Debugging with GDB

To debug, you must generate debugging info when compiling, and this requires you to pass the -g option to the compiler. If you don't know why, just think of how you debug with visual studio. (FAQ之 Debug单步调试)

• the doc for GDB can be obtained at GDB: The GNU Project Debugger.
• a cheatsheet canbe fond here: /usr/share/doc/gdb/refcard.ps.gz

I'll illustrate how to debug with GDB with a simple C program.

#include <stdio.h>

void main () 
{
    /*inteded to transfer temprature format
    from Fahrenheit to Celsius*/

    float far, cel, inp;
    const int up=300, low=0, step=20;

    far=low;

    puts("This is a list for temprature conversion:\n\n");
    printf("%s\t%s\n\n", "Fahrenheit", "Celsius");

    while(far <= up)
    {
        cel = 5 * (far-32) / 9.;
        printf("%5.0f F \t%9.2f C\n", far, cel);
        far += step;
    }

    //Now, we'll read input
    printf("Please enter the value of Fahrenheit\n");
    scanf("%f", &inp);
    printf("You have entered Fahrenheit temprature %f F\n", inp);
    printf("The Celsius temprature is %f C\n", 5 * (inp-32) / 9.);
}

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1. The debug mode and release mode

compile the C source with/out debug info

I run the compiler with release mode and debug mode, respectively. Accordingly, executable file are named with release and debug. As it shown in the picture above, the release version is smaller than the debug one, which is resulted from the lack of debug info.

---

2. debug with GDB

To start the debugging, type gdb exe_name. Note the name here is the debugged executable file, not the source code.

• If you debug the source file:
  the debugger finds it being a non-executabe file, debug failed
  

  if you debug the source file ...

• if you debug the release format:
  the debugger tries to find the debug info but in vain.
  

  if you debug the release exe

• when you debug the right file:
  the debugger successfully find the debug info, and debugging is ready
  

  right way to evoke debug

• when entering the debugging mode successfully, we can set breakpoints at lines as they appear in the source file 'cause the debug info contains all the contens (comments may not included, however, I am not very sure) of the source file.

break to set breakpoints, e.g., break 20 to set at the 20th line.

• when breakpoints set, remember to run the program

the command is run

• when the program reaches to the breakpoint, or, put it in another way, technically, trigger the breakpoint, the program will pause.

You can type continue to let the program go on.

---

3.Below is the full info in the terminal for debugging.

mark@ASUS:~/Desktop$ gdb debug
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1
Copyright (C) 2016 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-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from debug...done.
(gdb) break 20
Breakpoint 1 at 0x4006ec: file temprt.c, line 20.
(gdb) continue
The program is not being run.
(gdb) run
Starting program: /home/mark/Desktop/debug 
This is a list for temprature conversion:


Fahrenheit  Celsius

    0 F        -17.78 C

Breakpoint 1, main () at temprt.c:20
warning: Source file is more recent than executable.
20          far += step;
(gdb) continue
Continuing.
   20 F         -6.67 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
   40 F          4.44 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
   60 F         15.56 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
   80 F         26.67 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  100 F         37.78 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  120 F         48.89 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  140 F         60.00 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  160 F         71.11 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  180 F         82.22 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  200 F         93.33 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  220 F        104.44 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  240 F        115.56 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  260 F        126.67 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  280 F        137.78 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
  300 F        148.89 C

Breakpoint 1, main () at temprt.c:20
20          far += step;
(gdb) continue
Continuing.
Please enter the value of Celsius
3
You have entered Fahrenheit temprature 3.000000 F
The Celsius temprature is -16.111111 C
[Inferior 1 (process 23884) exited normally]
(gdb)