Laboratory Exercises, C++ Programming

16 Tools for Practical C++ Development
(gdb) is a command prompt.
The first step is to run the program inside the debugger. Enter the command run and the program
arguments. First run the program without arguments, like this:
(gdb) run
Starting program: /h/dd/c/.../reciprocal
Program received signal SIGSEGV, Segmentation fault.
*__GI_____strtol_l_internal (nptr=0x0, endptr=0x0, base=10, group=0, loc=0xb7f1c380)
at strtol_l.c:298
...
The error occurred in the strtol l internal function, which is an internal function that atoi
calls. Inspect the call stack with the where command:
(gdb) where
#0 *__GI_____strtol_l_internal (nptr=0x0, endptr=0x0, base=10, group=0, loc=0xb7f1c380)
at strtol_l.c:298
#1 0xb7e07fc6 in *__GI_strtol (nptr=0x0, endptr=0x0, base=10) at strtol.c:110
#2 0xb7e051b1 in atoi (nptr=0x0) at atoi.c:28
#3 0x080487b0 in main (argc=1, argv=0xbfa1be44) at reciprocal.cc:9
You see that the atoi function was called from main. Use the up command (three times) to go to
the main function in the call stack:
...
(gdb) up
#3 0x080487b0 in main (argc=1, argv=0xbfa1be44) at reciprocal.cc:9
9 int i = std::atoi(argv[1]); // atoi: "ascii-to-integer"
Note that GDB finds the source code for main, and it shows the line where the function call
occurred.
You can examine the value of a variable using the print command:
(gdb) print argv[1]
$1 = 0x0
This confirms a suspicion that the problem is indeed a 0 pointer passed to atoi. Set a breakpoint
on the first line of main with the break command:
(gdb) break main
Breakpoint 1 at 0x80487a0: file reciprocal.cc, line 9.
Now run the program with an argument:
(gdb) run 5
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /h/dd/c/.../reciprocal 5
Breakpoint 1, main (argc=2, argv=0xbff7aae4) at reciprocal.cc:9
9 int i = std::atoi(argv[1]); // atoi: "ascii-to-integer"
The debugger has stopped at the breakpoint. Step over the call to atoi using the next command: