Use struct from header file in main (in C)

Question

I have the following header file:

#ifndef COMPLEX_H_INCLUDED
#define COMPLEX_H_INCLUDED

typedef struct {
    double r; //real part
    double i; //imag part
} complex;

complex make(double r,double i);

#endif // COMPLEX_H_INCLUDED

and .c file:

#include <stdio.h>
#include <math.h>
#include "complex.h"

complex make(double re,double im)
{
    complex z;
    z.r=re;
    z.i=im;
    return z
}

Now, when I try to create a complex in a main file, I don't seem to be able to print a complex number that I've created.

#include <stdio.h>
#include <stdlib.h>
#include "complex.h"

int main()
{
    double a,b;
    printf("Enter real, then imaginary part:");
    scanf("%f %f",a,b);

    complex z;
    z=make(a,b);

    printf("The number is: %f%+fi",z.r,z.i);
    return 0;
}

I get an error: undefined reference to make.

Answer 1

The "undefined reference" error comes from the fact that you're not linking the oobject file produced by compiling complex.c into your executable. You need to add it to your linking command line or project settings, depending on how you develop. We have a FAQ that deals with this aspect of linking in great detail: What is an undefined reference/unresolved external symbol error and how do I fix it? (The FAQ is for C++, but this part of linking is the same in C).

---

In addition to this, however, you code has the following issue:

Building with warnings enabled (which you should always, always do) produces these warnings (among others) from your code:

main.cpp: In function 'main':

main.cpp:28:13: warning: format '%f' expects argument of type 'float *', but argument 2 has type 'double' [-Wformat=]
     scanf("%f %f",a,b);
            ~^     ~
main.cpp:28:16: warning: format '%f' expects argument of type 'float *', but argument 3 has type 'double' [-Wformat=]
     scanf("%f %f",a,b);
           ~^    ~

[Live example]

You're passing doubles where a float* is expected, so the program has Undefined Behaviour (and, as in the live example above, will most likely crash).

The correct form is to use pointers as arguments, and format specifiers for the correct type (lf for double):

scanf("%lf %lf",&a,&b);

[Live example]