GRASS Debugging:

 Print debugging message if variable DEBUG
 is set to level equal or greater

g.gisenv set="DEBUG=3"

Levels: (recommended levels)
 * 0 - silence
 * 1 - message is printed once or few times per module
 * 3 - each row (raster) or line (vector)
 * 5 - each cell (raster) or point (vector)

Further hints:
> How can I force gcc, to complain about missing header file?
 
'-Wimplicit-function-declaration'
'-Werror-implicit-function-declaration'
     Give a warning (or error) whenever a function is used before being
     declared.

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To debug  TCL code, run (example):

GRASS:~> g.gisenv set="DEBUG=1"
GRASS:~> d.text.freetype > dtf_tcl.txt

then edit 'dtf_tcl.txt'  to remove the "| wish &" from the end.

then '. dtf_tcl.txt > dtf.tcl' to get rid of the echo-proofing chars.

then 'wish < dtf.tcl'  to test it.

References:
"Is white space significant in Tcl"  http://wiki.tcl.tk/981
"Tcl quoting hell"  http://wiki.tcl.tk/1726
"1 minute intro to tcl"  http://www.pconline.com/~erc/tcl.htm

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To make gcc less tolerant in accepting errors and code flaws, compile
with:

export CFLAGS='-g -ansi -Wall -D_BSD_SOURCE=1 -D_SVID_SOURCE=1 -D_POSIX_SOURCE=1 -D_POSIX_C_SOURCE=199506L'

Also nice to emulate gcc/MacOSX compiler:
CFLAGS='-fno-common'

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A good idea is maybe to use '-Wall -Werror' for the gcc options. This means
it treats the warnings as errors, i.e. it stops on them. So you have time
to read them then.

The gcc switch -Wimplicit-function-declaration (implied by -Wall) should
report missing prototypes (use -Werror-implicit-function-declaration to
make it an error rather than a warning).
 
The linker switch --warn-common (e.g. LDFLAGS='-Wl,--warn-common') might
be useful for catching multiply-defined symbols.
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C Code debugging:

1) Graphical debugger
      ddd `which r.plane`
      RUN -> here enter Parameters/Flags

2) Text/CMD line debugger
      gdb `which r.plane`
      r <flags> <parameters>
      bt
      
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Debugging on Mac OS X

 The 'ldd' command doesn't exist, but
  otool -L 
 does almost the same job.