How it works?

[match88]

I have a question about the implementation&interface in different free libraries, such as GNU Scientific Library. But this is just a common question, not concerned about this particular package.

I'm using the example from the official GNU web site: http://www.gnu.org/software/gsl/manual/html_node/An-Example-Program.html#An-Example-Program

#include <stdio.h>
#include <gsl/gsl_sf_bessel.h>

int
main (void)
{
  double x = 5.0;
  double y = gsl_sf_bessel_J0 (x);
  printf ("J0(%g) = %.18e\n", x, y);
  return 0;
}

And everything works fine...

But my question here is about implementation: I looked into gsl_sf_bessel.h header file and there is only function prototypes here (for example for our function: double gsl_sf_bessel_J0(const double x);), and that's it. No implementation...

Is it possible to see the implementation (is there gsl_sf_bessel.cpp files for example?). And if it's not: How it works?
How could we use only header files (interface) to use those functions? Where the system gets the information about the interface of this function?

[match88]

I'm feeling, that probably it's somehow related to the libraries, which also installed when I did the installation of GNU Scientific, but I'm not sure...
So, is it only header files and implementaion is somewhere else (like in some dynamic library where I can't see the implementation)? So I can't change the implementation if I wanted to, for example, but only use as-is, right?..

[snowright]

The implementations are in normal .c files just like usually, there just does not exists file "gsl_sf_bessel.c" exactly. The implementation of your example function "gsl_sf_bessel_J0" for example is found in file "bessel_J0.c". I can say from the specific header file that the implementations of its content are divided to quite a few source files but almost all of their names start with "bessel_" so they are not exactly hard to find even by manually searching. The files in question are in the "specfunc" folder if you want to take a look.

[xismn]

Most of the time, the implementations are housed in static-libraries (.a or .lib extensions), or in dynamic (link) libraries (also known as .dll).