The macros F77_FALSE and F77_TRUE expand to the numerical values that FORTRAN treats as false and true (e.g. 0 and 1). They should be used when setting logical values to be returned to the calling FORTRAN routine. There are also macros F77_ISFALSE and F77_ISTRUE that should be used when testing a function argument for truth or falsehood.
CNF and F77 Mixed Language Programming -- FORTRAN and C