Using the Gnu Debugger gdb

The Free Software Foundation ("Gnu") has also developed an advanced debugging environment called "gdb" (for Gnu debugger). This debugger is useful for tracing the progress of a program that has compiled "successfully" (that is, it has no syntax errors), but is not running the way the coder expects it to run. This page shows a couple of basic commands to help debug an input file. Suppose we have a file hello-oops.c containing the following lines of code

When we compile this input file using the Gnu C compiler gcc (see the C Programming page), we see no error messages, so the syntax is apparently okay). But suppose this is part of a larger, more complex piece of code, and we need to understand exactly what is going on when these few lines are executed. It turns out we can trace the progress of the execution using gdb by using the "-g" option flag when compiling the code:

This inserts code in the executable file, helloworld-oops.exe, which enables the tools in gdb to work. In the meantime, once the OK button is clicked, the compilation process looks no different than it did without the "-g" flag:

Now we go to work on the executable by invoking gdb as a DOS command:

A DOS box appears in response:

Designate the file to debug by indicating the executable file which we compiled via the "-g" option by using the gdb "file" command. The response looks like the following:

Keep the program listing open in the editor so that you are able to see line numbers. Since we are just starting, set a "breakpoint" near the top of the main() routine by placing it at line 7 using the gdb break command:

Now run the program, with the understanding that execution will stop at the breakpoint, using the gdb run command:

Note that the program has stopped prior to executing line 7, the for() loop. While line 6 has executed, declaring variable i to be an integer, no value has been assigned to it because, again, line 7 has yet to be executed. But just for grins, let's take a look at what value it contains anyway. We'll use the gdb print command:

It shows a nonsensical value of 39320736, which is probably the number left from whatever was going on in the particular address of memory now being reserved for the integer i. The gdb print command can be used to show current values of any declared variables at any time. It is one of the most important tools to use when debugging larger programs.

Words of wisdom:  Be sure to refer often back to the open file in your editor so you can keep track of where execution is, using the line numbers

For now, though, let's begin single stepping starting from the breakpoint using the step or s command:

We have now moved past the initialization i=0 of the for() loop header, on to line 8. Let's print the value of integer variable i now...

Note:  gdb allows the abbreviation "p" for the print command.

Now integer variable i contains the value 0. We can continue to step through the program, one line at a time by continuing to invoke the gdb "s" command several times...

Note what has happened: We single-stepped past the printf() command to the next executable statement and the result Hello, world! appeared in the gdb session, just as it would if the program were being run normally. Execution stops at the top of the for() loop, since there are no more executable statements in the remainder of the block over which execution is looping. Single-step one more time to arrive at executable line 8 again. Now before going on, let's show the current value of integer variable i using the print command:

Note that the increment i++ portion of the for() loop has now been executed, raising the value of integer variable i from 0 to 1.

Now the user can continue this process, using any of the many commands available in gdb until a thorough understanding has been reached about just how the program is executing. The coder can use this information to debug his or her code.

For now, we'll stop by using the gdb continue command to reach the end of the program:

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