Calling C functions from fortran - c

Question

I'm trying to call C functions using fortran (need this for a project).
So first was trying to simply call a non parametrized, void function via fortran.
Kindly help me resolve the following errors in the given code.
C Code for matrix multiplication:
#include <stdio.h>
extern "C"
{ void __stdcall mat();
}
void mat()
{
int m, n, p, q, c, d, k, sum = 0;
printf("Enter the number of rows and columns of first matrix\n");
scanf("%d%d", &m, &n);
int first[m][n];
printf("Enter the elements of first matrix\n");
for ( c = 0 ; c < m ; c++ )
for ( d = 0 ; d < n ; d++ )
scanf("%d", &first[c][d]);
printf("Enter the number of rows and columns of second matrix\n");
scanf("%d%d", &p, &q);
int second[p][q];
if ( n != p )
printf("Matrices with entered orders can't be multiplied with each other.\n");
else
{
printf("Enter the elements of second matrix\n");
for ( c = 0 ; c < p ; c++ )
for ( d = 0 ; d < q ; d++ )
scanf("%d", &second[c][d]);
int multiply[m][q];
for ( c = 0 ; c < m ; c++ )
{
for ( d = 0 ; d < q ; d++ )
{
for ( k = 0 ; k < p ; k++ )
{
sum = sum + first[c][k]*second[k][d];
}
multiply[c][d] = sum;
sum = 0;
}
}
printf("Product of entered matrices:-\n");
for ( c = 0 ; c < m ; c++ )
{
for ( d = 0 ; d < q ; d++ )
printf("%d\t", multiply[c][d]);
printf("\n");
}
}
}
int main()
{
mat();
return 0;
}
Also, the code for calling the function mat from fortran that I wrote is :
program mat_mult !This is a main program.
call mat()
stop
end
On executing the C file I get the following error:
matrix_mult.c:5: error: expected identifier or ‘(’ before string constant
On executing the fortran file using the F77 compiler, I get the following error:
/tmp/ccQAveKc.o: In function MAIN__':
matrix_mult.f:(.text+0x19): undefined reference tomat_'
collect2: ld returned 1 exit status
Kindly help me in identifying the error/correct code.
Thank you.

Answer 1

A few things first
1) I'm assuming you're using a C99 compiler because the C code you've written will not work on a C89 compiler.
2) extern "C" is for C++ programs: not C programs.
Not really sure which compiler you're using. Just assuming you're using gcc and gfortran since it looks like you're on a Linux based system.
gcc just adds a leading _ to the symbols: so mat becomes _mat.
gfortran adds both a leading and trailing _ to the symbols: so mat becomes _mat _.
To make C and Fortran talk
a) Remove the main function from the C code
b) Remove extern "C" declaration. This is a C++ declaration that tells the compiler that the routine mat should not have any name mangling.
c) Since you don't have any parameters, just assume _cdecl and change void mat() to void mat(). If you have to use stdcall, then you need to compile with --enable-stdcall-fixup. stdcall will be needed if the Fortran program has to pass parameters to the C program but that is a different ball game. Instead of _mat _, the compiler generates mat#0
d) Your C routine will now look like
#include <stdio.h>
void mat_()
{
...
}
/* No main */
e) Since the routine mat is not declared in the Fortran code, the compiler needs to know that it is external.
program mat_mult
external mat
call mat()
stop
end
f) Compile and link (say the C program is called mat.c and the fortran program is called matmul.f)
gcc -g -c mat.c
gfortran -g matmul.f mat.o -o matmul
There will probably be a whole load of comments recommending that you use F2003 or F2008 but if you have been told that you have to use F77, then you have to use F77.