What happens to memory after free()?

Question

I know that on your hard drive, if you delete a file, the data is not (instantly) gone. The data is still there until it is overwritten. I was wondering if a similar concept existed in memory. Say I allocate 256 bytes for a string, is that string still floating in memory somewhere after I free() it until it is overwritten?

Answer 1

Your analogy is correct. The data in memory doesn't disappear or anything like that; the values may indeed still be there after a free(), though attempting to read from freed memory is undefined behaviour.

Answer 2

Generally, it does stay around, unless you explicitly overwrite the string before freeing it (like people sometimes do with passwords). Some library implementations automatically overwrite deallocated memory to catch accesses to it, but that is not done in release mode.