"C" program creates Seg fault

320mb · 06-23-2006, 08:10 PM

does anyone see a problem in my code here, it compiles without error, but when I run it........it Seg faults, thx

Code:

#include <stdio.h>

int main(int argc, char *argv[1]){
	char password[5] = "linux";
	char *pass=password;
    
    *strcpy(pass,argv[1]);
    	
     while(*pass){
		*pass=((*pass-'Z')*'A')%26+'Z';
		pass++;
	}
	printf("%s\n",password);
	return 0;
}

spooon · 06-23-2006, 08:14 PM

Quote:

Originally Posted by 320mb

Code:

	char password[5] = "linux";

you need at least 6 (for the 5 letters and the null terminator)

also, password must have enough space to store the string copied from argv[1]

taylor_venable · 06-23-2006, 08:16 PM

Quote:

Originally Posted by 320mb

Code:

char password[5] = "linux";

Here's your problem. The C string literal "linux" actually has six characters in it: the five you can read plus the null character \0 (value of zero) which indicates the end of all C strings.

This causes your string literal to go outside the bounds of the array you're assigning it into (six bytes for five slots).

Edit: Sorry; looks like I'm a slower typist than Spooon.

xhi · 06-24-2006, 10:49 AM

if you use g++ to compile it will catch an error such as the one that was mentioned.

i wonder if that is causing the segfault though. depending on what you are passing in for argv[1], that could also be a problem.

taylor_venable · 06-24-2006, 02:31 PM

Hmm, yes; looking closer, I see what might be a few more problems. Normally, the main method's signature is expressed as

Code:

int main(int argc, char** argv)

or

int main(int argc, char* argv[])

I'm also unsure about the meaning of the star (*) in front of strcpy(). While I'm certain that writing out of array bounds will (or rather, should) cause a segmentation fault, xhi is also correct. Passing argv[1] as "somethingverylong", or any string more than four characters, would cause the same problem. That's why you should normally use something at least as secure as strncpy() when dealing with possibly untrustworthy input.