How to get full assembler output in gcc?

Question

I know that I can get the assembler source code generated by the compiler using:

gcc -S ... 

although this annoyingly does not give me the object file as part of the process.

But how can I get everything about compiled code? I mean addresses, bytes created, etc.

The instructions output by gcc -S do not tell me anything about the length of the instructions or the encodings that I want to see.

Answer 1

I like objdump for this, but the most useful options are not obvious - especially if you use it in an object file that contains repositions and not the final binary.

objdump -d some_binary does a reasonable job.

objdump -d some_object.o less useful because external function calls do not get exposed:

 ... 00000005 <foo>: 5: 55 push %ebp 6: 89 e5 mov %esp,%ebp 8: 53 push %ebx ... 29: c7 04 24 00 00 00 00 movl $0x0,(%esp) 30: e8 fc ff ff ff call 31 <foo+0x2c> 35: 89 d8 mov %ebx,%eax ... 

call is actually printf() ... adding the -r flag helps with this; he marks the move. objdump -dr some_object.o gives:

 ... 29: c7 04 24 00 00 00 00 movl $0x0,(%esp) 2c: R_386_32 .rodata.str1.1 30: e8 fc ff ff ff call 31 <foo+0x2c> 31: R_386_PC32 printf ... 

Then it will be useful for me to see each line annotated as <symbol+offset> . objdump has a convenient option for this, but it has the annoying side effect of disabling dumping the actual bytes - objdump --prefix-addresses -dr some_object.o gives:

 ... 00000005 <foo> push %ebp 00000006 <foo+0x1> mov %esp,%ebp 00000008 <foo+0x3> push %ebx ... 

But it turns out that you can undo this by providing another obscure option, finally arriving at my favorite objdump spell:

objdump --prefix-addresses --show-raw-insn -dr file.o

which gives a conclusion as follows:

 ... 00000005 <foo> 55 push %ebp 00000006 <foo+0x1> 89 e5 mov %esp,%ebp 00000008 <foo+0x3> 53 push %ebx ... 00000029 <foo+0x24> c7 04 24 00 00 00 00 movl $0x0,(%esp) 2c: R_386_32 .rodata.str1.1 00000030 <foo+0x2b> e8 fc ff ff ff call 00000031 <foo+0x2c> 31: R_386_PC32 printf 00000035 <foo+0x30> 89 d8 mov %ebx,%eax ... 

And if you created using debugging symbols (i.e. compiled with -g ) and you replace -dr with -Srl , it will try to annotate the output with the corresponding source lines.

Answer 2

The easiest way to get a quick list is to use the -a option for assembler, which you can do by placing -Wa,-a on the gcc command line. You can use various modifiers for option a to precisely influence what comes out - see the man page as (1).

Answer 3

gcc will create the assembly language source file. You can then use as -a yourfile.S to create a list including offsets and encoded bytes for each command. -a also has some sub-options to control what is displayed in the listing file ( as --help will list them along with other available options).

Answer 4

It seems to me that you want a disassembler. objdump is pretty much the standard ( otool on Mac OS X); according to any information on the map file that your linker gives you, parsing your object file should provide you with everything you want.

Answer 5

 nasm -f elf xx.asm -l x.lst gcc xx.c xx.o -o xx 

generates the file 'list' x.lst, which is only for xx.asm

for xx.c along with xx.asm you can compile both of them and then use "gdb" - gnu debugger