float fmodf(float_x, float_y) function, math.h, c++

Question

hello I was hoping someone could help me with this.

I am using the float fmodf(float_x, float_y) function from math.h.

i am able to code with it properly but I was just wanted to know does anyone know the functions exact code is, so i can understand it better

Answer 1

The exact code depends on your math library (probably included in the same package as your C library). You can look at the glibc implementation if you want to see one implementation. This is the function that gets used in glibc (from sysdeps/ieee754/flt-32/e_fmodf.c).

float
__ieee754_fmodf (float x, float y)
{
    int32_t n,hx,hy,hz,ix,iy,sx,i;

    GET_FLOAT_WORD(hx,x);
    GET_FLOAT_WORD(hy,y);
    sx = hx&0x80000000;     /* sign of x */
    hx ^=sx;        /* |x| */
    hy &= 0x7fffffff;   /* |y| */

    /* purge off exception values */
    if(hy==0||(hx>=0x7f800000)||        /* y=0,or x not finite */
       (hy>0x7f800000))         /* or y is NaN */
        return (x*y)/(x*y);
    if(hx<hy) return x;         /* |x|<|y| return x */
    if(hx==hy)
        return Zero[(u_int32_t)sx>>31]; /* |x|=|y| return x*0*/

    /* determine ix = ilogb(x) */
    if(hx<0x00800000) { /* subnormal x */
        for (ix = -126,i=(hx<<8); i>0; i<<=1) ix -=1;
    } else ix = (hx>>23)-127;

    /* determine iy = ilogb(y) */
    if(hy<0x00800000) { /* subnormal y */
        for (iy = -126,i=(hy<<8); i>=0; i<<=1) iy -=1;
    } else iy = (hy>>23)-127;

    /* set up {hx,lx}, {hy,ly} and align y to x */
    if(ix >= -126)
        hx = 0x00800000|(0x007fffff&hx);
    else {      /* subnormal x, shift x to normal */
        n = -126-ix;
        hx = hx<<n;
    }
    if(iy >= -126)
        hy = 0x00800000|(0x007fffff&hy);
    else {      /* subnormal y, shift y to normal */
        n = -126-iy;
        hy = hy<<n;
    }

    /* fix point fmod */
    n = ix - iy;
    while(n--) {
        hz=hx-hy;
        if(hz<0){hx = hx+hx;}
        else {
        if(hz==0)       /* return sign(x)*0 */
            return Zero[(u_int32_t)sx>>31];
        hx = hz+hz;
        }
    }
    hz=hx-hy;
    if(hz>=0) {hx=hz;}

    /* convert back to floating value and restore the sign */
    if(hx==0)           /* return sign(x)*0 */
        return Zero[(u_int32_t)sx>>31];
    while(hx<0x00800000) {      /* normalize x */
        hx = hx+hx;
        iy -= 1;
    }
    if(iy>= -126) {     /* normalize output */
        hx = ((hx-0x00800000)|((iy+127)<<23));
        SET_FLOAT_WORD(x,hx|sx);
    } else {        /* subnormal output */
        n = -126 - iy;
        hx >>= n;
        SET_FLOAT_WORD(x,hx|sx);
        x *= one;       /* create necessary signal */
    }
    return x;       /* exact output */
}