c - malloc double free behaviour

Question

The GNU manual page for malloc defines that when free() called twice with the same pointer (that was previously allocated by malloc()) "undefined behaviour occurs".

What kind of behaviour is that?
Where can I find more Information about this kind of error?

Answer 1

The C99 standard states the following for undefined behaviour (under the Terms and definitions section):

Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

In summary: anything can happen. The worst case is for the program to behave as expected.