Run 1: badprog.c
% gcc -g badprog.c #-- compile program with -g flag
% gdb a.out #-- invoke gdb with the executable
GNU gdb 6.4.90-debian
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i486-linux-gnu"...Using host libthread_db library "/lib/tls/libthread_db.so.1".
(gdb) break main #-- set a breakpoint at the begining of the program's execution
Breakpoint 1 at 0x8048436: file badprog.c, line 36.
(gdb) run #-- run the program
Starting program: /home/newhall/public/gdb_examples/a.out
Breakpoint 1, main () at badprog.c:36 #-- gdb stops at breakpoint
36 int arr[5] = { 17, 21, 44, 2, 60 };
(gdb) list #-- list the source code near the break point
31 return 0;
32 }
33
34 int main(int argc, char *argv[]) {
35
36 int arr[5] = { 17, 21, 44, 2, 60 };
37
38 int max = arr[0];
39
40 if ( findAndReturnMax(arr, 5, max) != 0 ) {
(gdb) list 11 #-- list source code around line 11
11 // this function should find the largest element in the array and
12 // "return" it through max
13 // array: array of integer values
14 // len: size of the array
15 // max: set to the largest value in the array
16 // reuturns: 0 on success and non-zero on an error
17 //
18 int findAndReturnMax(int *array1, int len, int max) {
19
20 int i;
(gdb) list #-- list the next few lines of code
21
22 if(!array1 || (len <=0) ) {
23 return -1;
24 }
25 max = array1[0];
26 for(i=1; i <= len; i++) {
27 if(max < array1[i]) {
28 max = array1[i];
29 }
30 }
(gdb) next #-- execute the next instruction
38 int max = arr[0];
(gdb) #-- hitting Enter executes the previous command (next in this case)
40 if ( findAndReturnMax(arr, 5, max) != 0 ) {
#-- also you can use the up and down arrows to scroll through previous commands
(gdb) print max #-- print out the value of max
$1 = 17
(gdb) p arr #-- p is short for the print command
$2 = {17, 21, 44, 2, 60}
(gdb) step #-- step into the function call
#-- if we had entered 'next' the entire function call would have been executed
findAndReturnMax (array1=0xbfc5cb3c, len=5, max=17) at badprog.c:22
22 if(!array1 || (len <=0) ) { #-- 'step' takes us to the entry point of findAndReturnMax
(gdb) print array1[0] #-- lets see what the param values are
$3 = 17
(gdb) p max
$4 = 17
(gdb) list
17 //
18 int findAndReturnMax(int *array1, int len, int max) {
19
20 int i;
21
22 if(!array1 || (len <=0) ) {
23 return -1;
24 }
25 max = array1[0];
26 for(i=1; i <= len; i++) {
(gdb) break 26 #-- set a breakpoint inside findAndReturnMax
Breakpoint 2 at 0x80483e7: file badprog.c, line 26.
(gdb) cont #-- continue the execution
Continuing.
Breakpoint 2, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=17) #-- gdb hits the breakpoint
at badprog.c:26
26 for(i=1; i <= len; i++) {
(gdb) p i
$5 = 0
(gdb) n #-- n is short for next
27 if(max < array1[i]) {
(gdb) display max #-- display will print out the value everytime we hit a breakpoint
1: max = 17
(gdb) display array1[i]
2: array1[i] = 21
(gdb) break 27 #-- set a breakpoint inside the loop
Breakpoint 3 at 0x80483f0: file badprog.c, line 27.
(gdb) cont #-- continue execution
Continuing.
Breakpoint 3, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=21)
at badprog.c:27
27 if(max < array1[i]) {
#-- display prints these out:
2: array1[i] = 44
1: max = 21
(gdb) cont
Continuing.
Breakpoint 3, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=44)
at badprog.c:27
27 if(max < array1[i]) {
2: array1[i] = 2
1: max = 44
(gdb) cont
Continuing.
Breakpoint 3, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=44)
at badprog.c:27
27 if(max < array1[i]) {
2: array1[i] = 60
1: max = 44
(gdb) cont
Continuing.
Breakpoint 3, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=60)
at badprog.c:27
27 if(max < array1[i]) {
2: array1[i] = 17
1: max = 60 #-- so max is 60 here
(gdb) where #-- show the stack frames
#-- findAndReturnMax is the active function at line 27, it was called by main at line 40:
#0 findAndReturnMax (array1=0xbfd043ec, len=5, max=60) at badprog.c:27
#1 0x08048479 in main () at badprog.c:40
frame 1 #-- move into main's calling context (stack frame 1) to examine main's state
#1 0x08048479 in main () at badprog.c:40
40 if ( findAndReturnMax(arr, 5, max) != 0 ) {
(gdb) print max #-- in main's stack frame max is 17
$1 = 17
(gdb) cont #-- continue execution
Continuing.
max value in the array is 17 #-- main prints out value of max after function call
#-- This looks like a bug:"
#-- findAndReturnMax set max to 60, but 60 isn't getting "passed back" to main after the call
#-- to fix this we need either have findAndReturnMax return the value of max or pass max by reference
(gdb) quit #-- quit gdb
The program is running. Exit anyway? (y or n) y
Run 2: segfaulter.c
% gdb segfaulter
GNU gdb 6.4.90-debian
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i486-linux-gnu"...Using host libthread_db library "/lib/tls/libthread_db.so.1".
(gdb) run #-- just run segfaulter and let it seg fault
Starting program: /home/newhall/public/gdb_examples/segfaulter
Failed to read a valid object file image from memory.
Program received signal SIGSEGV, Segmentation fault.
0x080483e1 in initfunc (array=0x0, len=100) at segfaulter.c:15
15 array[i] = i;
(gdb) where #--- let's see where it segfaulted
#0 0x080483e1 in initfunc (array=0x0, len=100) at segfaulter.c:15
#1 0x0804846e in main () at segfaulter.c:38
(gdb) list #--- let's see code around segfaulting instruction
10 int initfunc(int *array, int len) {
11
12 int i;
13
14 for(i=1; i <= len; i++) {
15 array[i] = i;
16 }
17 return 0;
18 }
19
(gdb) p array[0] #--- let's print out some values and see what's going on
Cannot access memory at address 0x0
#-- it looks like array is a bad address (0x0 is NULL)
(gdb) p array
$1 = (int *) 0x0
(gdb) frame 1 #--- let's see what main is passing to this funtion
#1 0x0804846e in main () at segfaulter.c:38
38 if(initfunc(arr, 100) != 0 ) {
(gdb) print arr #--- print out arr's value (what we pass to initfunc)
$2 = (int *) 0x0
#--- oops, we are passing NULL to initfunc...we forgot to initialize arr to point to valid memory