GDB Tutorial

Gdb is a debugger for C (and C++). It allows you to do things like run the program up to a certain point then stop and print out the values of certain variables at that point, or step through the program one line at a time and print out the values of each variable after executing each line. It uses a command line interface.

This is a brief description of some of the most commonly used features of gdb.


To prepare your program for debugging with gdb, you must compile it with the -g flag. So, if your program is in a source file called memsim.c and you want to put the executable in the file memsim, then you would compile with the following command:
gcc -g -o memsim memsim.c

Invoking and Quitting GDB

To start gdb, just type gdb at the unix prompt. Gdb will give you a prompt that looks like this: (gdb). From that prompt you can run your program, look at variables, etc., using the commands listed below (and others not listed). Or, you can start gdb and give it the name of the program executable you want to debug by saying
gdb executable

To exit the program just type quit at the (gdb) prompt (actually just typing q is good enough).



Gdb provides online documentation. Just typing help will give you a list of topics. Then you can type help topic to get information about that topic (or it will give you more specific terms that you can ask for help about). Or you can just type help command and get information about any other command.


file executable specifies which program you want to debug.


run will start the program running under gdb. (The program that starts will be the one that you have previously selected with the file command, or on the unix command line when you started gdb. You can give command line arguments to your program on the gdb command line the same way you would on the unix command line, except that you are saying run instead of the program name:
run 2048 24 4
You can even do input/output redirection: run > outfile.txt.


A ``breakpoint'' is a spot in your program where you would like to temporarily stop execution in order to check the values of variables, or to try to find out where the program is crashing, etc. To set a breakpoint you use the break command.

break function sets the breakpoint at the beginning of function. If your code is in multiple files, you might need to specify filename:function.

break linenumber or break filename:linenumber sets the breakpoint to the given line number in the source file. Execution will stop before that line has been executed.


delete will delete all breakpoints that you have set.

delete number will delete breakpoint numbered number. You can find out what number each breakpoint is by doing info breakpoints. (The command info can also be used to find out a lot of other stuff. Do help info for more information.)


clear function will delete the breakpoint set at that function. Similarly for linenumber, filename:function, and filename:linenumber.


continue will set the program running again, after you have stopped it at a breakpoint.


step will go ahead and execute the current source line, and then stop execution again before the next source line.


next will continue until the next source line in the current function (actually, the current innermost stack frame, to be precise). This is similar to step, except that if the line about to be executed is a function call, then that function call will be completely executed before execution stops again, whereas with step execution will stop at the first line of the function that is called.


until is like next, except that if you are at the end of a loop, until will continue execution until the loop is exited, whereas next will just take you back up to the beginning of the loop. This is convenient if you want to see what happens after the loop, but don't want to step through every iteration.


list linenumber will print out some lines from the source code around linenumber. If you give it the argument function it will print out lines from the beginning of that function. Just list without any arguments will print out the lines just after the lines that you printed out with the previous list command.


print expression will print out the value of the expression, which could be just a variable name. To print out the first 25 (for example) values in an array called list, do
print list[0]@25