Thread: Why put the prototype inside the function definition?

The following is part of the code found on page 110 in the book "The C Programming Language" by K & R.

Code:

void qsort(char *v[], int left, int right)
{
     int i, last;
     void swap(char *v[], int i, int j);

     /*rest of the code*/
}

What's the reasoning for having the prototype for swap() inside the function definition of qsort()? Ie, why isn't the prototype for swap() before main()?

Originally Posted by vart

to show that swap function is called only from the qsort probably

Yeah, I think you're right. Here is what happens when I put the prototype printagain() inside the function definition printme1().

Code:

[cdalten@localhost oakland]$ more prot.c
#include <stdio.h>

void printme1(void)
{
  char arr[] = "test";
  void printagain(char *);
  printf("This is inside printme1()\n");

  printagain(arr);
}


void printme2(void)
{
  char arr[] = "test";
  printf("This is inside printme2()\n");

  printagain(arr);
}


void printagain(char *s) {
  printf("%s\n", s);
}

int main(void)
{

  printme1();
  printme2();

  return 0;
}

[cdalten@localhost oakland]$ gcc -g -Wall prot.c -o prot
prot.c: In function ‘printme2’:
prot.c:18: warning: implicit declaration of function ‘printagain’
prot.c:6: warning: previous declaration of ‘printagain’ was here
prot.c:18: error: incompatible implicit declaration of function ‘printagain’
prot.c:6: error: previous implicit declaration of ‘printagain’ was here
[cdalten@localhost oakland]$

And now here is what happens when I put the prototype for printagain() at the very top. Ie, not in the function definitions.

Code:

[cdalten@localhost oakland]$ more prot.c
#include <stdio.h>

void printagain(char *); /*this is no longer inside the function defintion*/

void printme1(void)
{
  char arr[] = "test";
  printf("This is inside printme1()\n");

  printagain(arr);
}


void printme2(void)
{
  char arr[] = "test";
  printf("This is inside printme2()\n");

  printagain(arr);
}


void printagain(char *s) {
  printf("%s\n", s);
}

int main(void)
{

  printme1();
  printme2();

  return 0;
}

[cdalten@localhost oakland]$ gcc -g -Wall prot.c -o prot
[cdalten@localhost oakland]$ ./prot
This is inside printme1()
test
This is inside printme2()
test
[cdalten@localhost oakland]$

In the 1st program in post #4, have you tried calling the function printagain from the main function or placing the function printme2 after the definition of function printagain??

I guess in the above two cases, the errors that were listed will not appear. It all depends on how you place the code.

Originally Posted by neandrake

But isn't the function made available after it's defined? So would you have includes at the bottom of your C files to include "hidden" functions? Or is there another model for using them?

Prototypes are scoped, so putting a prototype inside a function does not make it world visible.

On page 297 of Prata's "C Primer" he prototypes a function twice, once in the header and then in the main function. No explanation why, confused the hell out of me.

Originally Posted by neandrake

I understand, but the function definition puts that function at global scope, correct?

It puts that definition at file scope. If no prototype exists in the header that other source files include, those other source files have no way of calling the function.

Originally Posted by neandrake

Then why not just put the prototypes at the top of your c file, why make them only inside the function? I'm missing the point of being able to do this. Also, prototypes have no effect on the stack/heap right; they're just notifications to the compiler?

That is true; prototypes make no memory demands.

To be honest, I've always done as you described (put the helper function before the function that calls it); the only scenario I'm coming up with where it makes a difference is if function A has a variable called foo and function B needs to call a function called foo and people want to avoid confusion. I can't imagine that happens often, though.

I also need help with an answer for this, How would you use the c library function to solve a programming problem? Please give three examples.
Thank you