segmentation fault in c++

drigz · 06-14-2004, 02:17 PM

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>

using namespace std;

char* newpwd(char* oldpwd)
{
	char* newpw;
	for (int i=0; i<8; i++)
		newpw[i] = (char) (97+rand()%26);
	newpw[8]='\0';
	strcpy(oldpwd, newpw);
	
	return oldpwd;
}

main ()
{
	char entered[80];
	char real[80];
	srand((unsigned int) time (NULL));
	printf("New password: %s\n", newpwd(real));
	for ( ; ; )
	{
	scanf("%s", entered);
	if (strcmp(entered, "new") == 0)
		printf("New password: %s\n", newpwd(real));
	else if (strcmp(entered, real) == 0)
		printf("Correct!\n");
	else
		printf("INCORRECT!\n");
	}
}

This program is to train me to use a new password, and to find one i can type easily.

however, i get a segmentation fault whenever i run it

what is wrong with it?

KneeLess · 06-14-2004, 03:23 PM

Well there are a few things to do, and note what you wrote is C not C++. (Subtle difference,

) To maintain compatibility with your main() you'll want to not make newpw a pointer, rather an array... like this

Code:

char* newpwd(char* oldpwd)
{
	char newpw[8]; /* See? */
	for (int i=0; i<8; i++)
		newpw[i] = (char) (97+rand()%26);
	newpw[8]='\0';
	strcpy(oldpwd, newpw);
	
	return oldpwd;
}

The problem was that char *newpw is just a pointer to a char, it does not automatically allocate space. if you wanted to do that you'd have to do this.

Code:

#include <malloc.h>

char* newpwd(char* oldpwd)
{
	char* newpw = calloc(8, sizeof(char*));
	for (int i=0; i<8; i++)
		newpw[i] = (char) (97+rand()%26);
	newpw[8]='\0';
	strcpy(oldpwd, newpw);
	
	return oldpwd;
}

And this program suffers from a buffer overflow too in main() from char entered[80]. What if I enter a password that is 1024 characters long? Think of a solution for that.

Also note that a bash script that uses mkpasswd would probably be easier to write, more effient, and more secure.

drigz · 06-14-2004, 03:54 PM

thanks, it works now.

btw it doesnt compile as c because of hte for (int i=0..... so i have to use c++ to compile it so its c++ in my opinion.

KneeLess · 06-14-2004, 04:25 PM

Okay, whatever.

One thing I forgot is if you're using the one with calloc() make sure to free() your pointer!

aluser · 06-14-2004, 06:16 PM

you should allocate 9 chars if you're going to use an index like [8]. If you only allocate 8, it may segfault for some compilers, or overwrite stuff you don't want to overwrite etc.

blanks · 06-15-2004, 11:04 AM

Quote:

btw it doesnt compile as c because of hte for (int i=0..... so i have to use c++ to compile it so its c++ in my opinion.

just declare a variable, i, in that function, as in:

Code:

char* newpwd(char* oldpwd)
{
	char newpw[8]; 
        int i;   /**********here********/

	for (i=0; i<8; i++)
		newpw[i] = (char) (97+rand()%26);
	newpw[8]='\0';
	strcpy(oldpwd, newpw);
	
	return oldpwd;
}

now you have straight C and can compile with a C compiler.

drigz · 06-15-2004, 11:11 AM

i know, but why bother?

aluser · 06-15-2004, 11:50 AM

Quote:

i know, but why bother?

portability. There are more C compilers out there than c++ compilers. Once upon a time the gnu c++ compiler generated slower code, even when invoked on valid C -- I don't know if that's still true. Still, if you are going to require a c++ compiler you might was well make use of the stl <string> class or at least switch to operator "new" and "delete" instead of malloc/calloc/free. If your code gets linked to other peoples' projects, it's doubly important for it to not be schizophenic about it's c or c++ness, as they link differently.

drigz · 06-15-2004, 12:43 PM

im not gonna distribute it - who needs it - it only took me more than 2 mins to make cos of the seg fault.

aluser · 06-15-2004, 03:38 PM

well sure, but I'm guessing part of the reason for writing it was to help move on to bigger and better things : )

drigz · 06-16-2004, 03:06 PM

nope. i just needed a way to get a new password which i liked.