Inconsistent value returned by function

915086731 · 01-28-2016, 09:49 AM

when I call the function default_ratio_check(...), I get confusing value.

Code:

const int COMP_READ_COMPRESS = 1;
const int COMP_WRITE_COMPRESS = 0;
const int COMP_TRACE_USE_DEFAULT_RATIO = 2;
double COMP_DEFAULT_RATIO = 0.20;

compress_struct* compress_global;

double default_ratio_check( char** s, int i)
{
    if( s == NULL ){
        if(COMP_TRACE_USE_DEFAULT_RATIO ) {
            return COMP_DEFAULT_RATIO;
        }
    }
    if( COMP_READ_COMPRESS || COMP_WRITE_COMPRESS ){
        if(COMP_TRACE_USE_DEFAULT_RATIO ) {
            COMP_DEFAULT_RATIO = atof( s[i] );
            return COMP_DEFAULT_RATIO;
        }
    }
    return 0;
}

I assign the result to the double type "ratio".

Code:

   ratio =  default_ratio_check( (char**)0, 0 );

the value assigned to ratio is 2.0, which is the value of COMP_TRACE_USE_DEFAULT_RATIO instead of COMP_DEFAULT_RATIO. So I change the value of COMP_TRACE_USE_DEFAULT_RATIO,correspondingly, the returned value to ratio is also followed. I have use gdb to step into the function, however , I find everything works correctly. But the return value is wrong after the default_ratio_check() returned.
Thanks for your helping. gcc version 5.2.1 20151010; ubuntu 15.10

Keith Hedger · 01-28-2016, 02:29 PM

I think the fault is in how you are calling the function here

Code:

ratio =  default_ratio_check( (char**)0, 0 );

I assume you are trying to to pass the address of a "0" character ( as you convert that char to a float with atof ), you are actually passing a NULL pointer to the function, s will always = NULL in this case and so the first if statement is always true.

Try

Code:

char *buff1="1";
char *buff2="2";
double ratio;
ratio=  default_ratio_check(&buff1, 0 );
printf("result=%f\n",ratio);
ratio=  default_ratio_check(&buff2, 0 );
printf("result=%f\n",ratio);

Result is

Code:

result=1.000000
result=2.000000

That is if I understand what you are trying to do!
I asumne this is a fragment of a larger peice of code as there are easier ways of passing the args.

rtmistler · 01-28-2016, 02:53 PM

I agree with Keith Hedger.

In the debugger check what it's doing to (char **)0 and I think you'll find that it either pointing to a null, or bad location.

Instead create a character string such as:

Code:

static char zero_string[2] = "0\0";

And then pass the address of that.

I don't know why you need a pointer to a pointer either. A simple character pointer should do fine. You're not modifying the passed string after all.