[ranjithsutari]

In this program function parameter is an array of characters(string). In this code the size of function parameter is 20 bytes and the size of local variables in function is only 4 bytes.
Please explain why this happens
why the size of local variable is only 4 bytes why not less or more
why buffer overflow does not happens

/*
*	Sample Code to demonstrate the size of different variables
*
*	By Ranjith Sutari, 2007
*/
#include<stdio.h>

/*Function to Read the String 
* the size of the local variable in this function is only 4 bytes
*/
int read_string(char prompt[], char answer[], int max)
{

	fputs(prompt, stdout);
	fflush(stdin);
	fgets(answer, max, stdin);
	printf("size of the local variable answer in function read_string is %d\n", sizeof answer);
	return 0;

}

/* Function to print the string
*	the size of the local variable in this function is 4 bytes only
*/

int print_string(char prompt[], char answer[])
{
	fputs(prompt, stdout);
	printf("%s", answer);
	printf("Size of the local variable answer in function print_string is %d", sizeof answer);
	return 0;
}

int main()
{
	char STRING[20];

	/*size of the STRING is 20 bytes
	*/
	printf("Size of The variable string in function main at beginning is %d\n\n", sizeof string);
	
	read_string("What is your Name : ", STRING, sizeof STRING); /* STRING IS PASSED AS FUNCTION
																PARAMETER*/
	print_string("Your name is ", string);						
	
	/* size of the string is 20 bytes*/
	printf("\n\nSize of The variable string in function main at end is %d\n", sizeof string);
	fflush(stdin);
	getchar();

	return 0;
}

Thanks in Advance
Ranjith Sutari

[Salem]

First off, fflush(stdin) is undefined. Flushing a stream is only valid for output streams, or update streams when the last operation was a write.
http://faq.cprogramming.com/cgi-bin/...&id=1043284351

As for your main question, the sizeof is applied to the parameter value (in this case, a pointer). The sizeof operator can't work out how much data a pointer points to (see example). Using [ ] instead of pointer notation in your function declaration doesn't change anything.

Eg.

void foo ( const char *a ) {
  printf("%lu\n", sizeof(a) );
}
int main ( ) {
  char a[] = "hello";
  char b[] = "this is a longer string";
  foo(a);
  foo(b);
  return 0;
}

Also see http://c-faq.com/aryptr/aryparmsize.html