CGI bin file downloads instead of executing through HTML Browser

[hrlabs]

I am Embedded Software Engineer, I am new to Embedded Web Server programming, I wrote the HTML code which is simple form as the code shown below :

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Code HTML5 - [expand]
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<form action="/cgi-bin/gpio.cgi" method="post">
<div><label>Multiplicand 1: <input name="m"></label></div>
<div><label>Multiplicand 2: <input name="n"></label></div>
<div> <input type="submit"   value="Multiply" ></div>
</form>

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and i wrote the following gpio.cgi code in C langugae to run it on Linux through my HTML :

here is the code shown below :

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Code C - [expand]
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int main() {
char *data ;
long  m,n;
printf("%s %c%c\n ", "Content-Type:text/html;charset=iso-8859-1",13,10);
printf("<title>Multiplication results</title>\n");
printf("<h3>Multiplication results</h3>\n");
data=getenv("QUERY_STRING");
if (data==NULL) 
 
printf("<P>Error! Error in passing data from to Script");
 
else if (sscanf(data,"m=%ld&n=%ld",&m,&n)!=2)
 
printf("<P>Error! invalid Data. ");
 
else printf("<P>The Product of %ld and %ld is %ld.",m,n,m*n);
 
return 0;
 
}

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Than i compiled my above program by typing following command in teminal in Linux:

gcc -o gpio.c gpio.cgi

and i put this gpio.cgi file in /cgi-bin directory the same path as i wrote in HTML code,

but when i click Submit button after entering data , my gpio.cgi file gets download instead of execution, while i tested gpio.cgi file independently and it works fine but not with HTML browser.

please help me i am really stucked with it.