malloc_chunk

Notice how the data of an allocated chunk uses the first attribute () of the next chunk. is the pointer which is returned to the user.

Free chunk

P (PREV_INUSE): 0 when previous chunk (not the previous chunk in the linked list, but the one directly before it in memory) is free (and hence the size of previous chunk is stored in the first field). The very first chunk allocated has this bit set. If it is 1, then we cannot determine the size of the previous chunk.

A (NON_MAIN_ARENA): 0 for chunks in the main arena. Each thread spawned receives its own arena and for those chunks, this bit is set.