Re: error calling c functions from gfortran

[FX Coudert <fxcoudert at gmail dot com>]

> I get the wrong answer calling c programs from a gfortran program.  
> The same c
> functions work when called from a c main program. I guess I need  
> some kind of
> interface statement to call c functions from gfortran. Can anyone  
> help me?

A few hints:
  1. Fortran passes arguments by reference, so your C functions need  
to be changed into:

int lshift_(int *Num2Shift, int *NumBits ) { return(*Num2Shift <<  
*NumBits); }
int rshift_(int *Num2Shift, int *NumBits ) { return(*Num2Shift >>  
*NumBits); }

  2. The rshift and lshift in your Fortran code are declared  
external, but are not given a type. You need to specify that they  
return integer values, by adding "integer rshift, lshift". Still  
better, you should ask for implicit typing not to be used (it causes  
much trouble, especially to beginners), by adding the line "implicit  
none" just before your type declarations:

      program ingrid
      implicit none
      integer rshift, lshift
      external rshift,lshift
      integer ans
c      read(*,*) i,j
c rshift(i,j): i=# to shift, j=#bits to shift
      ans=rshift(8,1)
      print *,"rshift(8,1)=",ans
      ans=lshift(8,1)
      print *,"lshift(8,1)=",ans
c      write(*,*) ans
      end

With that, it should work:

$ gfortran a.c a.f && ./a.out
rshift(8,1)=           4
lshift(8,1)=          16


FX