R – How to get GNU AS to emit per-line debugging info or GDB to single step lines in (AVR) Assembly

assemblyavr-gccembeddedgccgdb

I cannot figure out how to get GAS to emit line number debugging information for gdb in assembly. Note that I'm using the avr-gcc cross compiler, but doubt that is relevant.

I'm using the following command line options to assemble:
avr-gcc -g -ggdb -Os -mmcu=atmega644 -ffunction-sections -fsigned-char -Winvalid-pch -Wall -Wno-long-long -x assembler-with-cpp -Wa,-ggdb -c -o z813.o z813.s

When I use a nearly identical set of arguments to compile C code, I can see the line number and source file augmentation using 'objdump -l -d'.

However the assembly objects have none. ie


000000d4 <run_timetable>:
      d4:       0e 94 57 00     call    0xae    ; 0xae <run_timetable_row>
      d8:       0e 94 b4 00     call    0x168   ; 0x168 <delay>

vs

00000f9c :
main():
/home/braddock/work/tviki/tviki/scratch/z813-vid.c:68
     f9c:       0e 94 ae 07     call    0xf5c   ; 0xf5c <init>
/home/braddock/work/tviki/tviki/scratch/z813-vid.c:70
     fa0:       0e 94 6a 00     call    0xd4    ; 0xd4 <run_timetable>

When in avr-gdb (via simulavr remote gdb connection), I cannot get the debugger to single step through my assemble code, although it does recognize the symbol names and information. I assume this is related.

I put in good effort trying to find this in info/man/google and permuting the likely flags. Any help appreciated!

Best Answer

When you "compile" assembler source code, invoking the C compiler is somewhat overkill. The compiler will recognize that the input is a C file, and ignore any options that you pass it that affect C compilation, such as -g and -Os. Pass "-v" to the compiler line to see the options that get actually passed to the assembler.

When I assemble with

avr-as --gstabs -mmcu=atmega644 a.s -o a.o

then I get nice-looking source line information in the object file. My version of avr-as (2.18.0) doesn't support a -ggdb option at all, it only has -g, --gstabs, --gstabs+, --gdwarf-2. With stabs, objdump is able to display the lines. With dwarf-2, it's not - I'm not sure whether that's a bug in objdump or in as.

If you absolutely insist on invoking the assembler as "gcc", you should pass -Wa,--gstabs.

Related Solutions

Linux – Using gdb to single-step assembly code outside specified executable causes error “cannot find bounds of current function”

Instead of gdb, run gdbtui. Or run gdb with the -tui switch. Or press C-x C-a after entering gdb. Now you're in GDB's TUI mode.

Enter layout asm to make the upper window display assembly -- this will automatically follow your instruction pointer, although you can also change frames or scroll around while debugging. Press C-x s to enter SingleKey mode, where run continue up down finish etc. are abbreviated to a single key, allowing you to walk through your program very quickly.

   +---------------------------------------------------------------------------+
B+>|0x402670 <main>         push   %r15                                        |
   |0x402672 <main+2>       mov    %edi,%r15d                                  |
   |0x402675 <main+5>       push   %r14                                        |
   |0x402677 <main+7>       push   %r13                                        |
   |0x402679 <main+9>       mov    %rsi,%r13                                   |
   |0x40267c <main+12>      push   %r12                                        |
   |0x40267e <main+14>      push   %rbp                                        |
   |0x40267f <main+15>      push   %rbx                                        |
   |0x402680 <main+16>      sub    $0x438,%rsp                                 |
   |0x402687 <main+23>      mov    (%rsi),%rdi                                 |
   |0x40268a <main+26>      movq   $0x402a10,0x400(%rsp)                       |
   |0x402696 <main+38>      movq   $0x0,0x408(%rsp)                            |
   |0x4026a2 <main+50>      movq   $0x402510,0x410(%rsp)                       |
   +---------------------------------------------------------------------------+
child process 21518 In: main                            Line: ??   PC: 0x402670
(gdb) file /opt/j64-602/bin/jconsole
Reading symbols from /opt/j64-602/bin/jconsole...done.
(no debugging symbols found)...done.
(gdb) layout asm
(gdb) start
(gdb)

How to link a gas assembly program that uses the C standard library with ld without using gcc

There are at least three things that you need to do to successfully use libc with dynamic linking:

Link /usr/lib/crt1.o, which contains _start, which will be the entry point for the ELF binary;
Link /usr/lib/crti.o (before libc) and /usr/lib/crtn.o (after), which provide some initialisation and finalisation code;
Tell the linker that the binary will use the dynamic linker, /lib/ld-linux.so.

For example:

$ cat hello.s
 .text
 .globl main
main:
 push %ebp
 mov %esp, %ebp
 pushl $hw_str
 call puts
 add $4, %esp
 xor %eax, %eax
 leave
 ret

 .data
hw_str:
 .asciz "Hello world!"

$ as -o hello.o hello.s
$ ld -o hello -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o -lc hello.o /usr/lib/crtn.o
$ ./hello
Hello world!
$

Best Answer

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How to link a gas assembly program that uses the C standard library with ld without using gcc

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