Simple C help!

L'prof

I have a header file, which contains a struct and a function:

struct point{
  double x, y;
};
typedef struct point point_t;

point_t read_point(void){
  point_t p;
  printf("Please enter a point: ");
  scanf("%f%f", &p.x, &p.y);
  return p;
}

And I just have a simple program to start with to check that my function works properly:

# include <stdio.h>
# include <math.h>
# include "point.h"

int main(void){
  point_t p = read_point();
  printf("(%f, %f)", p.x, p.y);
  return 0;
}

It doesn't seem to work...it just spits out the same values, no matter what input I give so I think I need to use pointers to access the values properly but I'm not sure how?

Any help would be great, thanks

lynchie

point_t read_point(void){
  [COLOR="Red"]point_t p;[/COLOR]
  printf("Please enter a point: ");
  scanf("%f%f", &p.x, &p.y);
  return p;
}

the point struct is be declared at the beginning of the function. Its allocated on the stack and therefore will cease to exist once the function has returned. Your best bet is to declare the point_t in your c file and pass a pointer to it down as an argument to the read_point method.

L'prof

I thought that this line in my c file would account for that?

point_t p = read_point();

But then again, if I knew what I was talking about, I wouldn't be on here!

fasty

jasonorr wrote: »

I thought that this line in my c file would account for that?

point_t p = read_point();

But then again, if I knew what I was talking about, I wouldn't be on here!

That doesn't work for C, as lynchie say, p is on the heap and once the function returns it's out of scope and gone.

Try:

void read_point(point_t* p){
  printf("Please enter a point: ");
  scanf("%f%f", p->x, p->y);
}

Call the function with

point_t p;
read_point(&p);

L'prof

I'll tried that just now and it doesn't seem to work, the program just freezes! but, I was told that the function doesn't actually need any arguments and, how would it be implemented then?

ianhobo

It doesn't seem to work...it just spits out the same values

What do you mean?
That compiled and worked for me.
It prints out the values that you entered.
What do you want to happen?

ianhobo

I copy and pasted your original code into one file,compiled it with gcc, and ran it. it works

L'prof

# include <stdio.h>
# include <math.h>
# include "point.h"

int main(void){
  point_t p = read_point();
  printf("(%f, %f)", p.x, p.y);
  return 0;
}

struct point{
  double x, y;
};
typedef struct point point_t;

point_t read_point(void){
  point_t p;
  printf("Please enter a point: ");
  scanf("%f%f", &p.x, &p.y);
  return p;
}

So these lines of code work for you?

When I compile it and enter 1 2 as input, I get:

(0.000000, 196297699578146160000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
0000)

If I enter 1 3 I get:

(0.000000, 196297699728296500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
0000)

I'm using Visual Studio 2005 as a compiler!

fasty

That code is the same as you posted earlier. It won't work!

You need to pass a pointer to a point_t struct to your read_point function as a parameter.

L'prof

That code is the same as you posted earlier. It won't work!

You need to pass a pointer to a point_t struct to your read_point function as a parameter.

I was replying to this:

I copy and pasted your original code into one file,compiled it with gcc, and ran it. it works

I thought he was trying to tell me that the original code without the pointers was working but, I can't get it to work with the pointers either...the program just freezes, I think when that's happened before I ran the same program on a Linux machine and I got a segmentation fault error!

daymobrew

jasonorr wrote: »

I thought he was trying to tell me that the original code without the pointers was working but, I can't get it to work with the pointers either...the program just freezes, I think when that's happened before I ran the same program on a Linux machine and I got a segmentation fault error!

You have to change your function to accept and work with the pointer being sent.

Look at your use of scanf() - you pass pointers to it and it puts the data into the variables you passed.
Look at the declaration for scanf to see how describes the parameters it expects. Your function is currently not expecting any variables (the 'void' bit).

carveone

Let's take a deep breath

First of all, there's nothing at all wrong with declaring variables on the stack (auto variables are not on the heap!) and returning them. A copy of them mind, not a pointer to them. Your "return p" is fine. So ignore the pointer people! Sure you can do it that way, but that is not what is wrong...

struct point{
  [B]double x, y;[/B]
};

point_t read_point(void){
  point_t p;
  printf("Please enter a point: ");
  scanf([B]"%f%f"[/B], &p.x, &p.y);
  return p;
}

Look at the bolded lines. One is a double, the other is a float. They need to agree. scanf is a variadic function (one that takes a variable number of arguments). This means that scanf must trust you when you specify the arguments by means of the quoted string. The types of the arguments passed on the stack are not magically known... So in this case, you say %f for float, scanf takes you at your word and puts a float in there. Not good. Make the the string "%lf%lf" and you might have more luck.

Oh by the way, ditto with the printf... gcc probably works because its floating point mechanism might be different. Dunno. implementation specific as they say...

Conor.

carveone

jasonorr wrote: »

(0.000000, 196297699578146160000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00
0000)

Wow. That's so cool To me, that just screams "mismatched parameters types".

It's easier to comprehend what's going on if you imagine passing say, a pointer to a 32 bit int and insisting that it be treated as a 16bit short.

The integer starts life on the stack, so it is uninitialised garbage... Let's say: 0xF00FABCD. Then you take half of that 32 bits and put a number in there, say 0x01.

Now you have 0xF00F0001. Maybe. Depends on the machine! Now you print that out...

In the case of integers you might actually get away with this mistake because of the way the parts align. Floating point is anyone's guess!

L'prof

Make the the string "%lf%lf" and you might have more luck.

I love you, it works :cool:

I was using %f because I think that work on the linux machine in college!

carveone

You're welcome!

C has a number of traps waiting for the unwary and that's one of them. float and double have the same relationship as short and int. That is, depending on platform, they may be the same size. Or not. The C spec only says that double can't be smaller than a float. So you get the great experience of having your program work on linux, say, and then break horribly on windows.

Cheers,

Conor.

fasty

Well spotted! I wonder if there's a blush emoticon... Sorry for leading you down the wrong path, Jasonorr.

L'prof

No worries fasty, I'm sure it wasn't intentional...thanks for your help anyway!