linuxassemblygdbnasm

GDB complains No Source Available

I'm running on Ubuntu 12.10 64-bit.

I am trying to debug a simple assembly program in GDB. However, GDB's GUI mode (-tui) appears to be unable to locate the source code of my assembly file. I've rebuilt the project in the current directory and searched Google to no avail.

My commands:

nasm -f elf64 -g -F dwarf hello.asm
gcc -g hello.o -o hello
gdb -tui hello

Debug information appears to be loaded; I can set a breakpoint at main(), but the top half of the screen still displays '[ No Source Available ]'.

Here is hello.asm if you're interested:

;  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
Solution

  • The problem in this case is that the assembler isn't producing line-number information for the debugger. So although the source is there (if you do "list" in gdb, it shows a listing of the source file - at least when I follow your steps, it does), but the debugger needs line-number information from the file to know what line corresponds to what address. It can't do that with the information given.

    As far as I can find, there isn't a way to get NASM to issue the .loc directive that is used by as when using gcc for example. But as isn't able to take your source file without generating a gazillion errors [even with -msyntax=intel -mmnemonic=intel -- you would think that should work].

    So unless someone more clever can come up with a way to generate the .loc entries which gives the debugger line number information, I'm not entirely sure how we can answer your question in a way that you'll be happy with.