Question

The cod that I am using contains these snippets of code. I am calling ThetaG_JD with the argument 2455343.50000 which is just a sample Julian date. Every time I run the program, I receive a EXC_BAD_ACCESS on the indicated line. When using gdb and printing out the intermediary values and passing them through the floor function, I get no error, but when Frac() is used it always returns an error.

double Frac(double arg)
{
    /* Returns fractional part of double argument */
    return arg - floor(arg);
}

double ThetaG_JD(double jd)
{
    /* Reference:  The 1992 Astronomical Almanac, page B6. */

    double UT=0, TU=0, GMST=0;
    //THIS LINE
    UT=Frac(jd+0.5);
    // THAT ONE ^^
    jd=jd-UT;
    TU=(jd-2451545.0)/36525;
    GMST=24110.54841+TU*(8640184.812866+TU*(0.093104-TU*6.2E-6));
    GMST=Modulus(GMST+secday*omega_E*UT,secday);

    return (twopi*GMST/secday);
}

Answer 1

The EXC_BAD_ACCESS is somewhat puzzling to me, but this sounds suspiciously like a floating point exception. It’s been a while, but as I recall on x87 hardware, you could generate overflow/underflow/NaN and the processor wouldn’t let you know with an exception until the next FP operation which could be in a totally different part of the code. You could try something like jd += 0.5 instead of the call for Frac and see if it still dies.

Also the x87 status register will be able to show you if there’s an error state and you should be able to see that within gbd.