What's the advantage over an array though, which would allow you to better control the size without making assumptions about code generation.

edit: http://cm.bell-labs.co/who/dmr/primevalC.html (linked from another comment has the answer):

> A second, less noticeable, but astonishing peculiarity is the space allocation: temporary storage is allocated that deliberately overwrites the beginning of the program, smashing its initialization code to save space. The two compilers differ in the details in how they cope with this. In the earlier one, the start is found by naming a function; in the later, the start is simply taken to be 0. This indicates that the first compiler was written before we had a machine with memory mapping, so the origin of the program was not at location 0, whereas by the time of the second, we had a PDP-11 that did provide mapping. (See the Unix History paper). In one of the files (prestruct-c/c10.c) the kludgery is especially evident.

So I guess it has to be a function in order to be placed in front of main() so the buffer can overflow into the no longer needed code at the start of it.