PRB: Incorrect Results Returned, C Float Functions

ID: Q79381

The information in this article applies to:

Microsoft FORTRAN for MS-DOS, versions 4.1, 5.0, 5.1
Microsoft FORTRAN for OS/2, versions 4.1, 5.0, 5.1

SYMPTOMS

When using the Microsoft FORTRAN Compiler version 4.1, 5.0, or 5.1, if FORTRAN code attempts to access a C float function, incorrect results may be returned from the C function.

CAUSE

FORTRAN expects a DOUBLE PRECISION number to be returned from both REAL and DOUBLE PRECISION functions. C functions of type float return only 4 bytes, and the FORTRAN run-time incorrectly interprets the value stored in the floating accumulator (__fac) as an 8-byte number.

RESOLUTION

This problem can be avoided by declaring the C function as type double, or by declaring the C function with the _fortran attribute.

MORE INFORMATION

Sample Code #1

The following code reproduces the problem:

FORTRAN Code


       interface to real function cthing[c](dog)
       real dog[value]
       end

       real dog
       real cthing,r
       dog=2
       r=cthing(dog)
       write(*,*) r
       end

C Code


#include <stdio.h>

float cthing(float dog)
{
   float cat;
   printf("%f \n",dog);
   cat=3.0;
   printf("%f \n",cat);
   return cat;
}

Output: Incorrect Results Generated Here

2.000000
3.000000
0.000000E+00

Sample Code #2

The following solution declares the C function with the _fortran attribute, and generates the expected output:

FORTRAN Code


       real dog,r
       real cthing
       dog=2.0
       r=cthing(dog)
       write(*,*) r
       end

C Code


#include <stdio.h>

float _fortran cthing(float *dog)
{
  float cat;
  printf("%f \n",*dog);
  cat=3.0;
  printf("%f \n",cat);
  return cat;
}

Output

2.000000
3.000000
3.000000

Additional query words: 5.00 5.10 mixed nofps language

Keywords :
Version : :4.1,5.0,5.1
Platform :
Issue type :