Example 1: Main Program in C, with Subroutines in C, C++, and FORTRAN
The C program is nothing out of the ordinary: it defines two variables, and calls various functions that change those variables' values. C requires that we use a "call by reference" syntax to make these changes persistent, rather than its default "call by value" method. Note that the name of the FORTRAN function called from the C program is ffunction_, a name we extracted via the nm command shown above. Note also that the C++ function has an extern "C" directive above the code of the function, indicating not that cppfunction() is written in C, but that it is called from a C-style interface instead of a C++ interface.
File cprogram.c:
#include <stdio.h>
int main(void) {
float a=1.0, b=2.0;
printf("Before running Fortran function:\n");
printf("a=%f\n",a);
printf("b=%f\n",b);
ffunction_(&a,&b);
printf("After running Fortran function:\n");
printf("a=%f\n",a);
printf("b=%f\n",b);
printf("Before running C++ function:\n");
printf("a=%f\n",a);
printf("b=%f\n",b);
cppfunction(&a,&b);
printf("After running C++ function:\n");
printf("a=%f\n",a);
printf("b=%f\n",b);
printf("Before running C function:\n");
printf("a=%f\n",a);
printf("b=%f\n",b);
cfunction(&a,&b);
printf("After running C function:\n");
printf("a=%f\n",a);
printf("b=%f\n",b);
return 0;
}
File ffunction.f:
subroutine ffunction(a,b)
a=3.0
b=4.0
end
File cppfunction.C:
extern "C" {
void cppfunction(float *a, float *b);
}
void cppfunction(float *a, float *b) {
*a=5.0;
*b=6.0;
}
File cfunction1.c:
void cfunction(float *a, float *b) {
*a=7.0;
*b=8.0;
}
Compilation Steps: each program is compiled into an object file using the appropriate compiler with the -c flag. After all the object files are created, the final gcc command links the object files together into a single executable:
gcc -c cprogram.c
g77 -c ffunction.f
g++ -c cppfunction.C
gcc -c cfunction1.c
gcc -o cprogram cprogram.o ffunction.o cppfunction.o cfunction1.o
Though this example problem does not require it, many of the math functions (for example, sin, cos, pow, etc.) require that you also link in the libm math library. Add a -lm flag to the final gcc command above to link in the math library.
Results:
./cprogram
Before running Fortran function:
a=1.000000
b=2.000000
After running Fortran function:
a=3.000000
b=4.000000
Before running C++ function:
a=3.000000
b=4.000000
After running C++ function:
a=5.000000
b=6.000000
Before running C function:
a=5.000000
b=6.000000
After running C function:
a=7.000000
b=8.000000