I have a code example like this:

#+BEGIN_SRC C
#include <stdio.h>
#include <math.h>

/* define complex struct */
struct complex_struct {
  double x, y;
};

/* some helper functions on complex struct */
double real_part(struct complex_struct z) {
  return z.x;
}
double img_part(struct complex_struct z) {
  return z.y;
}
double magnitude(struct complex_struct z) {
  return sqrt(z.x * z.x + z.y * z.y);
}
double angle(struct complex_struct z) {
  return atan2(z.y, z.x);
}

/* helper functions to construct complex variable */
struct complex_struct make_from_real_img(double x, double y) {
  struct complex_struct z;
  z.x = x;
  z.y = y;
  return z;
}

struct complex_struct make_from_mag_ang(double r, double A) {
  struct complex_struct z;
  z.x = r * cos(A);
  z.y = r * sin(A);
  return z;
}

/* implement complex arithemtic */
struct complex_struct add_complex(struct complex_struct z1, struct
complex_struct z2) {
  return make_from_real_img(real_part(z1) + real_part(z2), img_part(z1) +
img_part(z2));
}

int main(int argc, char *argv[]) {
  struct complex_struct z1, z2 = {1.1, 2.4};

  struct complex_struct z;
  z = add_complex(z1, z2);

  printf("%f", z);
  return 0;
}
#+END_SRC

But evaluate it got error:

```
/tmp/cckFlXlJ.o: In function `magnitude':
C-src-18467gDZ.c:(.text+0xa8): undefined reference to `sqrt'
/tmp/cckFlXlJ.o: In function `angle':
C-src-18467gDZ.c:(.text+0xfe): undefined reference to `atan2'
/tmp/cckFlXlJ.o: In function `make_from_mag_ang':
C-src-18467gDZ.c:(.text+0x174): undefined reference to `cos'
C-src-18467gDZ.c:(.text+0x190): undefined reference to `sin'
collect2: error: ld returned 1 exit status
zsh:1: no such file or directory: /tmp/babel-18467-Yn/C-bin-18467tNf
```

So I think `ob-C.el` doesn't support to load included header files.

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