Objective
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To learn how to debug a C program using gdb
What is gdb?
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GNU Debugger
The most popular debugger for UNIX systems to debug C and C++ programs.
Features provided by gdb
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Set up breakpoints
Control the execution : Continue, Stepping over and in
Printing the variable values
Miscellaneous
Set up breakpoints
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break line_number
E.g. Set up a breakpoint at line 10
Command: break 10
Demo
(gdb) break 10
Breakpoint 1 at 0x400605: file debug.c, line 10.
Control the execution
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run (or r) : start a new execution until a breakpoint or to the end if no breakpoints.
continue( or c) : continue executing until the next breakpoint
next (or n) : execute next line as a single instruction (step over)
step (or s) : same as next, but does not treat the function as a single instruction, instead it goes into function and executes it line by line (step in)
Printing the variable values
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print variable_name
E.g. print i print j
Or simply use p instead of print p i p j
Print backtraces of all stack frame
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where
Shows stack: sequence of function calls executed so far
Good for pinpointing location of a program crash
up
Goes up one level in the stack
Display the values of expressions at each step
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display expression
E.g. Show the value of expression of b + 1 at each time your program stops display b+1
Demo
(gdb) display b+1
1: b+1 = 2
(gdb) n
7 scanf("%d",b);
1: b+1 = 2: file debug.c, line 10.
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Step 1 : Compile the debug.c with –g debugging option
$gcc -o debug -g debug.c
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Step 3: Set up breakpoint at line 6
(gdb) break 6
Fall 2017
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Step 3: List the source code
(gdb) list
1 #include<stdio.h>
2
3 int main(void){
4
5 int b=1,c=2;
6 printf("Please enter the value of b:");
7 scanf("%d",b);
8 c=b/3;
9 printf("%d/3=%d ",b,c );10 }
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Step 4 :Run from start until the first breakpoint
(gdb) r
Starting program: /home/ylong4/public/debug
Breakpoint 1, main () at debug.c:6
6 printf("Please enter the value of b:");
Step 5: Print out value stored in b
(gdb) p b
$1 = 1
Control the execution
Print backtraces of all stack frame
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Step 7 : Show stack and go up one level
(gdb) where
#0 0x00007ffff7a72122 in __GI__IO_vfscanf () from
/lib64/libc.so.6
#1 0x00007ffff7a80b59 in __isoc99_scanf () from
/lib64/libc.so.6
#2 0x00000000004005d6 in main () at debug.c:7
(gdb) up
#1 0x00007ffff7a80b59 in __isoc99_scanf () from
/lib64/libc.so.6
(gdb) up This line generated
#2 0x00000000004005d6 in main () at debug.c:7error.
7 scanf("%d",b); What is the error?
Other Resources
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Online GDB https://www.onlinegdb.com
RMS’s gdb debugging tutorial http://www.unknownroad.com/rtfm/gdbtut
/gdbtoc.html
GDB Quickstart https://www.youtube.com/watch?v=5yZIF mplXsw
