Thread: Newbie question

Code:

#include <stdio.h>
#include <string.h>
#include <cstdlib>

int length, i, intNumber, sum, power = 1;
char number[8], sWhich[1], which = 'n';

int main(){
	printf("Type in a number: ");
	gets(number);
	length = strlen(number);
	
	if (number[0] == '0'){
		which = 'b';
		for (i=1; i<length; i++){
			if (number[i] > 49 || number[i] < 48){
				printf("That is an invalid number!");
				exit(1);
			}
		}
	}
	
	for (i=0; i<length; i++){
		if (number[i] > 49 && number[i] < 58){
			if (which = 'n') {
				which = 'd';
			}
		}else if (number[i] < 48 || number[i] > 57){
			printf ("That is an invalid number!");
			exit(1);
		}
	}

	if (which == 'n'){
		printf("Is this number decimal (d) or binary (b)? ");
		gets(sWhich);
		which = sWhich[0];
		number[0] = 49;    /* Why is this necessary? number[0] becomes null otherwise */
	}
	
	sum = 1;
	for (i=length-1; i>=0; i--){
		intNumber += ((number[i] - 48) * sum);
		sum *= 10;
	}

	if (which == 'd' && intNumber > 255){
		printf("That is an invalid number!");
		exit(1);
	}
	
	if (which == 'b'){
		sum = 0;
		power = 1;
		for (i=length - 1; i>=0; i--){
			if (number[i] == 49){
				sum += power;
			}
			power *= 2;
		}
		printf("Converting binary to decimal. Answer is: %d", sum);
	}
	
	if (which == 'd'){
		printf("Converting decimal to binary. Answer is: ");
		sum = 128;
		for (i=0; i<8; i++){
			if (intNumber >= sum){
				printf("1");
				intNumber -= sum;
			}else{
				printf("0");
			}
			sum /= 2;
		}
	}
}

This is a simple binary/decimal converter. As per the comment, if I don't include the assignment "number[0] = 49", number[0] resets to null. If the program doesn't encounter "which = 'd'", then number[0] stays at whatever value the user assigned to it. I don't understand how or why gets(sWhich) or which=sWhich[0] is resetting number[0].

It works, but I'd like to avoid this in future, so any help much appreciated. Also don't be afraid to criticize my likely redundancies

Wait, the overflow runs into other global variables?? I had no idea. I'll be sure to steer clear of gets in future. Thanks again.

The overflow can "safely" overrun anything your program owns in memory. The only reason why the program should crash is if it tries to write to protected memory.

For example, if you use gets() on the stack:

Code:

char array[5];
char array2[5];

gets(array);

And enter a ten-character string, the rest will most likely end up in array2.

If you have it on the stack with nothing to catch it:

Code:

char array[5];

gets(array);

It will most likely end up overwriting the stack base pointer and return variable, as stack variables are stored backwards as an offset of the base pointer:

Code:

push %rbp /* pushes base to stack */
movl %rsp, %rbp /* base ptr = stack ptr */
movl $0, -0x4(%rbp) /* your array, offset by 4 (first four = return variable) */

/* if I wanted to access array[4] */
movl $0, -0x5(%rbp) /* stored backwards, series of five one-byte types */

/* so if gets() overwrites that, into array[6], for example.. */
movl $0, -0x3(%rbp) /* wut! this is modifying the return variable! */

/* now lets say it keeps going, and writes into array[9] */
movl $0, (%rbp) /* now, you just overwrote your base pointer */

You can see what happens if this continues.

Mayhem. Thanks for clarifying. I think I'll be sticking to fgets in future