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GDB - Debugging Example1
Let us write a program to generate a core dump.
#include <iostream> using namespace std; int divint(int, int); int main() { int x = 5, y = 2; cout << divint(x, y); x =3; y = 0; cout << divint(x, y); return 0; } int divint(int a, int b) { return a / b; }
To enable debugging, the program must be compiled with the -g option.
$g++ -g crash.cc -o crash
NOTE: We are using g++ compiler because we have used C++ source code.
Now, when you run this program on your linux machine, it will produce the following result:
Floating point exception (core dumped)
You will find a core file in your current directory.
Now to debug the problem, start gdb debugger at the command prompt:
$gdb crash # Gdb prints summary information and then the (gdb) prompt (gdb) r Program received signal SIGFPE, Arithmetic exception. 0x08048681 in divint(int, int) (a=3, b=0) at crash.cc:21 21 return a / b; # 'r' runs the program inside the debugger # In this case the program crashed and gdb prints out some # relevant information. In particular, it crashed trying # to execute line 21 of crash.cc. The function parameters # 'a' and 'b' had values 3 and 0 respectively. (gdb) l # l is short for 'list'. Useful for seeing the context of # the crash, lists code lines near around 21 of crash.cc (gdb) where #0 0x08048681 in divint(int, int) (a=3, b=0) at crash.cc:21 #1 0x08048654 in main () at crash.cc:13 # Equivalent to 'bt' or backtrace. Produces what is known # as a 'stack trace'. Read this as follows: The crash occurred # in the function divint at line 21 of crash.cc. This, in turn, # was called from the function main at line 13 of crash.cc (gdb) up # Move from the default level '0' of the stack trace up one level # to level 1. (gdb) list # list now lists the code lines near line 13 of crash.cc (gdb) p x # print the value of the local (to main) variable x
In this example, it is fairly obvious that the crash occurs because of the attempt to divide an integer by 0.
To debug a program 'crash' that has crashed and produced a core file named 'core', type the following at the command line:
gdb crash core
As this is mostly equivalent to starting gdb and typing the 'r' command, all of the commands above could now be used to debug the file.