How can I include debug information with nasm?

Question

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.

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. — Jester
– Jester, Commented Jan 2, 2015 at 19:41
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. — Jester
– Jester, 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 :) — Koray Tugay
– Koray Tugay, Commented Jan 2, 2015 at 19:49

dev_null · Accepted Answer · 2015-01-02 20:11:57Z

8

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.

answered Jan 2, 2015 at 20:11

dev_null

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3 Comments

Frank Kotler Over a year ago

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...

Koray Tugay Over a year ago

@FrankKotler Thanks for the information. _start made ld not complain anymore. But what is the cause for this? Any source I can read?

Frank Kotler Over a year ago

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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