I recently posted a request for help, but had the problem solved
via another channel. The problem was that ACLPC would send floats into a C
function, but would not get them out.

The problem is that ACLPC will only allow :single-float as a return type
for C functions, but C is always passing back 'double's on the stack (even
if the function is declared 'float').  The sol'n is to declare a dummy 4
byte structure, 'LispFloat', (which C will pass back on the stack), cast
the single 'float' pointer to a 'LispFloat' pointer and then dereference
the LispFloat pointer as the return value.