yet another segmentation fault :)

[yougene]

These segmentation faults are making me feel very humble about my C skills. I have a feeling I don't remember something here.

(gdb) run
Starting program: /home/eugene/cfiles/a.out 

Program received signal SIGSEGV, Segmentation fault.
0x08048495 in string2list (sPtr=0xbf8bbdf0, string=0xbf8be000 <Address 0xbf8be000 out of bounds>) at 12.9.c:62
62			insert( sPtr, *string );

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

struct listnode {
	char data;
	struct listnode *nextPtr;
};


typedef struct listnode ListNode;


void insert( ListNode **sPtr, char input );
void display( ListNode *sPtr );
void string2list( ListNode **sPtr, char *string );

int main()
{
	ListNode *startPtr = NULL;
	char string[ 10 ] = "a test";


	string2list( &startPtr, string );
//	insert( &startPtr, 'q' );
//	insert( &startPtr, 'z' );
//	insert( &startPtr, 'a' );
	display( startPtr );


	return 0;
}





void string2list( ListNode **sPtr, char string[] )
{
	
	while ( string != '\0' ) {
		insert( sPtr, *string );
		++string;
	}

}




void display( ListNode *sPtr )
{
	ListNode *currentPtr = NULL;
	currentPtr = sPtr;

	while ( currentPtr != NULL ) {
		printf( "%c", currentPtr->data );
		currentPtr = currentPtr->nextPtr;
	}
	printf( "\n" );
}


void insert( ListNode **sPtr, char input )
{
	ListNode *newPtr = NULL, *previousPtr = NULL, *currentPtr = NULL;

	newPtr = malloc( sizeof( ListNode ) );


	if ( newPtr != NULL ) {
		newPtr->data = input;
		newPtr->nextPtr = NULL;

		currentPtr = *sPtr;

		while ( currentPtr != NULL ) {
			previousPtr = currentPtr;
			currentPtr = currentPtr->nextPtr;
		}

		if ( currentPtr == *sPtr ) {
			newPtr->nextPtr = *sPtr;
			*sPtr = newPtr;
		}
		else {
			previousPtr->nextPtr = newPtr;
			newPtr->nextPtr = currentPtr;
		}
	}

}

[samf]

You may want to allocate some memory for the node before you start copying things into it.

ListNode *startPtr = NULL;

Your pointer isn't pointing to anything when you pass it to string2list

string2list( &startPtr, string );

[vart]

You may want to allocate some memory for the node before you start copying things into it.

ListNode *startPtr = NULL;

Your pointer isn't pointing to anything when you pass it to string2list

string2list( &startPtr, string );

allocation of the nodes is made inside the insert function

[vart]

		while ( currentPtr != NULL ) {
			previousPtr = currentPtr;
			currentPtr = currentPtr->nextPtr;
		}

		if ( currentPtr == *sPtr ) {
			newPtr->nextPtr = *sPtr;
			*sPtr = newPtr;
		}
		else {
			previousPtr->nextPtr = newPtr;
			newPtr->nextPtr = currentPtr;
		}

Note that when while loop is ended the currentPtr is NULL
Using this knowledge you can make the red part simplier...

[samf]

allocation of the nodes is made inside the insert function

Yes, that would be true if you actually called that function. But not only did you comment out the calls to that function, but it follows the call to string2list, which is wrong as well.

	string2list( &startPtr, string );
//	insert( &startPtr, 'q' );
//	insert( &startPtr, 'z' );
//	insert( &startPtr, 'a' );
	display( startPtr );

[samf]

Sorry. My bad. I misread your code.

Try this:

        while ( *string != '\0' ) {
                insert( sPtr, *string );
                ++string;
        }

$ diff x.c.orig x.c
40c40
< while ( string != '\0' ) {
---
> while ( *string != '\0' ) {


$ gcc -Wall -g x.c

$ a.out
a test

[yougene]

Man, I'm a little dumb.

Thanks guys.

[yougene]

This is my first time using gdb could someone walk me through its output.

First of all, it says the bug is on the line after the while statement. Why is that?

Second, why is this an out of bounds error?

[rags_to_riches]

1. Because the access of the memory you don't own occurs after the coding error that caused it. In many cases the cause of bug is far from the location of the symptom, even in different modules.

2. Because string is always going to be != '\0', you continue walking the string until you've walked right past the end of it. Eventually, you'll run into memory you don't own and BAM....seg fault (or access violation).

[yougene]

ahh very interesting.

2. Because string is never dereferenced how can it be out of bound? The value of string be any multiple of sizeof( char ). Shouldn't an out of bounds error occur only if out of bounds memory is actually accessed?

[rags_to_riches]

string is dereferenced in the insert call.