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gdb provides a command "print localx" which prints the value stored in the localx variable. So, it basically must be using the symbol table to find the mapping (localx -> addressx on stack). I am unable to understand how this mapping can be created.

What I tried I studied the intermediate temporary files of gcc using -save-temps option, and observed that a local variable local1 was mapped to a symbol name "LASF8". However, the objdump utility tool did not show this symbol name.

Context : I am working on a project which requires building a pin-tool to print the accesses of local variables. Given a function, I would like to say that this address corresponds to this variable name. This requires reading the symbol table to correspond an address to a symbol table entry. GDB does the exact reverse mapping. Hence, I would like to understand the same.

Karishma Agarwal
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    This could help: http://eli.thegreenplace.net/2011/02/07/how-debuggers-work-part-3-debugging-information/ – dbrank0 Aug 25 '14 at 14:04

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The symbol table is contained in the debugging information. This debugging information is emitted by gcc -g. gdb reads the debugging information to get symbolic information, among other things.

Typically the debugging information is in DWARF format. See http://www.dwarfstd.org/ for the specification.

You can also see DWARF more directly using readelf. For example readelf -wi will show the main (".debug_info") debugging information for an ELF file.

Note that doing the mapping in reverse -- that is, assigning a name to every stack slot -- is not entirely easy. First, not every stack slot will have a name. This is because the compiler may spill temporaries to the stack. Second, many locals will have DWARF location expressions to represent their location. This means you'll need to write an expression evaluator (not hard but also not trivial); you could conceivably (unlikely in practice but possible in theory) run into expressions which cannot be evaluated without a real stack frame; and finally the names will therefore generally only be valid at a given PC.

I believe there's a feature request in gdb bugzilla to add this feature to gdb.

Tom Tromey
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  • Thanks for your prompt response. Could you please give details on how to write expression evaluator to represent local variable location? – Karishma Agarwal Aug 25 '14 at 19:13
  • I can't readily explain it. There are a lot of pieces to it, especially if you are using GCC and optimization, since there are also a number of GNU extensions (all documented -- but not in the main text). There's a whole section of the DWARF standard covering the basic expression syntax. But then there's also finding the expressions, etc. – Tom Tromey Aug 25 '14 at 20:35