmony‭
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re: "but such pointer should not be used" (2 comments)
re: "but such pointer should not be used"
alx‭ wrote 2 months ago · edited 2 months ago
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The non-null pointer returned by malloc(0) can be used. To be more precise, usual pointer arithmetic is allowed on such a pointer. You can do p + 0 or p - p, or p - 0.

Of course you cannot access any members, because there aren't any, but other than that, it's a valid pointer like any other.

alx‭ wrote 2 months ago
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In fact, this is different from NULL, and is what makes non-null a better behavior: it's not a special case.