Error in Lecture 10 Slide 16

[sciencemanx]

malloc(20) should take 32 bytes not 24 (24 to be 8 byte aligned and 8 for the metadata).

[0xTowel]

The slides are correct.

The system is 32bit, so an address takes 4 bytes, which is enough to store the chunk size (metadata).

An in-use chunk stores the chunk size, while a free chunk stores the prev_chunk size at the end of the chunk, written upon free(). This means when you malloc(20), you only need 24 bytes which is already aligned to a multiple of 8. See the in-use chunk diagram here.

Note that you don't need any more space for the flags since they are the 3 least-significant bits of the chunk size, which don't matter since everything is aligned to 8 (0b1000) making them always be zero.

@Lense this can be closed.

[Lense]

Thanks!