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I have this source code:

; hello.asm a first program for nasm for Linux, Intel, gcc ; ; assemble: nasm -f elf -l hello.lst hello.asm ; link: gcc -o hello hello.o ; run: hello ; output is: Hello World SECTION .data ; data section msg: db "Hello World",10 ; the string to print, 10=cr len: equ $-msg ; "$" means "here" ; len is a value, not an address SECTION .text ; code section global main ; make label available to linker main: ; standard gcc entry point mov edx,len ; arg3, length of string to print mov ecx,msg ; arg2, pointer to string mov ebx,1 ; arg1, where to write, screen mov eax,4 ; write command to int 80 hex int 0x80 ; interrupt 80 hex, call kernel mov ebx,0 ; exit code, 0=normal mov eax,1 ; exit command to kernel int 0x80 ; interrupt 80 hex, call kernel

This code is taken from here.

I am running ubuntu 12.04 32-bit on VirtualBox for learning purposes.

Steps I follow are:

  • nasm -f elf -g -F stabs hello.asm
  • ld -o hello hello.o
  • gdb hello -tui

Now when I only run hello it will run fine but gdb fails to show any source code. Why? When I tryp run in gdb I will see Hello World text just fine but it does not show the source.

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  • That's a bug somewhere in nasm or gdb. Note that if you start doing si for example, gdb will happily show you the source code. Commented Jan 2, 2015 at 19:41
  • what do you mean "start doing si" @Jester ? Commented Jan 2, 2015 at 19:42
  • Use the si command to single step instructions. gdb will correctly show the source code. Use start instead of run or place a breakpoint manually first. Commented Jan 2, 2015 at 19:43
  • @Jester Sorry, can you give a full example as an answer? I am very new to Assembly and I have no idea what "use the si command to single step instructions" mean.. Sorry :) Commented Jan 2, 2015 at 19:49
  • Excellent commenting style. Voted +1 for that factor alone Commented Jan 3, 2015 at 1:05

1 Answer 1

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It looks like stabs format doesn't work with GDB, try DWARF instead ( http://en.wikipedia.org/wiki/DWARF )

compile with

nasm -f elf -g -F dwarf hello.asm

then in gdb type

start

then

si

you will see sources with comments so on. as Koray Tugay said there is most probably a bug in gdb.

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3 Comments

May also help to put nop as the first instruction after... well, you're using main. Does ld not complain about the lack of an entrypoint? ld usually expects _start - gcc (like it says where you found the code) expects main. Tell ld -e main or change it to _start to shut ld up... or use gcc...
@FrankKotler Thanks for the information. _start made ld not complain anymore. But what is the cause for this? Any source I can read?
Some tutorials here: asm.sourceforge.net/resources.html#tutorials You might find that kind of information in the "teensy" tutorial, although writing obsessively short files is probably not something you want to do. Of course there's man ld...

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