Re:The answer here is yes!

Re:The answer here is yes!

Posted May 23, 2017 14:08 UTC (Tue) by anselm (subscriber, #2796)
In reply to: Re:The answer here is yes! by mirabilos
Parent article: The "too small to fail" memory-allocation rule

This sort of thing may work most of the time, but just for the record, while calloc() fills the whole structure in question with all-bits-zero bytes, there is no guarantee in the C standard that an individual structure entry like foo->a will in fact turn out to be a valid null pointer afterwards. (The C language does not require the null pointer to be “all bits zero”, even though expressions like “!(foo->a = geta())” must still return 1, as in “true”, if the geta() call yields a null pointer.)

If you're unlucky this means that if, say, you error out when trying to allocate foo->b, the “free(foo->d);” at the beginning of the out: path might try to free something at the all-bits-zero-address-that-isn't-a-null-pointer that hasn't previously be allocated, which isn't allowed. This shortcut looks enticingly convenient and probably works on most platforms today but people who are interested in safe, portable, and standard-conforming C code shouldn't be using it.

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