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To debug a C program using gdb effectively it is required to compile it first using the command:

gcc -g programName.c

otherwise on using simply

gcc programName.c

and running we are unable to provide gdb the Source Code i.e.programName.c resulting in an error message -

dl-debug.c: No such file or directory.

on demand for command like list in gdb.

Please Explain the Cause, Why it is like this.

  • 3
    Have you read the manual or did any other research, yet? – johannes Sep 29 '14 at 9:41
  • 1
    the -g flag tells the compiler to add debug info that allows gdb to function, it is disabled by default to 1 prevent bloat and 2 prevent reverse engineering – ratchet freak Sep 29 '14 at 9:41
  • @ratchetfreak it doesn't really prevnt reverse enginieering, maybe it adds a tiny bit of work, but there's still enough info in the binary and good tools ... – johannes Sep 29 '14 at 9:53
3

A debugger needs:

  • to refer to variable and function names, not their addresses (e.g. i not 0xdeadbeef and main() not 0x4B1D).
  • to associate the address of machine code with a line in the source code.

To this purpose the compiler builds (-g) a (augmented) symbol table (it may be embedded into the program or stored as a separate file). Sometimes other ancillary information are generated.

There are different formats to store these information (stabs, coff, dwarf...).

Contrarily to popular beliefs, many compilers (GCC for sure) write some symbols to an object file even in release mode (with the -O3 switch). That's why even with a release binary, you can do:

$ gcc -O3 programName.c
$ gdb a.out
[...]
Reading symbols from a.out...(no debugging symbols found)...done.

Of course this isn't enough for a good debugging experience.

Symbols can be removed from the binary using the strip command:

$ strip -s a.out
$ nm a.out
nm: a.out: no symbols

and now:

$ gdb a.out
[...]
(gdb) b main
Function "main" not defined.

Also consider that there is an interaction between code optimization and debugging:

  • some variables you declared may not exist at all;
  • the flow of control may briefly move where you did not expect it;
  • some statements may not be executed because they compute constant results or their values were already at hand
  • some statements may execute in different places because they were moved out of loops
  • ...

So, with many compilers, you cannot turn on both optimization and debugging (GCC has the -Og switch for debugging optimized code).

Further details:

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