pow in non main function.

[sandbucket]

i cant get the pow() function in math.h to work in a function other than main. heres my code

 #include <stdio.h>                                                    
#include <math.h>

int bintoint(char * bin);
int main()
{
    printf("%lf", pow(2, 8));
    
    return 0;
}

int bintoint(char * bin)
{    
    int count = 0, total = 0, temp = 1;
    while (bin[count] != '\0')
    {
        count++;
    }
    
    while(count-- != 0)
    {
        printf("%lf", pow(2, count));
    }    
    return total;
}

(note its not complete)
can you tell me how to fix this?

[qny]

In C89 (Microsoft's Visual Studio is a C89 compiler), use "%f" to print a value of type double; use "%Lf" to print a value of type long double (note difference with scanf()).
The function pow() returns a double.

Change the printf format string.

Using "%lf" in a printf format string, in C89, is Undefined Behaviour.

[Matticus]

[edit] answered by qny

[christop]

Could the problem be that you don't ever call bintoint from main?

[c99tutorial]

I don't see how this is specific to Microsoft's Compilers. To printf a double, use %f, and to scanf a double, use %lf. If you're dealing with long doubles, however, one might have to pay attention to the compiler and/or C runtime which is being used.

[qny]

Many compilers today compile C99.
In C99, using "%lf" in printf with a corresponding double value is accepted ... (7.19.6.1/7 l (ell) ... has no effect on a following ... f ... conversion specifier).

So the original code behaved differently (UB vs defined behaviour) when compiled/run in different implementations.

[sandbucket]

gcc -Wall -o "1" "1.c" -std=c99 (in directory: /home/sandbucket/C/chapter 15)
1.c: In function ‘bintoint’:
1.c:14:28: warning: unused variable ‘temp’ [-Wunused-variable]
/tmp/cczXWO9g.o: In function `bintoint':
1.c.text+0x95): undefined reference to `pow'
collect2: error: ld returned 1 exit status
Compilation failed.
this is the error message i get.

[qny]

You need to include the math library proper in the linking stage. The header is not enough.
Add the option -lm to your compiler invocation

gcc -Wall -o 1 1.c -std=c99 -lm

You don't get the error in main because the compiler itself is doing the calculation (*), just as if you had writen

    printf("%lf", 256);

(*) gcc (4.3) uses MPFR

[sandbucket]

ohh true macros thanks alot bud

[sandbucket]

ohh true macros thanks alot bud

idk if that is 100% right. i i dont want to sully my name on this forum right once i first join.

[qny]

idk if that is 100% right

Almost 100%
It's no more a macro than the compiler calculating some multiplications ...

timestamp = 6549864651;
days = timestamp / (24*60*60); // compiler emits code to divide by 86400

[christop]

Almost 100%
It's no more a macro than the compiler calculating some multiplications ...

timestamp = 6549864651;
days = timestamp / (24*60*60); // compiler emits code to divide by 86400

And at a better optimization level the compiler might emit code to set days to 75808, without a division.