Variable not taking any value (c)

Question

It may be a dumb thing that I didn't saw but I've got the a really strange behavior on my program.

Here's my code

#include <stdio.h>
void spectral_color(double r,double g,double b,double l); //https://stackoverflow.com/a/22681410

int main(int argc, char **argv)
{
    double r,g,b;
    spectral_color(r,g,b,600);
    //printf("%lf,%lf,%lf",r,g,b);
}

void spectral_color(double r,double g,double b,double l) // RGB <0,1> <- lambda l <400,700> [nm]
    {
    double t;  r=0.0; g=0.0; b=0.0;
    if ((l>=595.0)&&(l<650.0)) { 
        t=(l-595.0)/(650.0-595.0); 
        r=(double)0.98+(0.06*t)-(0.40*t*t); 
    }
    printf("%lf,%lf,%lf",r,g,b);
}

It prints 0.000000,0.000000,0.000000 and I tried this in the gdb

15          t=(l-595.0)/(650.0-595.0); 
(gdb) p l
$8 = 600
(gdb) p r
$9 = 1.7802772279180279e-307
(gdb) p t
$10 = 2.4846118548532686e+264
(gdb) n
16          r=(double)0.98+(0.06*t)-(0.40*t*t); 
(gdb) p t
$11 = 0.090909090909090912
(gdb) p r
$12 = 1.7802772279180279e-307
(gdb) n
18      printf("%lf,%lf,%lf",r,g,b);
(gdb) p t
$13 = 0.090909090909090912
(gdb) p r
$14 = 1.7802772279180279e-307

Here, r do not take the value it should and I have no idea why it would do that. Do someone have an explanation ?

Thanks in advance

edit:

I passed the value of r to another variable and changed the %lf to %f, the result is that r is never equal to the new variable but the correct value ends up in this new variable

double d = r; //this is line 18
printf("%f,%f,%f",d,g,b);

gdb result:

20  }
(gdb) p d
$1 = 0.98214876033057852
(gdb) p r
$2 = 1.7802772279180279e-307
(gdb)

You have Undefined Behaviour. You are using uninitialised variables. — Ajay Brahmakshatriya, Jun 15 '17 at 02:39
I think your compiler can not handle `lf` properly (or mismatch library). — BLUEPIXY, Jun 15 '17 at 02:46
It shows the correct value with the print but if I look with gdb, `r` is never equal to the value printed — Tix'at, Jun 15 '17 at 02:48
To be clear, you are talking about the printf inside the `spectral_color` function? The one you have commented out in `main` obviously won't work , even if you uncomment it — M.M, Jun 15 '17 at 03:21
`0.000000` is the correct output for `%f` (or `%lf`) and the value `1.7802772279180279e-307` — M.M, Jun 15 '17 at 03:23
With `double r,g,b; spectral_color(r,g,b,600);`, what value do you think `r` has when `spectral_color(r,...)` is called? — chux - Reinstate Monica, Jun 15 '17 at 03:26
to understand this compiler behavier, you probably need to read assembly file that compiler generated.`gcc -S file.c` — K.Juce, Jun 15 '17 at 03:33
what's the point of passing r, g and b when you set them as 0 anyway? — phuclv, Jun 15 '17 at 09:38

Paul · Answer 1 · 2017-06-15T09:31:51.997

-1

Once I have been bothered by this problem until I read the manual

This is the explanation of the l length modifier

(ell) A following integer conversion corresponds to a long int or unsigned long int argument, or a following n conversion corresponds to a pointer to a long int argument, or a following c conversion corresponds to a wint_t argument, or a following s conversion corresponds to a pointer to wchar_t argument.

Maybe a picture could be more illustrating

There would be a type promotion or conversion performed if you use the length modifier of l for a double. That's probably the reason of your confusion.

edited Jun 15 '17 at 09:31

answered Jun 15 '17 at 08:49

Paul

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*"By the way, I always use `L` for a `float` or `double` conversion."* That makes no sense at all. As shown in your table, uppercase `L` is reserved for `long double`. – user694733 Jun 15 '17 at 09:18
*"There would be a type promotion or conversion performed if you use the length modifier of `l` for a `double`"* Length modifiers don't affect type promotion. `double` is always passed as `double` to `printf` unless there is some non-standard compiler behaviour going on. – user694733 Jun 15 '17 at 09:25
I don't see how you using `long double` is relevant to the problem. Question doesn't use `long double`. – user694733 Jun 15 '17 at 09:28
@user694733 Maybe I was totally wrong. Could you please tell me why the `l` length modifier causes the `double` to `0`? – Paul Jun 15 '17 at 09:45

Variable not taking any value (c)

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