memset sets a block of memory to a given value. It is often used to initialize an uninitialized block of memory to 0. So often, in fact, that this bug where it always initialized the memory to 0 survived so long.

memset's job is to fill a block of memory with a given byte. The most common fill byte is 0, to initialize or otherwise clear a buffer. It looks like it is so common, in fact, that a programmer accidentally made Android's memset _always_ fill with 0, no matter what byte was passed in, and presumably no one noticed for a while. (I can't figure out when this patch was introduced.)

Memset (according to the c++ reference) is supposed to: "Sets the first num bytes of the block of memory pointed by ptr to the specified value (interpreted as an unsigned char)."

So, give it a pointer to some memory, give it a value and the number of bytes you want to set and it'll go wild. In this case, memset was just blindly dropping in zeroes into the memory block

The function was always setting the memory to 0 instead of what was passed in. I think most uses of memset are to initialize memory to 0, but any other use would have had unexpected results.