Passing a variable to function by reference

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nubechecorre

Posted on: 2018/7/14 21:21 #1

Just can't stay away

I try to pass a variable by reference to a function and change its value. Inside that function it works but when I print the value of the variable outside the function the value is different.. I'd like to understand what I am doing wrong as I need it for my program..

I declare this variable as global plus the definition of two functions outside (before) the main function:


int a;

 

void change_sound_1(int *);

void change_sound_2(int *);

 

.....

inside the main function i have this code:


switch(code)

{

     case 0:

     IDOS->Printf("Sound rate is: 11025\n");

     change_sound_1(&sound_code);

     IDOS->Printf("sound_code is:%d\n", sound_code);

     break;

 

     case 1:

     IDOS->Printf("Sound rate is: 22050\n");

     change_sound_2(&sound_code);

     IDOS->Printf("sound_code is:%d\n", sound_code);

     break;

}

 

break;

These are the two functions that I use to modify the int variable:


void change_sound_1(int *a){

    *a=0;

    printf("sound code is:%d\n", *a);

     *a=*a+1;

    printf("sound code is:%d\n", *a);

}

 

void change_sound_2(int *b){

    *b=0;

    printf("sound code is:%d\n", *b);

    *b=*b+2;

    printf("sound code is:%d\n", *b);

}

But when i print the variable, for the second time, in the main function, it prints 0 (zero).. Shouldn't it print the the same value that the two functions print? as I have modified its value using the pointer??

thomas

Re: Passing a variable to function by reference

Posted on: 2018/7/15 0:11 #2

Not too shy to talk

For IDOS->Printf %d is a 16 bit integer, but the compiler puts a 32 bit integer on the parameter, so you print only the upper half of the 32 bits which is always zero for numbers below 65536. You have to use %ld to print it properly.

You should consequently use either printf or Printf everywhere, but don't mix them. You only get confused like in this case.

nubechecorre

Re: Passing a variable to function by reference

Posted on: 2018/7/15 10:50 #3

Just can't stay away

thanks for the answer, enlightening!

salass00

Re: Passing a variable to function by reference

Posted on: 2018/7/15 11:07 #4

Just can't stay away

@nubechecorre

Generally I would recommend using printf() over IDOS->Printf() since the former will with newer gcc versions give you a warning if the format string and parameters don't match.

Of course you should also always compile with warnings enabled (-Wall) and ideally -Werror as well which makes gcc treat warnings as errors.

nubechecorre

Re: Passing a variable to function by reference

Posted on: 2018/7/15 12:53 #5

Just can't stay away

Thanks Salass00

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