How to forward typedef’d struct in .h

Move the typedef struct Preprocessor Prepro; to the header the file and the definition in the c file along with the Prepro_init definition. This is will forward declare it for you with no issues.

Preprocessor.h

#ifndef _PREPROCESSOR_H_
#define _PREPROCESSOR_H_

#define MAX_FILES 15

typedef struct Preprocessor Prepro;

void Prepro_init(Prepro* p);

#endif

Preprocessor.c

#include "Preprocessor.h"

#include <stdio.h>

struct Preprocessor {
    FILE fileVector[MAX_FILES];
    int currentFile;
};

void Prepro_init(Prepro* p) {
    (*p).currentFile = 0;
}