Diary Of An x264 Developer

x264 keeps a set of pointers to various arrays used for custom quantization matrices (CQMs), deadzone biases, etc. These are stored in the primary x264_t struct as follows:

uint16_t (*quant4_bias[4])[16]; /* [4][52][16] */
uint16_t (*quant8_bias[2])[64]; /* [2][52][64] */

They are malloced at the start of the program and deleted when x264 finishes. Unfortunately the delete code looks something like this:

for( i = 0; i < 6; i++ )
{
    …
    x264_free( h->quant4_bias[i] );
}

A small part of me died when I read this code. Yes, that’s right, its overflowing the array of pointers intentionally because they’re arranged sequentially in the struct. Apparently, according to Loren Merritt, this simplifies the deletion code.