Question

How do you perform pass by variable on an interface function provided by a C library?

For example, I have the following interface function:


void f( double *d ) { *d = 7.5; }


In my application, I link through that library and call the function:


double p = 0;
f( &p );


However, I noticed p is still 0 and not 7.5.

Answer 1

Do you have the prototype for f in scope, in your application?

If you haven’t the compiler converts the address of p to an int; and if they aren’t the same size or use the same passing conventions or whatever, you have problems.

You should #include "the_proper_header.h" or you may directly add the prototype in your application, right before you call the function.

double p = 0;
void f(double *);
f( &p );