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sources/tech/20140210 Three steps to learning GDB.md

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+Three steps to learning GDB
+============================================================
+
+Debugging C programs used to scare me a lot. Then I was writing my [operating system][2] 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][3] 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 `disassemble`command! 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 `x`and `break` got me pretty far. You can read the [documentation for examining memory][4].
+
+--------------------------------------------------------------------------------
+
+via: https://jvns.ca/blog/2014/02/10/three-steps-to-learning-gdb/
+
+作者：[Julia Evans ][a]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]:https://jvns.ca
+[1]:https://jvns.ca/categories/spytools
+[2]:http://jvns.ca/blog/categories/kernel
+[3]:https://twitter.com/mgedmin
+[4]:https://ftp.gnu.org/old-gnu/Manuals/gdb-5.1.1/html_chapter/gdb_9.html#SEC56