Thread: Pointers and Strings (simple question)

Hi,

I am trying copy a string over another string and I wrote the following code:

Code:

/* Params: toAssign: The string to overwrite. 
	   value: The new value of the string  should have
	   valuelengt: The length of the value*/
int assignValue(char *toAssign, char *value, int valuelength)
{

if ((toAssign = malloc(valuelength*sizeof(char)))==NULL) return (0);

int i;

for (i=0;i<valuelength;i++)
	toAssign[i]=value[i];

toAssign[valuelength] = '\0';

//printf("toassogn: %s",toAssign);
return(1);
}

int main()
{

char string[] = "test1";
char string2[] = "blablabla";

/*Trying to copy string2 over string  */
if (!assignValue(string, string2, sizeof string2)){
	printf("Something went wrong...");
	return (EXIT_FAILURE);
}

printf("%s\n", string);

return(EXIT_SUCCESS);
}

In this case I am trying to copy string2 ("blablabla") over string ("test1")

If I get it correctly when calling assignValue the pointers of the first elements of "string" and "string2" are passed into the function. In the function I allocate new memmory space and I make the pointer toAssign to point to that space. Then I copy the elements of the second array to the newly allocated space.

When assignValue exits and the control is returned to main "string" should point to the newly allocated memory, right? Well, it doesn't (i.e. the printf) statement in main() prints "test1"). So I guess something is wrong in the way I think about it?

Hmmm.. I think I know what is wrong. The pointer cannot change because it is passed as a parameter to the function (in the same way a value cannot change if it is passed as a parameter to a function).

Maybe if I pass as a parameter a pointer to the pointer and manipulate that in the assignValue? I'll give it a shot now.

Thanks for the reply kmdv. I am just preparing for a project in C and I am playing around with various C concepts (haven't used C since my bachelor's 12 years ago!).

Originally Posted by kmdv

You don't need to pass valueLength, because the string is (shall ... ) be ended with a NULL character.

That's a very good point, thank you.

Originally Posted by kmdv

And everything you allocate you must FREE.

Thanks for reminding....it shows I have a strong C# background!

I found out that I am not going to go very far with this approach.The best way to do it is like strcpy...(i.e. avoid allocating memory in the function). Instead ask the caller to allocate the amount of memory required and my function to just do the copying.