Comment on page


Allocated Chunks

The chunk has two sections - the metadata of the chunk (information about the chunk) and the user data, where the data is actually stored.
size is fairly self-explanatory - it's the overall size of the chunk.
The flags A, M and P have special uses. P is the PREV_INUSE flag, which is 0 when the previous adjacent chunk (the chunk ahead) is free. M and P are used for more sophisticated heap attacks, so we won't worry about them just yet.
prev_size is set if the previous adjacent chunk is free, as calculated by P. If it is not, the heap saves space and prev_size is part of the previous chunk's user data. If it is, then prev_size stores the size of the previous chunk.

Free Chunks

A free chunk looks a bit different:
The first part is the same, but FD and BK pointers have been added. This is for the purpose of sorting them into bins, which we will come onto soon.