## Using gdb

Any errors reported by the library are passed to the function `gsl_error`. By running your programs under gdb and setting a breakpoint in this function you can automatically catch any library errors. You can add a breakpoint for every session by putting

    break gsl_error

into your ``.gdbinit'` file in the directory where your program is started.

If the breakpoint catches an error then you can use a backtrace (`bt`) to see the call-tree, and the arguments which possibly caused the error. By moving up into the calling function you can investigate the values of variable at that point. Here is an example from the program `fft/test_trap`, which contains the following line,

    status = gsl_fft_complex_wavetable_alloc (0, &complex_wavetable);

The function `gsl_fft_complex_wavetable_alloc` takes the length of an FFT as its first argument. When this line is executed an error will be generated because the length of an FFT is not allowed to be zero.

To debug this problem we start `gdb`, using the file ``.gdbinit'` to define a breakpoint in `gsl_error`,

    bash$ gdb test_trap

    GDB is free software and you are welcome to distribute copies
    of it under certain conditions; type "show copying" to see
    the conditions.  There is absolutely no warranty for GDB;
    type "show warranty" for details.  GDB 4.16 (i586-debian-linux),
    Copyright 1996 Free Software Foundation, Inc.

    Breakpoint 1 at 0x8050b1e: file error.c, line 14.

When we run the program this breakpoint catches the error and shows the reason for it.

    (gdb) run
    Starting program: test_trap

    Breakpoint 1, gsl_error (reason=0x8052b0d
        "length n must be positive integer",
        file=0x8052b04 "c_init.c", line=108, gsl_errno=1)
        at error.c:14
    14        if (gsl_error_handler)

The first argument of `gsl_error` is always a string describing the error. Now we can look at the backtrace to see what caused the problem,

    (gdb) bt
    #0  gsl_error (reason=0x8052b0d
        "length n must be positive integer",
        file=0x8052b04 "c_init.c", line=108, gsl_errno=1)
        at error.c:14
    #1  0x8049376 in gsl_fft_complex_wavetable_alloc (n=0,
        wavetable=0xbffff778) at c_init.c:108
    #2  0x8048a00 in main (argc=1, argv=0xbffff9bc)
        at test_trap.c:94
    #3  0x80488be in ___crt_dummy__ ()

We can see that the error was generated in the function `gsl_fft_complex_wavetable_alloc` when it was called with an argument of n=0. The original call came from line 94 in the file ``test_trap.c'`.

By moving up to the level of the original call we can find the line that caused the error,

    (gdb) up
    #1  0x8049376 in gsl_fft_complex_wavetable_alloc (n=0,
        wavetable=0xbffff778) at c_init.c:108
    108   GSL_ERROR ("length n must be positive integer", GSL_EDOM);
    (gdb) up
    #2  0x8048a00 in main (argc=1, argv=0xbffff9bc)
        at test_trap.c:94
    94    status = gsl_fft_complex_wavetable_alloc (0,
            &complex_wavetable);

Thus we have found the line that caused the problem. From this point we could also print out the values of other variables such as `complex_wavetable`.