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Translating by Torival Three steps to learning GDB
Debugging C programs used to scare me a lot. Then I was writing my operating system and I had so many bugs to debug! I was extremely fortunate to be using the emulator qemu, which lets me attach a debugger to my operating system. The debugger is called gdb.
I’m going to explain a couple of small things you can do with gdb, because I found it really confusing to get started. We’re going to set a breakpoint and examine some memory in a tiny program.
1. Set breakpoints
If you’ve ever used a debugger before, you’ve probably set a breakpoint.
Here’s the program that we’re going to be “debugging” (though there aren’t any bugs):
#include <stdio.h>
void do_thing() {
printf("Hi!\n");
}
int main() {
do_thing();
}
Save this as hello.c. We can debug it with gdb like this:
bork@kiwi ~> gcc -g hello.c -o hello
bork@kiwi ~> cat
bork@kiwi ~> gdb ./hello
This compiles hello.c with debugging symbols (so that gdb can do better work), and gives us kind of scary prompt that just says
(gdb)
We can then set a breakpoint using the break command, and then run the program.
(gdb) break do_thing
Breakpoint 1 at 0x4004f8
(gdb) run
Starting program: /home/bork/hello
Breakpoint 1, 0x00000000004004f8 in do_thing ()
This stops the program at the beginning of do_thing.
We can find out where we are in the call stack with where: (thanks to @mgedmin for the tip)
(gdb) where
#0 do_thing () at hello.c:3
#1 0x08050cdb in main () at hello.c:6
(gdb)
2. Look at some assembly code
We can look at the assembly code for our function using the disassemblecommand! This is cool. This is x86 assembly. I don’t understand it very well, but the line that says callq is what does the printf function call.
(gdb) disassemble do_thing
Dump of assembler code for function do_thing:
0x00000000004004f4 <+0>: push %rbp
0x00000000004004f5 <+1>: mov %rsp,%rbp
=> 0x00000000004004f8 <+4>: mov $0x40060c,%edi
0x00000000004004fd <+9>: callq 0x4003f0
0x0000000000400502 <+14>: pop %rbp
0x0000000000400503 <+15>: retq
You can also shorten disassemble to disas
3. Examine some memory!
The main thing I used gdb for when I was debugging my kernel was to examine regions of memory to make sure they were what I thought they were. The command for examining memory is examine, or x for short. We’re going to use x.
From looking at that assembly above, it seems like 0x40060c might be the address of the string we’re printing. Let’s check!
(gdb) x/s 0x40060c
0x40060c: "Hi!"
It is! Neat! Look at that. The /s part of x/s means “show it to me like it’s a string”. I could also have said “show me 10 characters” like this:
(gdb) x/10c 0x40060c
0x40060c: 72 'H' 105 'i' 33 '!' 0 '\000' 1 '\001' 27 '\033' 3 '\003' 59 ';'
0x400614: 52 '4' 0 '\000'
You can see that the first four characters are ‘H’, ‘i’, and ‘!’, and ‘\0’ and then after that there’s more unrelated stuff.
I know that gdb does lots of other stuff, but I still don’t know it very well and xand break got me pretty far. You can read the documentation for examining memory.
via: https://jvns.ca/blog/2014/02/10/three-steps-to-learning-gdb/
作者：Julia Evans 译者：译者ID 校对：校对者ID