I'd like to point out that when calling a function that might set errno in C,

it's usually good practice to set errno to zero immediately before the call,

so as to be sure that any subsequent non-zero errno value was actually

a consequence of the function call:

errno = 0; /* clear errno before the function call */

rtnval = my_func(...);

if (errno != 0) {...}

This should probably be taken into account if you're extending

the FFI to provide access to errno.