what can cause free to segfault

[Irrwahn Grausewitz]

Or *before* the block (e.g. past something else, seemingly completely
unrelated), probably even more likely and definitely more difficult to track
down. Believe, been there, done that

.... got the t-shirt, wore it out. ;-)
IMHO these are among the worst kinds of errors one can think of.

 

[Dan Pop]

Does that, then, suggest a macro wrapper for free() such as

#define FREE( ptr ) free(ptr); ptr=NULL;

If it looks like a function call, it must behave like a function call,
at the syntactical level, at least, so the proper definition is:

#define FREE(ptr) (free(ptr), ptr = NULL)

and, due to its limitations, ptr must not be an expression with side
effects or not evaluating to a modifiable lvalue.

The anal programmer may want to write (ptr) = NULL instead ;-)

If you're wondering what's wrong with your version, consider:

if (foo) FREE(p);

What is the value of p after this "statement" if foo evaluates to 0?

Dan

 

[Dan Pop]

... got the t-shirt, wore it out. ;-)
IMHO these are among the worst kinds of errors one can think of.

Therefore, they're worth paying the additional attention necessary to
avoid making them in the first place.

Dan