C Code problem

ar1 · 05-09-2005, 03:15 AM

I wrote a program to give the permutations of a string, and get a segmentation fault in the function "swap".

Now, the program runs fine if I declare the string as "char str[10] = "ABCD" rather than as char *str = "ABCD".

Can anyone tell me why the segmentation fault occurs in the first case? I tried using a debugger but everything seemed to be the same in both cases, yet the error :/

Thanks

Code:

/*
 *	perms.c : List the permutations of a string P(n,c)
 */


int N = 4;
int C = 4;

void perms(int, char *);
void swap(char *, char *);

int main()
{
	char *str = "ABCD";
	perms(0, str);
	return 0;
}

void perms(int i, char *str)
{
	int j;
	
	if (N == i)
		printf("%s\n", str);
	else
	{
		for (j = i; j < N; j++)
		{
			swap(str + i, str + j);
			perms(i+1, str);
			swap(str + i, str + j);
		}
	}
}

void swap(char *s, char *t)
{
	char tmp;
	tmp = *s;
	*s = *t;
	*t = tmp;
}

Hivemind · 05-09-2005, 04:55 AM

When you write a string literal, i.e.:

Code:

"hello"

that string lives in static storage and is of type

Code:

const char *

So the string in your code is of really of type const char * and not of type char *. A decent compiler should
issue a warning if try to declare a string literal to have the type char *.
If you're using gcc, I suggest always compiling with -Wall -W, and sometimes -ansi and -pedantic may also be warranted.
You already have the solution to your problem, you want a string to be initialised and modifiable and the correct way
is to use an array:

Code:

char foo[] = "hello";

The compiler will fill in the size for you (six characters in this case).
Trying to modify string literals is a common mistake.

ar1 · 05-09-2005, 06:22 AM

Hivemind, thanks for your reply. Actually, gcc is not giving the error even with -Wall -W, nor with -ansi and -pedantic.

What do you mean by "static storage"? Do you mean the stack? If so, then all variables initialized in the program are also stored there but can be modified :/

Plus, it seems to me that even if

Code:

char foo[] = "hello"

is dynamic,

Code:

 char foo[10] = "hello"

should not be dynamic, yet it doesn't give an error in my program.

Can you explain further? thanks.

Hivemind · 05-09-2005, 07:04 AM

I already did explain it. If a variable has static storage it means it exists throughout the lifetime of the program, and string literals have static linkage and are not modifiable. Programs living on the stack or heap do not automatically exist through the lifetime of a program and whether they can be modified are usually controllable by the programmer (by declaring them const or not).