16 July 2009

Printing arrays and gsl_matrix data in 2D form within gdb

Because I'm lousy with indexing, I often have to watch loops unfurl live in gdb. Here are several quick user-defined gdb commands that make inspecting 2-dimensional data stored in a 1-dimensional array much easier.

Place the following commands in your ~/.gdbinit:

# Modified from http://sourceware.org/ml/gdb/2007-07/msg00140.html
define p_array2d_generic
    set $array=$arg0
    set $ibound=$arg1
    set $jbound=$arg2
    set $ilda=$arg3
    set $jlda=$arg4

    set $i = 0
    set $j = 0
    while $i<$ibound
        while $j<$jbound
            printf " "
            printf "%9.4g", $array[$i*$ilda+$j*$jlda]
            set $j = $j + 1
        printf "\n"
        set $j = 0
        set $i = $i + 1

document p_array2d_generic
Print an array in 2D form: p_array2d array ibound jbound ilda jlda

define p_array2d_cm
    p_array2d_generic $arg0 $arg1 $arg2 1 $arg1

document p_array2d_cm
Print an array in column-major 2D form: p_array2d array ibound jbound

define p_array2d_rm
    p_array2d_generic $arg0 $arg1 $arg2 $arg2 1

document p_array2d_rm
Print an array in row-major 2D form: p_array2d array ibound jbound
Now you can use p_array2d_cm and p_array2d_rm to print out column- and row-major double arrays, respectively.

Using the above and some information about the memory layout of gsl_matrix instances, it's now easy to write one line function to dump a gsl_matrix:

define p_gsl_matrix
    p_array2d_generic $arg0->data $arg0->size1 $arg0->size2 $arg0->tda 1

document p_gsl_matrix
Print a gsl_matrix in 2D form: p_gsl_matrix (gsl_matrix *)

Question for you: I would like to have the p_array2d_generic take an additional argument which is a format specifier. A check using $argc would allow a reasonable, double-based default but would allow the user to specify anything he or she desires. However, I cannot get code like

set $format="%3.2g"
printf $format, 123
to work the way I think it should. I always run into a gdb error like <<Bad format string, missing '"'.>>. Anyone have any ideas how to provide a printf template parameter from a set variable in gdb?

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