When I try to compile my code in Visual Studio, I get these errors:

unresolved external symbol s_signal
unresolved external symbol s_list

I believe I have included the relevant m_pd.h header file, and have linked to pd.lib correctly.

This code should reproduce the same issue:

#include "m_pd.h"

#define _USE_MATH_DEFINES  // defines maths constants (such as M_PI)
#define MAX_BLOCK_SIZE    4096
#define MAX_ORDER 100

static t_class *example_class;

typedef struct _example {
    t_object x_obj;
    t_sample f_order;
    t_sample f;
    t_float prev[MAX_BLOCK_SIZE];
    t_float as[MAX_ORDER];
} t_example;

void *example_new(void)
{
    // constructor
    t_example *x = (t_example *)pd_new(example_class);

    // create inlets
    inlet_new(&x->x_obj,&x->x_obj.ob_pd,&s_list,gensym("setas"));

    // create outlets
    outlet_new(&x->x_obj, &s_signal);
    
    return (void *)x;
}

With https://github.com/pure-data/pure-data/blob/master/src/m_pd.h as my m_pd.h file.

I’m a total beginner to C, Visual Studio, and Pure Data, fyi, and the code is over 10 years old, given to me by someone else. I tried compiling it, and initially there were 20-something unresolved externals which I fixed by adding a linker to pd.lib. After that, it was just the two errors mentioned prior. I have looked for definitions of s_list and s_signal online with no luck.