, argc=1, argv=0xbffffa64, init=0x8048298 <_init>, fini=0x804841c <_fini>, rtld_fini=0x4000a350 <_dl_fini>, stack_end=0xbffffa5c) at ../sysdeps/generic/libc-start.c:78 This shows us that it died in main(), at line 5 of badcode.c. If we look at line 5 in badcode.c we find that we're trying to free memory that we never malloc'ed - a very common cause of seg faults. So now you can go and fix your code and finally get some sleep. Or if it's near enough to Dead Week you can get back to watching Bond on TNT. Now let's look at a more complicated example of using gdb. Let's say that you have a program in "morecode.c": int add_2(int value) { int retval; retval = value + 2; return(retval); } int main(void) { int A, B; A = 42; B = add_2(A); return(B); } Compile the code with the "-g" flag and we'll run gdb through its paces. [cs240]$ gcc -g -Wall morecode.c [cs240]$ gdb a.out gdb (and most debuggers) allow you to set breakpoints in the code - the program will automatically "pause" execution when it reaches a breakpoint. Breakpoints can be set at source code lines or function calls. To set a breakpoint at the main() function you can type "break main". (gdb) break main Breakpoint 1 at 0x804843e: file morecode.c, line 14. (gdb) You could have also typed "break 14" if you knew the line number you wanted to break on. Type "run" and the program will run until it hits a breakpoint. (gdb) run Starting program: /home/svec/cs240/a.out Breakpoint 1, main () at morecode.c:14 14 A = 42; (gdb) Use the command "list" to print out the source code with line numbers. (gdb) list 9 10 int main(void) 11 { 12 int A, B; 13 14 A = 42; 15 16 B = add_2(A); 17 18 return(B); (gdb) To step through the program you can use "next" and "step", or "n" and "s". Try stepping through this program using "next" and "step" until the program quits. Also try "continue". Also try "print A" and "print B" to see the values of these variables. Make sure you read the help on all these commands (like "help print") so you understand them. Now you know how to use gdb! You can run gdb on pretty much any UNIX machine in the world. You may also want to take a look at 'ddd' - it's a graphical version of gdb you can use if you're logged in at a graphical terminal. Don't forget the "help" command within gdb, or just type "man gdb" from any UNIX prompt to get more info. I hope that proves useful, I think it will. -svec