How to properly use 2 structs definition in one single C source file

Question

I'd need some help with structs in C.

Basically I'm trying to create a Student and a Group struct definition. Keep in mind that Group struct will contain Student structs previously implemented.

Below my structs definition:

Student Struct: student.h

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define DEFAULT_MATRICOLA 815010

typedef struct Student{
    int matricola;
    int voto_archi;
    char *turno;
}Student;

int generateVoto(){
    return (rand() % (30 - 18)) + 18;
}

char* generateTurno(int matricola){
    char *res = malloc(2*sizeof(char) + 1);
    if(matricola % 2 == 0)
        res = "T2";
    else    
        res = "T1";
    return res;
}

void initializeStudent(Student s, int matricola){
    s.matricola = matricola;
    s.voto_archi = generateVoto();
    if(s.matricola % 2 == 0){
        strcpy(s.turno, "T2");
    }
    else{
        strcpy(s.turno, "T1");
    }
}

void showStudent(Student s){
    printf("Matricola: %d Voto: %d Turno: %s\n", s.matricola, s.voto_archi, s.turno);
}

Student createStudent(int matricola){
    int voto = generateVoto();
    char *turno = generateTurno(matricola);
    Student s = {matricola, voto, turno};
    return s;
}

Group Struct: group.h

#include "headers.h"
#include "student.h"
#define MAX_MEMBERS 4

typedef struct Group{
    int groupId;
    Student *members;
    int numMembers;
    boolean closed;
}Group;

Group createGroup(int groupId){
    Group g;
    g.groupId = groupId;
    g.members = malloc(MAX_MEMBERS * sizeof(Student) + 1);
    g.numMembers = 0;
    g.closed = FALSE;
    return g;
}

void printGroup(Group g){
    int index = g.numMembers;
    if(index == 0)
        printf(RED "Group %d is EMPTY\n" RESET, g.groupId);
    else{
        for(int i = 0; i < MAX_MEMBERS; i++)
                showStudent(g.members[i]);
        printf("\n");
    }
}

Now even an empty main.c class containing only #include "student.h and #include "group.h would fail compiling but if we only add one of these two it works good.

Here's compiler's output:

Now, at last, my question:

How to create a main.c class using both student.h and group.h files? What am I doing wrong?

You need header guards. See https://stackoverflow.com/questions/27810115/what-exactly-do-c-include-guards-do — Fantastic Mr Fox, Dec 27 '18 at 14:32
Possible duplicate of [What exactly do C include guards do?](https://stackoverflow.com/questions/27810115/what-exactly-do-c-include-guards-do) — , Dec 27 '18 at 14:34
Nice _picture_ of _text_! Why not just copy & paste the text?! — Clifford, Dec 27 '18 at 14:39

score 3 · Answer 1 · answered Dec 27 '18 at 14:36

You need to wrap all your header files in "include guards" so that if the header content has already been included, in any subsequent inclusion the content is skipped to prevent redefinitions:

For example for group.h you might have:

#if !defined GROUP_H
#define GROUP_H

// all Header file content here...

#endif // GROUP_H

where the macro (GROUP_H) in this case must be unique throughout teh project - it is conventional to use a name based on the file name.

An alternative supported by many toolchaisn is to use the #pragma once directive:

#pragma once

// all Header file content here...

This is less portable, but more fool-proof that a traditional include guard.

score 2 · Accepted Answer · answered Dec 27 '18 at 14:37

Now even an empty main.c class containing only #include "student.h and #include "group.h would fail compiling but if we only add one of these two it works good.

Apparently you lack the guard to protect your header files, which look something like this (for each .h file):

#ifndef STUDENT_H
#define STUDENT_H

// your header file goes here

#endif STUDENT_H

Alternately, you can use #pragma once at the beginning of each header file (which is supposedly a better and shorter way).

How to properly use 2 structs definition in one single C source file

2 Answers2