cd cs31/weeklylab pwd mkdir week06 ls cd week06 pwd cp ~lammert/public/cs31/week06/* . ls Makefile simplefuncs.c
We are going to look again at how to use gcc to create an assembly version of this file, and how to create a object .o file, and how to examine its contents.
If you open up the Makefile you can see the rules for building .s, .o and executable files from simplefuncs.c. We will be compiling the 32-bit version of instructions, so we will use the -m32 flag to gcc:
gcc-4.4 -m32 -S simplefuncs.c # just runs the assembler to create a .s text file gcc-4.4 -m32 -c simplefuncs.c # compiles to a relocatable object binary file (.o) gcc-4.4 -m32 -o simplefuncs simplefuncs.o # creates a 32-bit executable fileTo see the machine code and assembly code mappings in the .o file:
objdump -d simplefuncs.oYou can compare this to the assembly file:
objdump -t simplefuncs # list symbol table in the executable (a.out) file nm --format sysv simplefuncs # list symbol table in the executable file
First, let's open up simplefuncs.c in vim. Then, let's try some things out in gdb:
gdb simplefuncs (gdb) break main (gdb) break func1 (gdb) runIn gdb you can disassemble code using the disass command:
(gdb) disass mainYou can set a break point at a specific instruction:
(gdb) break *0x08048477 # set breakpoint at specified addressAnd you can step or next at the instruction level using ni or si (si steps into function calls, ni skips over them):
(gdb) ni # execute the next instruction then gdb gets control again (gdb) ni (gdb) ni (gdb) ni (gdb) ni (gdb) disass (gdb) cont # continue to next break pointNow we are at the call to func1, let's step into this function using si (we also have a breakpoint at this function, let's see when it is hit):
(gdb) si (gdb) disass (gdb) ni (gdb) where (gdb) disass (gdb) contYou can print out the values of individual registers like this:
(gdb) print $eaxOr the memory contents at a given address, providing either the absolute numeric address or its value stored in registers:
(gdb) p *(int *)($ebp + 8) (gdb) x $ebp + 8 (gdb) x/d $ebp + 8 # x/d display as decimal valueYou can also view all register values:
(gdb) info registersYou can also use the display command to automatically display values each time a breakpoint is reached:
(gdb) display $eax (gdb) display $edxYou can use the examine command (x) to display the contents of a memory location either an address of via a register value (x is shorthand for examine, and p is shorthand for the print command):
x $esp-0x8 # see what p and x display for the same value p $esp-0x8 p *(int *)($ebp-0x8) # here is how to get what x gives you using print x $esp + 0x1c # here is examining the contents at a memory location x 0xffffd2fc # specifying the address in two different ways
ddd simplefuncsThe gdb prompt is in the bottom window. There are also menu options and buttons for gdb commands, but I find using the gdb prompt at the bottom easier to use.
You can view the register values as the program runs (choose Status->Registers to open the register window).
ddd a.out (gdb) break main (gdb) run 6 # run with the command line argument 6 (gdb) disass main # disassemble the main function (gdb) break sum # set a break point at the beginning of a function (gdb) cont # continue execution of the program (gdb) break *0x0804851a # set a break point at memory address 0x0804851a (gdb) ni # execute the next instruction (gdb) si # step into a function call (step instruction) (gdb) info registers # list the register contents (gdb) p $eax # print the value stored in register %eax (gdb) p *(int *)($ebp+8) # print out value of an int at addr (%ebp+8) (gdb) x/d $ebp+8 # examine the contents of memory at the given # address (/d: prints the value as an int) # display type in x is sticky: subsequent x commands # will display values in decimal until another type # is specified (e.g. x/x $ebp+8 # in hex)