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;
}

and then I call it as

Code:

myfunc(&othervariable);

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;

Then, I would initialize it like this?

from main:

Code:

*mystruct.value = &othervariable

from a function:

Code:

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

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

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.

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:

*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++
Get on with the times, since it seems all the tutorials are written in C++ these days
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:

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

Code:

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

and I create the slider with this call:

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

Thank you very much Elysia, you certainly made my day

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

I was just echoing what you mentioned. Sarcasm

Thank you very much Elysia, you certainly made my day

Not a problem.