A struct member as a pointer to a variable

Dear C experts,

First and foremost, please let me thank you for all your help in this forum. You are making my coding life much, much more pleasant :-)

Now, to the point. I need to create a struct in which member points to another (global) variable. As I am to pass this struct to functions, and pointers to pointers make me nervous, I thought of checking with the experts first.

Let me explain myself with a silly example. I know that if I define a function like this:

Code:

---

void myfunc(double *varvalue)
{
    *varvalue = somevalue;
}

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and then I call it as

Code:

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myfunc(&othervariable);

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then the global variable "othervariable" will get the value "somevalue". (I hope I am correct here).

Well, now I need to do that with structs. My guess (which could be totally wrong) is that I should do the following:

Code:

---

typedef struct _sType
{
    double *value
} sType;

sType mystruct;

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Then, I would initialize it like this?

from main:

Code:

---

*mystruct.value = &othervariable

---

from a function:

Code:

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void initfunc(sType *s)
{
    *s->value = &othervariable
}

---

Now, to update the value of "othervariable" should I do this?

from main:

Code:

---

*mystruct.value = somevalue;

---

and from within functions:

Code:

---

void myfunc(sType *s)
{
    *s->value = somevalue
}

---

Please let me now whether any (or all) of my assumptions are wrong.

Thanks so much, as usual, for your help

mc61

Quote:

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Originally Posted by mc61

Well, now I need to do that with structs. My guess (which could be totally wrong) is that I should do the following:

Code:

---

typedef struct _sType
{
    double *value
} sType;

sType mystruct;

---

Then, I would initialize it like this?

from main:

Code:

---

*mystruct.value = &othervariable

---

---

Nope.
You need to do

Code:

---

(*mystruct).value = &othervariable

---

Or just

Code:

---

mystruct->value = &othervariable

---

Quote:

---

from a function:

Code:

---

void initfunc(sType *s)
{
    *s->value = &othervariable
}

---

---

Nope. This would derefence the pointer itself, so you'll probably get a seg fault.
Just

Code:

---

s->value = &othervariable

---

Will do.

Quote:

---

Now, to update the value of "othervariable" should I do this?

from main:

Code:

---

*mystruct.value = somevalue;

---

---

Well, if mystruct is a pointer, then it's wrong, because you're only dereferencing the pointer to the struct and not the pointer inside the struct.
If mystruct is a struct on the stack, then yes, it is correct.

Also, it might be preferable to do

Code:

---

*(s->value) = somevalue

---

instead of

Code:

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*s->value = somevalue

---

To make it apparent that you are dereferncing the struct first and then the char* member.

The rest should be correct, I believe.
Also, what's the point of global variables if you pass pointers in structs?

Thank you Elysia, I will try what you told me as soon as I can.

To answer your question about why using global variables, I am implementing sliders and buttons on an OpenGL program, so this is in order that I can create a slider that would control the value of a variable in my model. Everything works for me now, except this last step that you kindly explained to me.

In case you are wondering why I am doing my own sliders, the vast majority of OpenGL GUIs out there are in C++, which I do not know, and the only one I found written in C (AntTweakBar) does not compile under OSX, which is what I use.

Thanks again for your help,

Marcelo

Well, you could learn C++ :p
Get on with the times, since it seems all the tutorials are written in C++ these days :D
Haha. Anyway, good luck on your project.

Tell me about those OpenGL C++ tutorials... I somehow managed to learn from them, but I guess I am too old to learn new tricks :)

At any rate, your advise worked wonderfully. Here is what I did:

Code:

---

typedef struct _sliderType
{
    (other stuff)
    double *variable;      // pointer to variable controlled by slider
} sliderType;

---

Code:

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void create_slider((other stuff),double *var, sliderType *s)
{
    ...
    s->variable  = var;
}

---

Code:

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void update_slider(sliderType *s)
{ 
  ...
  *(s->variable) = s->value;
}

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and I create the slider with this call:

create_slider( (other stuff), &somevariable, &myslider);

Thank you very much Elysia, you certainly made my day :cool:

Quote:

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Originally Posted by mc61

Tell me about those OpenGL C++ tutorials... I somehow managed to learn from them, but I guess I am too old to learn new tricks :)

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I was just echoing what you mentioned. Sarcasm ;)

Quote:

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Thank you very much Elysia, you certainly made my day :cool:

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Not a problem.