Can we use extern keyword with typedef datatype in header file?

I want to implement stack and its functions using static libraries in c. But even after declaring STACK in header file using extern i am getting errors that STACK is not declared.I have declared stack in a separate stack_1.c file and want other functions like push, pop etc to access this stack.Is it possible to do so?

List item

inside stack_main.c
```
#define MAX 5
#include "lib.h"

void push(int, STACK *);
int pop(STACK *);
void display(STACK *);

int main()
{
    int flag,ele,show;

    STACK s;
    s.top=-1;

    while(1)
    {
        printf("Enter 1:push 2:pop 3:display 4:exit\n");
        scanf("%d",&flag);

        switch(flag)
        {
           case 1:
            printf("Enter ele\n");
            scanf("%d",&ele);
            push(ele,&s);
            break;
```
inside lib.h
```
extern STACK;
int main();
void push(int , STACK *);
int pop(STACK *);
void display(STACK *);
```
inside push.c
```
#include "lib.h"       
void push(int ele, STACK* a)
{
     if(a->top!= MAX-1)
     {
          a->top++;
          a->stk[a->top]=ele;
     }
}
```
inside stack_1.c
```
#define MAX 5
#include "lib.h"

typedef struct {

        int stk[MAX] ;
        int top;       
}STACK;
```
ERRORS:

ani@ani-VirtualBox:~/Documents/ostl/week4$ gcc -c stack_main.c stack_1.c push.c pop.c display.c In file included from stack_main.c:4:0: lib.h:1:8: warning: type defaults to ‘int’ in declaration of ‘STACK’ [-Wimplicit-int] extern STACK; ^~~~~ lib.h:3:17: error: expected declaration specifiers or ‘...’ before ‘STACK’ void push(int , STACK *); ^~~~~ lib.h:4:9: error: expected declaration specifiers or ‘...’ before ‘STACK’ int pop(STACK *); ^~~~~ lib.h:5:14: error: expected declaration specifiers or ‘...’ before ‘STACK’ void display(STACK *); ^~~~~ stack_main.c:8:16: error: expected declaration specifiers or ‘...’ before ‘STACK’ void push(int, STACK *); ^~~~~ stack_main.c:9:9: error: expected declaration specifiers or ‘...’ before ‘STACK’ int pop(STACK *); ^~~~~ stack_main.c:10:14: error: expected declaration specifiers or ‘...’ before ‘STACK’ void display(STACK *); ^~~~~ stack_main.c: In function ‘main’: stack_main.c:16:11: error: expected ‘;’ before ‘s’ STACK s; ^ stack_main.c:17:5: error: ‘s’ undeclared (first use in this function) s.top=-1;

Solution

.I have declared stack in a separate stack_1.c file and want other functions like push, pop etc to access this stack.

No, you will have to add the definition of the type STACK in a header file which is included in stack_main.c and push.c.

Easiest option is to move the definition of the type STACK to the header file lib.h

What you can also do is to define the structure in the stack_1.c and the typedef in the header file. This can hide your internal workings of the STACK, from the external world. However in your case, i would go with the first approach.

e.g. in stack_1.c (or preferably in another header file)

struct stack1{

    int stk[MAX] ;
    int top;  
};

and in lib.h

typedef struct stack1 STACK;