Another GDB Short Tutorial:
GDB Tutorial
Last modi?ed February 22, 2012
GDB TutorialWhat is gdb?
“GNU Debugger”
A debugger for several languages, including C and C++ that allows you to inspect what the program is doing at a certain point during execution. Errors like segmentation faults may be easier to ?nd with the help of gdb.
GDB Tutorial Additional step when compiling program
Normally, you would compile a program like:
gcc [flags] <source files> -o <output file>
For example:
gcc -Wall -Werror -ansi -pedantic-errors prog1.c -o prog1.x
Now you add a -g option to enable built-in debugging support
(which gdb needs):
gcc [other flags] -g <source files> -o <output file>
For example:
gcc -Wall -Werror -ansi -pedantic-errors -g prog1.c -o prog1.x
GDB Tutorial Starting up gdb
Just try “gdb” or “gdb prog1.x.” from your terminal You’ll get a prompt that looks
like this:
(gdb)
If you didn’t specify a program to debug, you’ll have to load it in
now:
(gdb) file prog1.x
Here, prog1.x is the program you want to load, and “?le” is the command to load it.
GDB Tutorial Before we go any further
gdb has an interactive shell, much like the one you use as soon as you log into the Linux grace machines. It can recall history with the arrow keys, auto-complete words (most of the time) with the TAB key, and has other nice features.
Tip
If you’re ever confused about a command or just want more information, use the “help” command, with or without an argument:
(gdb) help [command]
You should get a nice description and maybe some more useful tidbits. . .
GDB Tutorial: Running the program
To run the program, just use:
(gdb) run
This runs the program. If it has no serious problems (i.e. the normal program didn’t get a segmentation fault, etc.), the program should run ?ne here too. If the program did have issues, then you (should) get some useful information like the line number where it crashed, and parameters to the function that caused the error:
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400524 in sum array region (arr=0x7fffc902a270, r1=2, c1=5, r2=4, c2=6) at sum-array-region2.c:12
GDB Tutorial: So what if I have bugs?
Okay, so you’ve run it successfully. But you don’t need gdb for that. What if the program isn’t working?
Basic idea
Chances are if this is the case, you don’t want to run the program without any stopping, breaking, etc. Otherwise, you’ll just rush past the error and never ?nd the root of the issue. So, you’ll want to step through your code a bit at a time, until you arrive upon the error. This brings us to the next set of commands. . .
GDB Tutorial: Setting breakpoints
Breakpoints can be used to stop the program run in the middle, at a designated point. The simplest way is the command “break.” This sets a breakpoint at a speci?ed ?le-line pair:
(gdb) break file1.c:6
This sets a breakpoint at line 6, of file1.c. Now, if the program ever reaches that location when running, the program will pause and prompt you for another command.
Tip
You can set as many breakpoints as you want, and the program should stop execution if it reaches any of them.
GDB Tutorial: More fun with breakpoints
You can also tell gdb to break at a particular function. Suppose you have a function my func:
int my func(int a, char *b);
You can break anytime this function is called:
(gdb) break my func
GDB Tutorial: Now what?
Once you’ve set a breakpoint, you can try using the run command again. This time, it should stop where you tell it to (unless a fatal error occurs before reaching that point). You can proceed onto the next breakpoint by typing “continue” (Typing run again would restart the program from the beginning, which isn’t very useful.)
(gdb) continue
You can single-step (execute just the next line of code) by typing “step.” This gives you really ?ne-grained control over how the program proceeds. You can do this a lot...
(gdb) step
GDB Tutorial: Now what?
Similar to “step,” the “next” command single-steps as well, except this one doesn’t execute each line of a sub-routine, it just treats it as one instruction.
(gdb) next
Tip
Typing “step” or “next” a lot of times can be tedious. If you just press ENTER, gdb will repeat the same command you just gave it. You can do this a bunch of times.
GDB Tutorial: Querying other aspects of the program
So far you’ve learned how to interrupt program ?ow at ?xed, speci?ed points, and how to continue stepping line-by-line. However, sooner or later you’re going to want to see things like the values of variables, etc. This might be useful in debugging. :)
The print command prints the value of the variable speci?ed, and print/x prints the value in hexadecimal:
(gdb) print my var
(gdb) print/x my var
GDB Tutorial: Setting watchpoints
Whereas breakpoints interrupt the program at a particular line or function, watchpoints act on variables. They pause the program whenever a watched variable’s value is modi?ed. For example, the following watch command:
(gdb) watch my var
Now, whenever my var’s value is modi?ed, the program will interrupt and print out the old and new values.
Tip
You may wonder how gdb determines which variable named my var to watch if there is more than one declared in your program. The answer (perhaps unfortunately) is that it relies upon the variable’s scope, relative to where you are in the program at the time of the watch. This just means that you have to remember the tricky nuances of scope and extent .
GDB Tutorial: Other useful commands
backtrace - produces a stack trace of the function calls that lead to a seg fault (should remind you of Java exceptions)
where - same as backtrace; you can think of this version as working even when you’re still in the middle of the program
finish - runs until the current function is ?nished
delete - deletes a speci?ed breakpoint
info breakpoints - shows information about all declared breakpoints Look at sections 5 and 9 of the manual mentioned at the beginning of this tutorial to ?nd other useful commands, or just try help.
GDB Tutorial: gdb with Emacs
Emacs also has built-in support for gdb. To learn about it, go here:
http://wiki.cs.earlham.edu/index.php/Pedagogical_Group_Provided_Tutorials
GDB Tutorial: More about breakpoints
Breakpoints by themselves may seem too tedious. You have to keep stepping, and stepping, and stepping. . .
Basic idea
Once we develop an idea for what the error could be (like dereferencing a NULL pointer, or going past the bounds of an array), we probably only care if such an event happens; we don’t want to break at each iteration regardless. So ideally, we’d like to condition on a particular requirement (or set of requirements). Using conditional breakpoints allow us to accomplish this goal. . .
GDB Tutorial: Conditional breakpoints
Just like regular breakpoints, except that you get to specify some criterion that must be met for the breakpoint to trigger. We use the same break command as before:
(gdb) break file1.c:6 if i >= ARRAYSIZE
This command sets a breakpoint at line 6 of ?le file1.c, which triggers only if the variable i is greater than or equal to the size of the array (which probably is bad if line 6 does something like arr[i]). Conditional breakpoints can most likely avoid all the unnecessary stepping, etc.
GDB Tutorial: Fun with pointers
Who doesn’t have fun with pointers? First, let’s assume we have the following structure de?ned:
struct entry {
int key;
char *name;
float price;
long serial_number;
};
Maybe this struct is used in some sort of hash table as part of a catalog for products, or something related.
GDB Tutorial: Using pointers with gdb I
Now, let’s assume we’re in gdb, and are at some point in the execution after a line that looks like:
struct entry * e1 = <something>;
We can do a lot of stu? with pointer operations, just like we could in C.
See the value (memory address) of the pointer:
(gdb) print e1
See a particular ?eld of the struct the pointer is referencing:
(gdb) print e1->key
(gdb) print e1->name
(gdb) print e1->price
(gdb) print e1->serial number
GDB Tutorial: Using pointers with gdb II
You can also use the dereference (*) and dot (.) operators in place of the arrow operator (->):
(gdb) print (*e1).key
(gdb) print (*e1).name
(gdb) print (*e1).price
(gdb) print (*e1).serial number
See the entire contents of the struct the pointer references (you can’t do this as easily in C!):
(gdb) print *e1
You can also follow pointers iteratively, like in a linked list:
(gdb) print list prt->next->next->next->data
