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Possible Duplicate:
C programming : How does free know how much to free?
A random thought occured to me that how comes free(myPtr) doesn’t need a length parameter? How does it know how much memory to free?
I can only guess that it keeps track of how much memory it allocates for each particular start address.
This is because malloc saves the information about the length of the allocated chunk, usually in a spot that precedes the address returned to your program. It is not unusual for implementations to allocate an extra storage for a size_t, put the size there, add sizeof(size_t), and return it to you as the malloc-ed pointer. The standard does not call for this implementation, thoug, so alternative implementations are possible, e.g. based on a hash table.