gdb myprog   # Start program in debugger
    b main     # Set breakpoint in function "main()"
    r abc      # "run" program with cmd-line arguments "abc"
    next       # "n": Single step (over functions)
    step       # "s": Single step (into functions)
    where      # Get stack traceback of current location in program
    p i        # Print the value of variable "i"
    del 1      # Delete breakpoint #1 (your "b main")
    h          # Gdb "help"
    q          # "Quit": Exit debugger

/*
 * Test SEGV
 *
 * 1) Microsoft compile/run:
 *    ---------------------
 *    - cl x.cpp => OK
 *    - x 123 =>
 *        Original strings: s1= abc, s2= abc...
 *        Updated strings: s1= 123, s2= 123...
 *        
 *    - x 123456 =>
 *        Original strings: s1= abc, s2= abc...
 *      <= FIRST ONE OK, SECOND DIES IMMEDIATELY
 *
 * 2) G++ compile/debug:
 *    -----------------
 *    - g++ -g -Wall -pedantic -o x x.cpp
 *      <= BE SURE TO INCLUDE "-g" SWITCH, FOR DEBUG INFO
 *    - gdb x
 *      (gdb) run
 *      <= THIS WILL PROMPT FOR USAGE
 *      (gdb) r 123
 *        Starting program: /home/paulsm/proj/temp/x 123
 *        Original strings: s1= abc, s2= abc...
 *        Program received signal SIGSEGV, Segmentation fault.
 *        0x4017f3d6 in strcpy () from /lib/tls/libc.so.6 *        
 *        <= THIS DIES...
 *      (gdb) where
 *        <= "WHERE" TELLS US OUR CURRENT TRACEBACK
 *        #0  0x4017f3d6 in strcpy () from /lib/tls/libc.so.6
 *        #1  0x08048518 in main (argc=2, argv=0xbffff3f4) at x.cpp:42
 *      (gdb) list
 *        <= "LIST" PRINTS THE CURRENT SOURCE
 *        42         strcpy (s1, argv[1]);
 *        43         strcpy (s2, argv[1]);
 *        44         printf ("Updated strings: s1= %s, s2= %s...\n", s1, s2);
 *        45
 *      (gdb) break main
 *        Breakpoint 1 at 0x80484c8: file x.cpp, line 34.
 *        <= "B MAIN" SETS A BREAKPOINT AT THE VERY BEGINNING OF THE PROGRAM
 *      (gdb) r 123
 *        <= START OVER
 *        Breakpoint 1, main (argc=2, argv=0xbffff3f4) at x.cpp:34
 *        34         if (argc != 2)
 *        (gdb) n
 *        41         printf ("Original strings: s1= %s, s2= %s...\n", s1, s2);
 *        (gdb) n
 *        Original strings: s1= abc, s2= abc...
 *        42         strcpy (s1, argv[1]);
 *        <= "NEXT" SINGLE-STEPS THROUGH A SOURCE LINE AT A TIME
 *        (gdb) p s1
 *        $1 = 0x8048678 "abc"
 *        (gdb) print argv[1]
 *        $2 = 0xbffff5b0 "123"
 *        <= "PRINT" PRINTS THE CURRENT VALUE/TYPE OF A VARIABLE
 */
#include <stdio.h>
#include <string.h>

int
main (int argc, char *argv[])
{
   static char *s1 = "abc";
   static char s2[] = "abc";

   // Check cmd-line
   if (argc != 2)
   {
     printf ("SAMPLE USAGE: x 123!\n");
     return 1;
   }   

   // Copy cmd-line into static string
   printf ("Original strings: s1= %s, s2= %s...\n", s1, s2);
   strcpy (s1, argv[1]);
   strcpy (s2, argv[1]);
   printf ("Updated strings: s1= %s, s2= %s...\n", s1, s2);

   // Done
   return 0;
}