Sorry for the trouble, I should have read the man pages :
malloc() allocates size bytes and returns a pointer to the allocated memory. The memory is not cleared. If size is 0, then malloc() returns either NULL, or a
unique pointer value that can later be successfully passed to free().
free() frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if
free(ptr) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.
It seems it is true at least for the gnu libc