I have a code example like this:

#+BEGIN_SRC C

#include <stdio.h>

#include <math.h>

/* define complex struct */

struct complex_struct {

=C2=A0 double x, y;

};

/* some helper functions on complex struct */

double real_part(struct complex_struct z) {

=C2=A0 return z.x;

}

double img_part(struct complex_struct z) {

=C2=A0 return z.y;

}

double magnitude(struct complex_struct z) {

=C2=A0 return sqrt(z.x * z.x + z.y * z.y);

}

double angle(struct complex_struct z) {<= /div>

=C2=A0 return atan2(z.y, z.x);

}

/* helper functions to construct complex variable */

struct complex_struct make_from_real_img(double x, double y) {

=C2=A0 struct complex_struct z;

=C2=A0 z.x =3D x;

=C2=A0 z.y =3D y;

=C2=A0 return z;

}

struct complex_struct make_from_mag_ang(double r, double A) {

=C2=A0 struct complex_struct z;

=C2=A0 z.x =3D r * cos(A);

=C2=A0 z.y =3D r * sin(A);

=C2=A0 return z;

}

/* implement complex arithemtic */

struct complex_struct add_complex(struct complex_struct z1, struct complex_struct z2) {

=C2=A0 return make_from_real_img(real_part(z1) = + real_part(z2), img_part(z1) + img_part(z2));

}

int main(int argc, char *argv[]) {

=C2=A0 struct complex_struct z1, z2 =3D {1.1, 2= .4};

=C2=A0=C2=A0

=C2=A0 struct complex_struct z;

=C2=A0 z =3D add_complex(z1, z2);

=C2=A0=C2=A0

=C2=A0 printf("%f", z);

=C2=A0 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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