betterexplaned-com--debugging-with-gdb.md

Debugging with GDB

source: BetterExplained.com

Getting Started: starting and stopping

• gcc -g myprogram.c, compiling program with debugging option to let you use variables and function names inside GDB, instead of raw memory locations

• gdb a.out opens a.out in debugger, doesn't start running

• can run help to list topics, with which help <topic> can be used for actual commands lists

• to run program in debugger, usually breakpoints are set before running

r             ## to plainly run

r arg1 arg2   ## to pass arguments while running

r < somefile  ## feed in a file with arguments

• q to quit GDB

Stepping through code

to trace path and reach point of origin for concern

• l list 10 lines of code at current code point, l 25 to use line:25 as center line, l myfunc to list the function

• next run until next line and pause, if current line is a function execute entire and then pause

• step runs the next instruction, it dives into function if next instruction happens to be function

• finish finish executing current function and pause, if wanna finish rest of function at once

Breakpoints and Watchpoints

to quickly enquire a certain code position, need to pause there

• break 35 sets breakpoint at line:35, break myfunction to break at myfunction

• watch x == 3 sets a watchpoint on x == 3, pauses the program when condition changes

• continue resume run until hit another breakpoint/watchpoint

• delete N to clear out a breakpoint

Setting Variables and Calling Functions

viewing and changing states at runtime, typically when paused

• print x to show value of var x, compile need to have debug information to use real names

• set x = 3 to change x to 3, set x = y to change value to variable y

• call myfunc() to call user-defined function, call urfunc(x) to call with arg; beware of calling buggy function

• display x to show value of x after every step or pause, undisplay x to stop the regular show

Backtrace and Changing Frames

shows frame of a single function call when in program

• bt prints current stack, bt full shows full backtrace with local variables

• up and down to move to next or previous frame respectively

• return to be back from current function

Crashes and Core Dumps

core dump can be read and give the line number of crash, argument passed and more

• gdb myprogram core to debug myprogram with core as dump file

• can use bt to examine details

Handling Signals

events thrown at certain events, GDB might pause and can be ignored

• handle <signalname> <action>

• varied levels of ingoring as

handle SIGUSR1 nostop

handle SIGUSR1 noprint

handle SIGUSR1 ignore

Tips

• prefer watchpoints over breakpoints

• printf works well for tracing, wrap printf in a log function for flexibility

• pass a log level with message (1 most important, 3 least, 0 off), tweak log function to persist errors

• use #define LOG_LEVEL LOG_WARN to display warning and above, #define LOG_LEVEL LOG_NONE to turn of debugging

#include

#define LOG_NONE 0
#define LOG_ERROR 1
#define LOG_WARN 2
#define LOG_INFO 3
#define LOG_LEVEL LOG_WARN


// shows msg if allowed by LOG_LEVEL
int log(char *msg, int level){
  if (LOG_LEVEL >= level){
    printf("LOG %d: %s\n", level, msg);
    // could also log to file
  }

  return 0;
}

int main(int argc, char** argv){
  printf("EHLO!\n");

  log("Really bad error!", LOG_ERROR);
  log("Warning, not so serious.", LOG_WARN);
  log("Just some info, not that important.", LOG_INFO);

  return 0;
}

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