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<title>Using gdb - GNU Scientific Library -- Reference Manual</title>
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Copyright (C) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 The GSL Team.
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Next: <a rel="next" accesskey="n" href="Examining-floating-point-registers.html#Examining-floating-point-registers">Examining floating point registers</a>,
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<h3 class="section">A.1 Using gdb</h3>
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<p><a name="index-gdb-2367"></a><a name="index-debugging-numerical-programs-2368"></a><a name="index-breakpoints-2369"></a>Any errors reported by the library are passed to the function
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<code>gsl_error</code>. By running your programs under gdb and setting a
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breakpoint in this function you can automatically catch any library
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errors. You can add a breakpoint for every session by putting
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<pre class="example"> break gsl_error
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<p class="noindent">into your <samp><span class="file">.gdbinit</span></samp> file in the directory where your program is
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<p>If the breakpoint catches an error then you can use a backtrace
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(<code>bt</code>) to see the call-tree, and the arguments which possibly
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caused the error. By moving up into the calling function you can
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investigate the values of variables at that point. Here is an example
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from the program <code>fft/test_trap</code>, which contains the following
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<pre class="smallexample"> status = gsl_fft_complex_wavetable_alloc (0, &complex_wavetable);
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<p class="noindent">The function <code>gsl_fft_complex_wavetable_alloc</code> takes the length of
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an FFT as its first argument. When this line is executed an error will
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be generated because the length of an FFT is not allowed to be zero.
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<p>To debug this problem we start <code>gdb</code>, using the file
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<samp><span class="file">.gdbinit</span></samp> to define a breakpoint in <code>gsl_error</code>,
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<pre class="smallexample"> $ gdb test_trap
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GDB is free software and you are welcome to distribute copies
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of it under certain conditions; type "show copying" to see
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the conditions. There is absolutely no warranty for GDB;
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type "show warranty" for details. GDB 4.16 (i586-debian-linux),
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Copyright 1996 Free Software Foundation, Inc.
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Breakpoint 1 at 0x8050b1e: file error.c, line 14.
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<p class="noindent">When we run the program this breakpoint catches the error and shows the
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<pre class="smallexample"> (gdb) run
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Starting program: test_trap
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Breakpoint 1, gsl_error (reason=0x8052b0d
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"length n must be positive integer",
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file=0x8052b04 "c_init.c", line=108, gsl_errno=1)
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14 if (gsl_error_handler)
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<p class="noindent">The first argument of <code>gsl_error</code> is always a string describing the
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error. Now we can look at the backtrace to see what caused the problem,
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<pre class="smallexample"> (gdb) bt
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#0 gsl_error (reason=0x8052b0d
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"length n must be positive integer",
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file=0x8052b04 "c_init.c", line=108, gsl_errno=1)
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#1 0x8049376 in gsl_fft_complex_wavetable_alloc (n=0,
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wavetable=0xbffff778) at c_init.c:108
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#2 0x8048a00 in main (argc=1, argv=0xbffff9bc)
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#3 0x80488be in ___crt_dummy__ ()
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<p class="noindent">We can see that the error was generated in the function
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<code>gsl_fft_complex_wavetable_alloc</code> when it was called with an
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argument of <var>n=0</var>. The original call came from line 94 in the
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file <samp><span class="file">test_trap.c</span></samp>.
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<p>By moving up to the level of the original call we can find the line that
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<pre class="smallexample"> (gdb) up
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#1 0x8049376 in gsl_fft_complex_wavetable_alloc (n=0,
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wavetable=0xbffff778) at c_init.c:108
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108 GSL_ERROR ("length n must be positive integer", GSL_EDOM);
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#2 0x8048a00 in main (argc=1, argv=0xbffff9bc)
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94 status = gsl_fft_complex_wavetable_alloc (0,
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&complex_wavetable);
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<p class="noindent">Thus we have found the line that caused the problem. From this point we
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could also print out the values of other variables such as
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<code>complex_wavetable</code>.
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