Call a C library from main Fortran code

Help
1

I have a C library that downloaded it from [here]. This library has some files in the “.dll” format and some files in the “.exe” format. I want to call it from Fortran code, probably as a subroutine. I want to know how can I introduce the library to Fortran that finds it easily. Which command and instruction are proper to call it in the main code.

I use IDE (Geany with the last version of gcc and Plato with an older version of gnu). Please, explain your solution in detail.

I will appreciate any comments that give me a little help and forgive me for writing shortcomings.
(qvoronoi -- Voronoi diagram)

2

It depends on the compiler you use - apparently you use gcc and gfortran on Windows. On Windows some compilers will require a so-called import library, but the Gnu compilers work directly with the DLL IIRC.
That said, the first taks will be to write an interface for the C routines you want to use. Not all this difficult, if you use the ISO_C_BINDING module and the bind() attribute. The Fortran Wiki (http://fortranwiki.org) has more information about that. Here is a page you may find useful: c_interface_module in Fortran Wiki

3

For these compilers, some C functions can be called without using the C interop features of Fortran 2003. For example, the C code

#include <stdlib.h>
void sum_abs_int_(int *in, int *num, int *out) {
    int i,sum;
    sum = 0;
    for (i=0; i < *num; ++i) {sum += abs(in[i]);}
    *out = sum;
    return;
}

void sum_abs_int_val_(int *in, int num, int *out) {
	int i,sum;
	sum = 0;
	for (i=0; i < num; ++i) {sum += abs(in[i]);}
	*out = sum;
	return;
}

void sum_abs_double_(double *in, int *num, double *out) {
    int i;
    double sum;
    sum = 0;
    for (i=0; i < *num; ++i) {sum += abs(in[i]);}
    *out = sum;
    return;
}

double twice_(double *x) {
	return *x*2;
}

double thrice(double *x) {
	return *x * 3;
}

can be called from

program xcallc
implicit none
integer, parameter :: n = 3, ivec(n)=[4,-2,7], dp = kind(1.0d0)
integer            :: isum
real(kind=dp)      :: dsum,twice
write (*,"(a,3(1x,i0))") "ivec =",ivec
call sum_abs_int(ivec,n,isum)
print*,"sum(abs(ivec))=",isum
call sum_abs_int_val(ivec,%val(n),isum) ! use %val() extension to pass by value
print*,"sum(abs(ivec))=",isum
call sum_abs_double(dble(ivec),n,dsum)
print*,"sum(abs(dble(ivec)))=",dsum
print*,twice(5.3d0)
end program xcallc

with the compilation script

gfortran -c xcallc.f90
gcc -c cforf.c
gfortran cforf.o xcallc.o
if exist a.exe a.exe

giving

ivec = 4 -2 7
 sum(abs(ivec))=          13
 sum(abs(ivec))=          13
 sum(abs(dble(ivec)))=   13.000000000000000     
   10.600000000000000

and the C function thrice() that does not have an underscore can be called from Fortran code that is compiled with the gfortran -fno-underscoring option. Admittedly the Fortran code is nonstandard since it uses the %val() function, and the above procedure may not work for other pairs of compilers.

4

Thanks for your comment. One part of my question is about:
I download a library from here. There is a bunch of code in there. I want to know how Fortran find the library and which file was needed for introducing it to Fortran?
I read the links. It described the preparation of variables not introducing it.

5

There is nothing magic about that .I am afraid: you will have to help the compiler. It does look like you are new to this - perhaps Building programs - Fortran Programming Language helps? I apologize if I underestimate your knowledge, though.

6

Another thing: I had a look at the documentation of the qhull library. It emphasizes the use of qhull via its C++ interface rather than via C. It is certainly possible to use the C API, but it seems it exposes you to the internal structures much more. An alternative is perhaps to interface with the program instead of the library - that would mean to write the relevant input files and read back in the results. That may be a quick(er) way to determine if qhull will help you solve the problems you want to study.

7

First of all, thanks for the link you sent. That link was very useful. In addition, I will always be eager to learn. So if you have additional resources please add them.

If I understand correctly:
I must first write my original source file.
Then I write my other files either as a module or as a subroutine.
Prepare the file related to the C program.
Compile the attached Fortran files separately. I can convert them to a library file if I want.
Convert file C to dynamic library.
Link them to the original file.
Then build the whole project. Building is done via using software (GUI or CMAKE).
I can finally run it.

My question is here:
It seems to be a subroutine interface with C library file. Then call this subroutine interface in the main program. It is true?