Thread: This compiler is driving me nuts

>>my ide/compile is driving me nuts
Yeah, and I expect your compiler is getting annoyed at you giving it bad code They have feelings too you know

Anyway, back in reality:
value is defined as an array of type char. It's local to the function. You cannot (shouldn't) return it.

What exactly are you trying to do? Explain, and someone will guide you.

If you got it to compile with a warning you did this probably.

Code:

char* copy(char data[], int start, int end) {
    char value[255];
    
    return value;
}

Noo!!!!! Don't do that!! You're trying to return a local variable. If you must do this , please make value static.

Code:

char* copy(char data[], int start, int end) {
    static char value[255];
    
    return value;
}

EDIT: Thanks hammer I rememered to change it on the first one to show him it'll give you a warning but was in too much of a hurry and forgot it on the correct one , doh.

value's data is an array. You can return a pointer to it, but only if you make it static first.

Or, you can create the memory for value dynamically (via malloc) and return that pointer instead. This will change the definition of value from char[] to char*. Then your calling function must free() the memory.

Or, and I don't recommend this one, you can put value inside a struct and return that. That way the char array will be passed back whole.


I am presuming you are wanting to return the complete array, not just the first character?

>>i know i have gotten this to compile before with only a warning
Doesn't always mean the code is correct though


[edit]
MrWizard, you're example is wrong, the function is returning a char, it should be a char*.

>>does that mean this is bad
Your some_function() example isn't dealing with an array, only a single int. That's the main difference that is causing you confusion, I expect.

Originally posted by martin_00
>i thought in c when ever you make a call or return a copy of the data is placed in a new varible.

am i wrong here

OK, let me try and explain a little:

Code:

int some_function(void) {
    int x = 1;

    return x;
}

Here we have some_function that returns an int. The value within x is returned at the end of the function. X itself only exists within this function.

Code:

int *another_func(void)
{
   static int x[10] = {100};
   return x;
}

Here we've changed things slightly. We deal with an array of ints rather than a single int. The variable x is now really a pointer in disguise, so when we return it, we are returning a pointer to the static array. We are NOT returning 10 ints. As the array is declared static, it exists for the life of the program, rather than this one function. That means another function can use a pointer to the array and read/write to those 10 ints.

If it wasn't declared static, you'll get compiler warnings, and the returned pointer will be useless, pointing to memory that has been made available for other things.

I'm not sure what it is you're trying to do within your function, so I won't go into too much more detail unless you ask.

Maybe if you tell us what your function is supposed to do (not how it is going to do it), we can advise you better. For example, are you trying to duplicate a string (like strdup)?

Originally posted by martin_00
i have a question already, did i have to copy data to another varible or is modifying an arguement alright? (i remember some one telling me about this)

If you return a static variable, you're really pointing at that variable inside that function call. You still have to end up copying that data somewhere (depending on how you want to use it) to actually make use of it.

Quzah.

Originally posted by martin_00
i have a question already, did i have to copy data to another varible or is modifying an arguement alright? (i remember some one telling me about this)

It depends. If you call your function like this:

>>copy("abcdefghijklmnopqrstuvwxyz", 5, 15)
You cannot safely modify the data pointed to by the data[] variable.

Maybe I'm being thick, but I still don't understand what it is you want your code to do. Can you explain, in English not C code, what you want. I don't want to correct your current code anymore, until I know what direction to steer you in.

Let's kick this out into the lake and see if it floats :P

Code:

#include <stdio.h>

void strNtoM(char [], int, int);

int main(void)
{
   char string1[80];
   
   printf("Enter your string: ");
   fgets(string1, 80, stdin);
   
   strNtoM(string1, 2, 5);
   
   return 0;
}
 
void strNtoM(char target[], int start, int end)
{
	int j;
	
	for(j = start-1; j < end; j++)
		printf("%c", target[j]);
}

Dunno if it's technically correct but I get "esti"