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GDB - Debugging Example 1

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Let us write a program which will 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 run this program on your linux machine, it produces the 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 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.

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