[GRASS-dev] d.thematic.area: need for a bit of help in C-programming/debugging

[Glynn Clements]

Moritz Lennert wrote:

> I'm having a problem with the d.thematic.area module I just committed in 
> a modified version.

> It seems to be linked to the  class_discont function in 
> lib/arraystats/class.c, as this only happens with this algorithm. 
> However, when I do the same test with d.thematic.areas -nl flags (i.e. 
> don't plot the map, but only output legend info), I don't get the 
> segfaults, so there is some problem between this function and the 
> display functions.

Or you just get lucky in that case.

> I seem to have tracked it down to the call to Vect_new_line_struct() on 
> line 32 in display/d.thematic.area/area.c and from there to (I think) 
> Vect__new_line_struct in lib/vector/Vlib/line.c.

Vect_new_line_struct() just malloc()s a block of memory. If it's
crashing there, then it's because the heap has been corrupted, by
overrunning another heap buffer.

I suggest using glibc's mprobe() function:

 -- Function: enum mcheck_status mprobe (void *POINTER)
     The `mprobe' function lets you explicitly check for inconsistencies
     in a particular allocated block.  You must have already called
     `mcheck' at the beginning of the program, to do its occasional
     checks; calling `mprobe' requests an additional consistency check
     to be done at the time of the call.

     The argument POINTER must be a pointer returned by `malloc' or
     `realloc'.  `mprobe' returns a value that says what inconsistency,
     if any, was found.  The values are described below.

 -- Data Type: enum mcheck_status
     This enumerated type describes what kind of inconsistency was
     detected in an allocated block, if any.  Here are the possible
     values:

    `MCHECK_DISABLED'
          `mcheck' was not called before the first allocation.  No
          consistency checking can be done.

    `MCHECK_OK'
          No inconsistency detected.

    `MCHECK_HEAD'
          The data immediately before the block was modified.  This
          commonly happens when an array index or pointer is
          decremented too far.

    `MCHECK_TAIL'
          The data immediately after the block was modified.  This
          commonly happens when an array index or pointer is
          incremented too far.

    `MCHECK_FREE'
          The block was already freed.

To use it, you need to link the program with -lmcheck (this ensures
that mcheck() is called before the first malloc()), then call mprobe()
on any malloc()d block which you suspect may be getting overrun (those
in class_discont() would seem like an obvious choice).

-- 
Glynn Clements <glynn at gclements.plus.com>