Why isn't a pointer
null
after calling
free?
How unsafe is it to use
(assign, compare)
a pointer value after it's been freed?
When you call free, the memory pointed to by the passed pointer is freed, but the value of the pointer in the caller remains unchanged, because C's pass-by-value semantics mean that called functions never permanently change the values of their arguments. (See also question 4.8.)
A pointer value which has been freed is, strictly speaking, invalid, and any use of it, even if is not dereferenced can theoretically lead to trouble, though as a quality of implementation issue, most implementations will probably not go out of their way to generate exceptions for innocuous uses of invalid pointers.
References:
ANSI Sec. 4.10.3
ISO Sec. 7.10.3
Rationale Sec. 3.2.2.3
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This page by Steve Summit // Copyright 1995 // mail feedback