CSCI 235 —

References

• The official (and extremely long) Debugging with GDB
• The very long Using GNU’s GDB Debugger
• Brown CSCI0330 six-page gdb Cheatsheet
• The popular Beej’s Quick Guide to GDB
• The printable two-page GDB Quick Reference
• RMS’s tutorial

Stuff to do

Read evaluate loop

Start up gdb and make sure you can do the first problems in last term’s exams. That will make the grading easier for me!

However, be sure you can do the problems without gdb for the exam.

Using a simple program

Download clect2.tar.gz, a gzip tar file illustrating some recent lectures, and untar (tar xfvz) it into your directory. Take a look at the Makefile and notice that (1) it’s simpler and (2) it has a new compiler option -g which causes your programs to be linked with debugging information. Go ahead and make the programs.

Stepping through the array program

Start up gdb on carray and begin by performing the “Mostly for fun” commands for appropriate values for xxx. (Use the up-arrow key to recall commands.)

//    print (void *)xxxNums
//    print (void *)&xxxNums[2]
//    print (void *)(xxxNums+2)

Now use each of the following commands at least once to control gdb.

• list to list the program
• break to set a breakpoint
• run to a break
• step to next statement
• fin to return from a function
• next to go to next statement and avoiding function calls i>
• continue on to the next breakpoint
• disassemble to see the machine code

Crashing the array program

Modify the main program and so that it passes 1000000 as the second argument to sumNumsPointers. Run it.

You should get a segmentation fault. Unfortunately, you have to enable core dumps to get an image of the crashed program. Do this with the following command.

ulimit -c unlimited

Now you should have core file. (Use the ls command.) You can load the core dump with the following command. After that you can examine its variables. Keep in mind that the core dump is only useful if you have compiled the program with debugging enabled.

gdb carray core

Look at the value of posV. I’m afraid it is a bit too large. Try to print *posV.

Using a more complex program

Now take a look at the cstruct.c program. Start gdb on cstruct and set a break at the first call to puts. Run it up to that point.

At this point we’re going to take a look at an old-fashioned CSCI 202 style linked list from the days when CSCI 202 was taught using C. You should be prepared to follow along with the instruction. This will involve printing the following values: NC, *NC, (*NC).name, NC->name, etc.

Now add a new state, say Oregon with population 4,093,465, between North Carolina and Rhode Island.