# 2014 February 3
#
# Using GDB (debugger) on final variation of reverse program (reverse4.c)
# 
# Added comments are preceded by a single # and a space.  Lines starting 
# with # followed immediately by a number and no space are output from GDB.
#

# 
# Compile with the -g option to fully utilize the debugger.
# Use GDB on the executable.
#

$ gcc -g -Wall -g -o reverse reverse4.c
$ gdb reverse
GNU gdb (GDB) Fedora 7.6.1-46.fc19
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /homes/campbl4/GDB_Demo/reverse...done.

#
# Use break to set a breakpoint at main
#

(gdb) break main
Breakpoint 1 at 0x4007a1: file reverse4.c, line 46.

#
# run the executable
#

(gdb) run
Starting program: /homes/campbl4/GDB_Demo/reverse 

Breakpoint 1, main () at reverse4.c:46
46	  printf("Please enter a string: ");
Missing separate debuginfos, use: debuginfo-install glibc-2.17-20.fc19.x86_64

#
# Use the step command to execute the next statement (line 46)
#

(gdb) step
47	  fgets(line, MAX_STR, stdin);

#
# Use the step command to execute the next statement (line 47)
#

(gdb) step
Please enter a string: hello
50	  line[strlen(line)-1] = '\0';

#
# print the current value of line (prior to executing line 50)
# the \n character is still at position 5
# the \0 character is at position 6
# strlen(line) is 6 (h-e-l-l-o-\n)
#

(gdb) print line
$1 = "hello\n\000\000`\343\377\377\377\177\000\000\070\001\301K3\000\000\000X\326\377\367\377\177\000\000}\000\000\000\000\000\000\000\001\000\000\000\000\000\000\000\001\000\000\000\000\000\000\000}\b@\000\000\000\000\000\277", '\000' <repeats 15 times>, "\060\b@\000\000\000\000\000\360\005@\000\000\000\000\000 \344\377\377"

(gdb) step
52	  rev_line = reverse(line);

#
# print the value of rev_line
# line 52 has not been executed yet so the assignment hasn't happened
#

(gdb) print rev_line
$2 = 0x0


(gdb) print line
$3 = "hello\000\000\000`\343\377\377\377\177\000\000\070\001\301K3\000\000\000X\326\377\367\377\177\000\000}\000\000\000\000\000\000\000\001\000\000\000\000\000\000\000\001\000\000\000\000\000\000\000}\b@\000\000\000\000\000\277", '\000' <repeats 15 times>, "\060\b@\000\000\000\000\000\360\005@\000\000\000\000\000 \344\377\377"

#
# step at line 52 of main will step into the function call for reverse
#

(gdb) step
reverse (s=0x7fffffffe2d0 "hello") at reverse4.c:15
15	  char * result = NULL;            /* the reversed string */

#
# allocate enough memory to hold the size of the string s, plus one more to hold \0
#

(gdb) step
20	  result = (char *) malloc(strlen(s)+1);

#
# about to perform the copy
#

(gdb) step
23	  strcpy(result,s);

#
# have performed the copy
#

(gdb) step
25	  L = 0;

#
# continue to the end of the program
# now the output in both line and rev_line has taken out the \n which indicated that the user was done 
# entering input, but was not actually counted as part of the input itself
#

(gdb) continue
Continuing.
The original string was:   >hello<
Backwards, that string is: >olleh<
Thank you for trying our program.
[Inferior 1 (process 23966) exited normally]

#
# quit gdb
#

(gdb) quit

$