Debugging with GDB

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1 Debugging with GDB

Debugging with GDB

This is a small LaTeX document I had prepared out of a talk. Hope this is useful to everyone. If something is illegible or difficult to understand then I'd be happy to explain it. thanks,

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1: Starting and invoking gdb

1. Inserting debugging information inside the output executables created after compilation and to start debugging session.

$ gcc -o fact fact.c -g
$ gdb fact

2. Giving shell commands from within gdb

(gdb) shell clear

3. Set breakpoint at the function main()

(gdb) break main

4. Delete break point number 1

(gdb) delete 1

Note: Pressing enter with no command executes the previous command

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2: Running and navigating in gdb

1. Run program to be debugged

(gdb) run

2. See where program stopped

(gdb) list

3. Execute next line of the program

(gdb) next
(gdb) n

4. Step inside

(gdb) step

5. Print stack trace

(gdb) where

(gdb) backtrace
    OR
(gdb) bt

(gdb) frame 0
(gdb) frame 1

6. Return back from function

(gdb) return

7. Continue execution until the next break point.

(gdb) continue

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3: Retrieving values of variables

1. Display the value of a variable "i"

(gdb) display

2. Set hardware/software watch point for variable "i"

(gdb) watch i

3. Print the value of variable "i"

(gdb) print i

4. Print the address of variable "i"

(gdb) print &i

5. Reassign a value to n

(gdb) set variable n=6
(gdb) continue

6. Call fact() function with different parameters.

(gdb) call fact(4)

7. Display the data type of a variable:

(gdb) ptype i
(gdb) whatis i

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4: Segmentation faults

1. Segmentation faults Here we compile and execute a program with results in a segmentation fault. The snapshot of memory is saved in a file called "core"

$ gcc -o demo demo.c -g
$ gdb demo core

(gdb) disassembly

notes:

 1)  sethi = an assembly instruction
 2)  A core file will not be generated if core file size limit is set to zero.
     To check, type 'ulimit -c' in a terminal. If the output is '0', a core file will not be generated.
     Set the core file size limit to the desired number of blocks using: 'ulimit -c <num-of-blocks>'

Anurag

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Debugging with GDB

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