Make functions call each other - C

Question

A noob question: I created a library called funcoes.h that have a menu() and other functions that can call menu(). An example:

void cifrar(){

 printf("\n\nDeseja cifrar outra mensagem? Digite 1 para Sim ou 2 para sair: ");
 scanf("%d", &exit);

 if(exit == 1){
      cifrar();
         }
 else{
      menu();
      }

}

void menu(){
     printf("Escolha uma das opcoes: ");         
     scanf("%d", &varMenu);
     switch(varMenu){
                     case 1:
                          system("cls");
                          cifrar();
                          break;
                     case 2:
                          system("cls");
                          decifrar();
                          break;
                     case 3:
                          system("cls");
                          sair();
                          break;
                     default:
                          system("cls");   
                          printf("Escolha uma opcao valida!\n\n");
                          menu();
                          break;
     }         
}

But when I compile, I have this error:

In function 'void cifrar()'
'menu' undeclared(first use this function)"

'void menu()' used prior to declaration

How to make them call each other without this error?

Thanks!

`library called funcoes.h`... is it really so? i'm interested. — Sourav Ghosh, Nov 21 '14 at 14:17
do you declare all those functions' prototypes at the begining of the `.h` file? — oopcode, Nov 21 '14 at 14:19
You need to define `void menu()` before you define any other function. Or forward declare its function signature, at least. — AndyG, Nov 21 '14 at 14:20
You have to declare the function prototypes in a header `.h` file (that's what it's called, not library) — bitcell, Nov 21 '14 at 14:20
Sorry for the question, I resolved following this link: http://stackoverflow.com/questions/14598812/how-to-have-two-functions-that-call-each-other-c — Victor Ide, Nov 21 '14 at 14:22
You need to declare void menu(); before using it in cifrar() function. — Vishal Gupta, Nov 21 '14 at 14:22
But Why I do have to declare void menu() empty before any other function? It doenst make sense to me — Victor Ide, Nov 21 '14 at 14:23
not before ANY other function, just before you use it. every function that you call has to be delcared BEFORE that call. it's as simple as that. — Philipp Murry, Nov 21 '14 at 14:25
Yeah I did. The problem was only not declaring it before using. — Victor Ide, Nov 22 '14 at 18:26

score 3 · Answer 1 · answered Nov 21 '14 at 14:28

every function that you call has to be declared BEFORE that call. you can do this by using a prototype of that function:

void menu();

void cifrar() {
  ...
}

void menu() {

  ..
}

or simply by putting the whole main function (with it's body) on top of cifrar.

score 2 · Answer 2 · answered Nov 21 '14 at 14:30

Well, maybe it would be nice to sum up what is in comments.

The compiler wants to know any function's prototype before this function is used somewhere else. 'Before' here means something like 'earlier in the source file'. You can, although, place all the prototypes in a separate .h file, include it in the .c file with actual code, and then place function implementations in whatever order you like - the compiler will not complain.

score 2 · Answer 3 · answered Nov 21 '14 at 14:41

What you should do, is to create a header file, which will have the signatures of all your functions and then you do not need to worry about where each function is located at the code, you will be able to use all of the functions all over the code.

Your code should look like this:

funcoes.h

void cifrar(void);
void menu(void);

funcoes.c

#include "funcoes.h"

void cifrar(void){

 printf("\n\nDeseja cifrar outra mensagem? Digite 1 para Sim ou 2 para sair: ");
 scanf("%d", &exit);

 if(exit == 1){
      cifrar();
         }
 else{
      menu();
      }
}

void menu(void){
     printf("Escolha uma das opcoes: ");         
     scanf("%d", &varMenu);
     switch(varMenu){
                     case 1:
                          system("cls");
                          cifrar();
                          break;
                     case 2:
                          system("cls");
                          decifrar();
                          break;
                     case 3:
                          system("cls");
                          sair();
                          break;
                     default:
                          system("cls");   
                          printf("Escolha uma opcao valida!\n\n");
                          menu();
                          break;
     }         
}

Another small tip, do not create functions without any arguments, such as:

void menu();

Always insert the arguments you want to pass. If you want the functions to get not arguments, just pass void.

void menu (void);

Make functions call each other - C

3 Answers3