floating point - absolute value - inline assembly - edited new code

I wrote a function named absD that i want to return the absolute value of its argument.. I am using GCC inline assembly with cygwin..

I dont see why its not working. i m loading into memory. then into st(0) where i am using fabs - absolute value. Do i have to allocate memory?

I am trying to learn assembly with C here so please be nice. Please give me good help. Thank you

heres the code and then the error:

#include <stdio.h>
#include <stdlib.h>
#define PRECISION   3

double absD (double n)
{


asm(
        "fldl %[nIn]\n"
        "fabs\n"
        "fstpl %[nOut]\n"
        : [nOut] "=m" (n)
        : [nIn] "m" (n)
);



return n;


}

int main (int argc, char **argv)
{
double  n = 0.0;

printf("Absolute value\n");
if (argc > 1)
    n = atof(argv[1]);

printf("abs(%.*f) = %.*f\n", PRECISION, n, PREC开发者_如何学编程ISION, absD(n));

return 0;
}

here is the output:

~ $ gc a3
gcc -Wall -g a3.c -o a3
~ $ ./a3
Absolute value
abs(0.000) = 0.000
~ $

Not outputing its absolute value... Thank you..

It is completely working..!! You are just forgetting to put values to argv. You can run the program with something like this:

./a3 -1.3

and it should return 1.3

you were done before posting it here...

fld (%eax) means "load a float from the value at address %eax". Obviously, the contents of %eax are a double, and not a pointer to a float, which is why you segfault.

Since the input is already on the stack (thus it has an address), there's no need to jump through hoops moving things around.

double absD(double input) {
    double output;
    asm(
            "fldl %[input]\n"
            "fabs\n"
            "fstpl %[output]\n"
            : [output] "=m" (output)
            : [input] "m" (input)
    );
    return output;
}

Also, your printf format is wrong: %f means float, but you're giving it a double; you want to use %g.