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This chapter describes some tips and tricks for debugging numerical programs which use GSL.
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
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
status = gsl_fft_complex_wavetable_alloc (0, &complex_wavetable);
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
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
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
The contents of floating point registers can be examined using the
info float (not available on all platforms).
(gdb) info float st0: 0xc4018b895aa17a945000 Valid Normal -7.838871e+308 st1: 0x3ff9ea3f50e4d7275000 Valid Normal 0.0285946 st2: 0x3fe790c64ce27dad4800 Valid Normal 6.7415931e-08 st3: 0x3ffaa3ef0df6607d7800 Spec Normal 0.0400229 st4: 0x3c028000000000000000 Valid Normal 4.4501477e-308 st5: 0x3ffef5412c22219d9000 Zero Normal 0.9580257 st6: 0x3fff8000000000000000 Valid Normal 1 st7: 0xc4028b65a1f6d243c800 Valid Normal -1.566206e+309 fctrl: 0x0272 53 bit; NEAR; mask DENOR UNDER LOS; fstat: 0xb9ba flags 0001; top 7; excep DENOR OVERF UNDER LOS ftag: 0x3fff fip: 0x08048b5c fcs: 0x051a0023 fopoff: 0x08086820 fopsel: 0x002b
Individual registers can be examined using the variables $reg, where reg is the register name.
(gdb) p $st1 $1 = 0.02859464454261210347719
It is possible to stop the program whenever a
point exception occurs. This can be useful for finding the cause of an
unexpected infinity or
NaN. The current handler settings can be
shown with the command
info signal SIGFPE.
(gdb) info signal SIGFPE Signal Stop Print Pass to program Description SIGFPE Yes Yes Yes Arithmetic exception
Unless the program uses a signal handler the default setting should be
changed so that SIGFPE is not passed to the program, as this would cause
it to exit. The command
handle SIGFPE stop nopass prevents this.
(gdb) handle SIGFPE stop nopass Signal Stop Print Pass to program Description SIGFPE Yes Yes No Arithmetic exception
Depending on the platform it may be necessary to instruct the kernel to
generate signals for floating point exceptions. For programs using GSL
this can be achieved using the
GSL_IEEE_MODE environment variable
in conjunction with the function
gsl_ieee_env_setup() as described
in see section IEEE floating-point arithmetic.
(gdb) set env GSL_IEEE_MODE=double-precision
Writing reliable numerical programs in C requires great care. The following GCC warning options are recommended when compiling numerical programs:
gcc -ansi -pedantic -Werror -Wall -W -Wmissing-prototypes -Wstrict-prototypes -Wtraditional -Wconversion -Wshadow -Wpointer-arith -Wcast-qual -Wcast-align -Wwrite-strings -Wnested-externs -fshort-enums -fno-common -Dinline= -g -O4
For details of each option consult the manual Using and Porting GCC. The following table gives a brief explanation of what types of errors these options catch.
-Wall, but it is not enough on its own.
-Wallrely on the optimizer to analyze the code. If there is no optimization then the warnings aren't generated.
-Wall, such as missing return values and comparisons between signed and unsigned integers.
unsigned int x = -1. If you need to perform such a conversion you can use an explicit cast.
-Wpointer-arith -Wcast-qual -Wcast-align
void, if you remove a
constcast from a pointer, or if you cast a pointer to a type which has a different size, causing an invalid alignment.
constqualifier so that it will be a compile-time error to attempt to overwrite them.
enumas short as possible. Normally this makes an
enumdifferent from an
int. Consequently any attempts to assign a pointer-to-int to a pointer-to-enum will generate a cast-alignment warning.
externdeclaration is encountered within an function.
inlinekeyword is not part of ANSI C. Thus if you want to use
-ansiwith a program which uses inline functions you can use this preprocessor definition to remove the
gdb. The only effect of debugging symbols is to increase the size of the file, and you can use the
stripcommand to remove them later if necessary.
The following books are essential reading for anyone writing and debugging numerical programs with GCC and GDB.
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