Passing linked list to function, segmentation fault

[Benkyou]

Hi there, this is my first post here so please tell, if im doing something wrong. Ok so heres the the struct

typedef struct _CMD
{
	char nome[100];
	char arg[100];
	struct _CMD *prox;
}*CMD;
typedef struct _sCMD
{
	int cont;
	CMD inicio;
	CMD fim;
}*sCMD;

The function i use to fill this list:

void add(sCMD com,char str[100],char param[100])
{
	CMD novo = (CMD)malloc(sizeof(struct _CMD));
	strcpy(novo->nome,str);
	strcpy(novo->arg,param);
	novo->prox = NULL;
	com->cont++;

	if(com->inicio == NULL)
	{
		com->inicio = novo;
		com->inicio->prox = com->fim;
	}
	if(com->fim != NULL)
	{
		com->fim->prox = novo;
		com->fim = novo;
	}
	if((com->inicio != NULL) && (com->fim == NULL))
	{
		com->fim = novo;	
	}

	free(novo);
}

After reading into the list, i tried writing everything i had there to see if it worked, and it did. Now the problem is, imagine i had to find the X element of that list, i wrote this function for the deed:

char *find(sCMD com,int i)
{
	int j=0;
	static char str[100];
 	CMD aux = (CMD)malloc(sizeof(struct _CMD));
        aux = com->inicio;
	
	while (j<i)
	{
	aux=aux->prox;
	j++;
	}
	
	strcpy(str,aux->nome);
	free(aux);
	return str;
}

To test it i run this inside a while, to try displaying all the strings "nome" of each element of the list.

i=0;
	while(i < com->cont)
	{	
		printf("%d",i);
		puts(find(com,i));
		i++;
	}

This works for i =0 but after, i get a segmentation fault when calling the function find. further tests showed me it was when i tried aux=aux->prox, now i have no idea why, but the second time this function is called, the list it receives as a parameter is "corrupted", full of trash. Now i hope this isnt too incoherent. Thanks in advance for any attempt of helping me.

[Narue]

>when i tried aux=aux->prox
...aux becomes invalid because aux->prox is an uninitialized pointer. Any further use of aux, except to point it to another address, is doomed to failure.

[Benkyou]

Thanks for the reply, really got to get this working...

I had this function before to show me everything i had on the list:

void lista(sCMD com) 
{
	int i=0;
	CMD aux = (CMD)malloc(sizeof(struct _CMD));
        aux = com->inicio;
	while(i<com->cont){
		puts(aux->arg);
		aux=aux->prox;
		i++;
	}	
}

Its close to the same thing i think... and this one worked ok, the problem is when im passing the linked list into the function. The first time i do that its good, when i=1 i checked what was inside com->inicio->nome and it was correct, i checked again but this time inside the function find and it was trash, so im thinking if its corruped then the pointer is probably pointing to what? no clue but i think thats why i get the seg fault...

[Benkyou]

Ok, aparently this is fixed... By the way the way function add was working was if i didnt do free(novo); so i removed free(aux); from function find and it seems to be working so far... but i would still like to know why, if anyone knows and doesn't mind loosing the time to explain, it would be much appreciated. Thanks in advance.

[kvprajapati]

This statement creates a heap and address of newly created heap is assigned to aux pointer variable. (This statement has no use in your code.)

CMD aux = (CMD)malloc(sizeof(struct _CMD));

Now, you are assiging an address of another heap to the aux pointer variable.

aux = com->inicio;

And at the end of function, you are freeing a heap which has search result.

free(aux);

[Benkyou]

Thanks for the reply. I think i understand it now.