What is symbol table and how is it integrated into the executable?

Question

When I tried to debug an executable:

(gdb) break +1
No symbol table is loaded.  Use the "file" command.

What does that mean exactly?

Is the symbol table appended to the executable?

Answer 1

Find the entry point of the application.

objdump -f main

    main:     file format elf32-i386
    architecture: i386, flags 0x00000112:
    EXEC_P, HAS_SYMS, D_PAGED
    start address 0x08048054

Put a breakpoint there using the gnu debugger

gdb

    exec-file main
    break *0x8048054
    set disassemble-next-line on
    run

Then step through the code

gdb

    stepi

Special Notes

If you are using the latest version of Ubuntu you would not be affected by this, but you may run into this bug if you are running Ubuntu 10.04 or older.

https://bugs.launchpad.net/ubuntu/+source/gdb/+bug/151518G

The solution would be to start debugging at the entry point address plus one.

Answer 2

It's because you didn't compile with debugging turned on. Try gcc -g file.c

Answer 3

The symbol table contains debugging information that tells a debugger what memory locations correspond to which symbols (like function names and variable names) in the original source code file. The symbol table is usually stored inside the executable, yes.

gdb is telling you that it can't find that table. If you compiled with gcc, unless you used the -g flag, it will not include the symbol table in the file. The easiest method is probably to recompile your file with -g. gdb should then automatically find the symbol table information.

Either add the -g flag to the command line arguments of gcc or to the Makefile that you used to compile the program. (A lot of times, there will be a variable called CFLAGS or similar inside the Makefile).

If you are trying to debug an arbitrary third-party program, a lot of times the information will have been "stripped" out of it. This is done to make reverse engineering harder and to make the size of the executable file smaller. Unless you have access to the source code and can compile the program yourself, you will have a very hard time using gdb on it.

Answer 4

There are two sets of symbols that gdb uses.

The -g set are debugging symbols, which make things a lot easier as they allow you to see your code and look at variables while debugging.

Another set of symbols is included by default when you compile. These are the linking symbols and live in the ELF (executable linkable format) symbol table. This contains a lot less info than the debug symbols, but contain the most important stuff, such as the addresses of the things in your executable (or library or object file). Without this information gdb won't even know where main is, so (gdb) break main would fail.

If you don't have the debugging symbols ( -g ) then you will still be able to (gdb) break main but you gdb will not have any concept of the lines of code in your source file. When you try to step through the code you will only advance 1 machine instruction at a time, rather than a line at a time.

The strip command is often used to strip off symbols from an executable (or other object file). This is often used if you don't want someone to be able to see the symbols or if you want to save space in the file. Symbol tables can get big. Strip removes both the debug symbols and the linker symbols, but it has several command line switches which can limit what it removes.

If you run the file command on your program one of the things it will tell you is weather or not the executable is has been stripped.

$ gcc my_prog.c -o my_prog
$ file my_prog
my_prog: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.15, not stripped
$ strip my_prog
my_prog: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.15, stripped
$