Output parameters in C
Factor handbook > C library interface > Passing data between Factor and C

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A frequently-occurring idiom in C code is the "out parameter". If a C function returns more than one value, the caller passes pointers of the correct type, and the C function writes its return values to those locations.
with-out-parameters ( c-types quot -- values... )

The idiom is commonly used for passing back an error message if the function calls fails. For example, if a function is declared in the following way:
FUNCTION: int do_frob ( int arg1, char** errptr )

Then it could return 1 on error and 0 otherwise. A correct way to call it would be:
1234 { c-string } [ do_frob ] with-out-parameters

which would put the functions return value and error string on the stack.