About the "info locals" command of gdb and python pretty printer

[asmwarrior <asmwarrior@gmail.com>]

Hi, I have some problems to show some uninitialized local variables.

For example, here is the test code:

void test()
{
     int aaaa = 1;
     int bbbb = 2;
     int cccc = 3;
}

int main()
{
     test();
     return 0;
}

Here, if I set a breakpoint at the first statement of the "test()", then 
run "info locals" command. gdb will plot all the information about both 
"aaaa, bbbb and cccc".

Things becomes more complex when bbbb and cccc are not simple type, but 
instead, they are stl containers like vector<int> or maps.
like:

void test()
{
     int aaaa = 1;
     vector <int> bbbb;
     map<int,string>cccc;
}

At this time, if we run the "info locals" with python stl pretty printer 
enabled, as you see, if you breakpoint is still at the first line "int 
aaa = 1;", then bbbb and cccc are not initialized, this may cause the 
python script to plot random values, some times, gdb or python will get 
crashed.
A more detailed discussion can be found :
http://sourceware.org/bugzilla/show_bug.cgi?id=11407

Also, this kind of problem is exist some other situations, In the 
QTCreator's manual:
http://doc.qt.nokia.com/qtcreator-1.2/creator-debugging.html
> The debug information provided by gcc does not include enough 
> information about the time when a variable is initialized. Therefore, 
> Qt Creator can not tell whether the contents of a local variable 
> contains "real data", or "initial noise". If a QObject appears 
> uninitialized, its value will be reported as "out of scope". However, 
> not all uninitialized objects can be recognized as such.

I asked this kind of question on the GCC maillist, some one replies in 
this post:
http://gcc.gnu.org/ml/gcc/2010-06/msg00633.html
He suggest that we can use the GCC option: -fvar-tracking

With this option, the variable location was recorded in the output file. 
I have just do a test on my first example code. (I tested under MinGW 
GCC 4.4.4)

objdump -W MyTarget.exe >> log.txt
will give all the debug information, like below:

-----------------------------------
...
<1><5f>: Abbrev Number: 2 (DW_TAG_subprogram)
<60>   DW_AT_external    : 1
<61>   DW_AT_name        : test
<66>   DW_AT_decl_file   : 1
<67>   DW_AT_decl_line   : 1
<68>   DW_AT_MIPS_linkage_name: _Z4testv
<71>   DW_AT_low_pc      : 0x4013d0
<75>   DW_AT_high_pc     : 0x4013ed
<79>   DW_AT_frame_base  : 0x0    (location list)
<7d>   DW_AT_sibling     : <0xb9>
<2><81>: Abbrev Number: 3 (DW_TAG_lexical_block)
<82>   DW_AT_low_pc      : 0x4013d6
<86>   DW_AT_high_pc     : 0x4013eb
<3><8a>: Abbrev Number: 4 (DW_TAG_variable)
<8b>   DW_AT_name        : aaaa
<90>   DW_AT_decl_file   : 1
<91>   DW_AT_decl_line   : 3
<92>   DW_AT_type        : <0xb9>
<96>   DW_AT_location    : 2 byte block: 75 74     (DW_OP_breg5: -12)
<3><99>: Abbrev Number: 4 (DW_TAG_variable)
<9a>   DW_AT_name        : bbbb
<9f>   DW_AT_decl_file   : 1
<a0>   DW_AT_decl_line   : 4
<a1>   DW_AT_type        : <0xb9>
<a5>   DW_AT_location    : 2 byte block: 75 78     (DW_OP_breg5: -8)
<3><a8>: Abbrev Number: 4 (DW_TAG_variable)
<a9>   DW_AT_name        : cccc
<ae>   DW_AT_decl_file   : 1
<af>   DW_AT_decl_line   : 5
<b0>   DW_AT_type        : <0xb9>
<b4>   DW_AT_location    : 2 byte block: 75 7c     (DW_OP_breg5: -4)
<1><b9>: Abbrev Number: 5 (DW_TAG_base_type)
<ba>   DW_AT_byte_size   : 4
<bb>   DW_AT_encoding    : 5    (signed)
<bc>   DW_AT_name        : int
...
----------------------------------------
So, you can see, there are some information about "DW_AT_decl_line", 
this information tells us that this variable is initialized after this 
line.

So, I think if GCC can use this information, then we can nicely solved 
the "info locals" command problem.

Any ideas?

By the way, there are other ways to give the "DW_AT_start_scope" 
information about a variable, but Currently, GCC don't have this kind of 
debug information generated. See here: 
http://gcc.gnu.org/ml/gcc/2007-04/msg00138.html


Thanks for reading my post.

asmwarrior ollydbg from codeblocks' forum