Question

As the title suggests, is there any way to read the machine code instructions as/after they have been executed? For example, if I had an arbitrary block of C code and I wanted to know what instructions were compiled and executed when that block was entered then would there be a way to do that? Thank you in advance for any pointers on the subject.

Edit: Some motivation as to what I’m trying to do: I want to have a program that roughly figures out how it has been compiled or what instructions it is currently running without actually needing to know how the machine code is made. I.e. I want to use the hard work that some compiler previously did in compiling a program so that I can copy and later use the machine code being executed.

Answer 1

Almost every debugger can do this.

For gdb, a useful trick to remember is: display/i $pc

Do that once, and then set a breakpoint on a function, the step through the function with stepi and nexti.

The instruction at the PC will be automatically displayed each time.

Ross-Harveys-MacBook-Pro:so ross$ cat > deb.c
int main(void) { return (long)main + 0x123; }
Ross-Harveys-MacBook-Pro:so ross$ cc -O deb.c
Ross-Harveys-MacBook-Pro:so ross$ gdb -q a.out
Reading symbols for shared libraries .. done
(gdb) break main
Breakpoint 1 at 0x100000f30
(gdb) display/i $pc
(gdb) r
Starting program: /Users/ross/so/a.out 
Reading symbols for shared libraries +. done

Breakpoint 1, 0x0000000100000f30 in main ()
1: x/i $pc  0x100000f30 <main+4>:   lea    -0xb(%rip),%rax        # 0x100000f2c <main>
(gdb) stepi
0x0000000100000f37 in main ()
1: x/i $pc  0x100000f37 <main+11>:  add    $0x123,%eax
(gdb) stepi
0x0000000100000f3c in main ()
1: x/i $pc  0x100000f3c <main+16>:  leaveq