Home/ Questions/Quick handling of the implicit declaration of function is invalid in c99 error.
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Vivian Singh
  • 14
Asked: 2022-05-18T17:07:10+00:00 In:

Quick handling of the implicit declaration of function is invalid in c99 error.

  • 14

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Here is the program I run:

#include <stdio.h>
 
 int main(int argc, const char * argv[])
 {
  int input;
  printf("Please give me a number : ");
  scanf("%d", &input);
  getchar();
  printf("The fibonacci number of %d is : %d", input, Fibonacci(input)); //!!!
 
 }/* main */
 
 int Fibonacci(int number)
 {
  if(number<=1){
  return number;
  }else{
  int F = 0;
  int VV = 0;
  int V = 1;
  for (int I=2; I<=getal; I++) {
  F = VV+V;
  VV = V;
  V = F;
  }
  return F;
  }
 }/*Fibonacci*/

After I run, it returns an error:

warning : Implicit declaration of function 'Fibonacci' is invalid in C99

Does anyone have any suggestions for the problem below: implicit declaration of function is invalid in c99 in the c – How to correct it?

implicit declaration
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2 Answers

  1. Best Answer
    Expert

    The cause:

    You get the warning message because you don’t declare the function before you call it.

    Solution:

    You can declare the function by some ways:

    • If the function is called from some source files, you need to write the header. Simply write the prototype.

      int Fibonacci(int number);

      In a file .h (such as myfunctions.h) and #include "myfunctions.h" in C code.

    • Before the function is called for the first time, let’s move it. You can do this by writting down this function:

      int Fibonacci(int number){..}

      Before you use main()

    • Before the function is called for the first time, you have to declare it clearly.

      This is the combination the two elements. Before you main() function, you should type the prototype function in the C file.

    I have an additional note for you: The function int Fibonacci(int number) must be used only in the file in which it is implemented. It should be declared static in order not to appear in other translation unit except that unit.

    • 9

  2. Before it can use the function, the compiler needs to know about it.

    Before you call the function, declare it 

    #include <stdio.h>

int Fibonacci(int number); //now the compiler knows, what the signature looks like. this is all it needs for now

int main(int argc, const char * argv[])
{
 int input;
 printf("Please give me a number : ");
 scanf("%d", &input);
 getchar();
 printf("The fibonacci number of %d is : %d", input, Fibonacci(input)); //!!!

}/* main */

int Fibonacci(int number)
{
//…
    • 11

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