How to use void pointer in function

[SilverMan]

I am trying to convert float/int/str information to string and store in an array.  Can I use a single function to do that?  I am getting errors under Labwindows 

82, 34 error: invalid operands to binary expression ('float *' and 'float')

I think I didn't use void pointer correctly, but why?  Thanks ahead.

main{

TestReport(Test_Description, 0.2,&MeasureValue,0.4, TESTREPORT_FORMAT_FLOAT,"Volt");

}

int TestReport(char description[], float Lowlimit, void *Result, float Highlimit, char datatype,
char unit[])
{

char TempTest[256];

strcat(Test_Report_Text, description); //report item header
switch (datatype){
case TESTREPORT_FORMAT_INT:
if (((int *)Result > (int)Lowlimit)&&((int *)Result < (int)Highlimit)) strcat(TempTest, "Pass");
strcat(TempTest, "*Fail");
break;
case TESTREPORT_FORMAT_FLOAT:
if (((float *)Result > Lowlimit)&&((float *)Result < Highlimit)) strcat(TempTest, "Pass");
strcat(TempTest, "*Fail");
break;
case TESTREPORT_FORMAT_STR:
break;
}
strcat(Test_Report_Text, TempTest);
return 0;
}

[RobertoBozzolo · Accepted solution]

The error is quite clear: you cannot compare a pointer (float *) to a float value. You should modify your expression this way:

if ((*(float *)Result > Lowlimit) && (*(float *)Result < Highlimit)) strcat (TempTest, "Pass");

Beware that the same applies to the int case: you don't get a warning since a pointer effectively is an integer value, but you are actually comparing the pointer (=address) value with the limits instead of the actual value of the variable addressed from the pointer.

[SilverMan]

Hi RobertoBozzolo,

Thank you so much for answering the questions, it is working now.  But I have an additional question:

In the function which contains the void * variable, I found in C language I have to use a extra variable in order to get the correct value(other than directly using the pointer variable). For example:

float Temp = *(float *)Result;

if(Temp > Lowlimit) do something;   //this will work

if(*(float *)Result > Lowlimit) do something; //this will not work

Can anyone explain why I have to the variable indirectly?

Thank you! 

[RobertoBozzolo]

I'm not aware of such a constraint: the code as I wrote should work correctly. Did you find some problems?

[SilverMan]

if I directly use the code:

if ((*(float *)Result > Lowlimit) && (*(float *)Result < Highlimit)) strcat (TempTest, "Pass");

 it wouldn't work. 

Unless I store the float value into another variable first and then it will work as it it suppose to be.

float tempresult = *(float *)Result;

if (tempresult > Lowlimit) && (temp < Highlimit) strcat (TempTest, "Pass");

I am just curious why the first line of code doesn't work, in my understanding, both types of C code have the same interpretation.

[AvatarAang]

What kind of error/behavior are you getting?

[rolfk]

How about when you try:

if (((*(float *)Result) > Lowlimit) && ((*(float *)Result) < Highlimit)) strcat (TempTest, "Pass");

It should be equivalent according to the C operator precedence standard, but this would make sure it is not some operator precedence messup.