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())

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.