GDB internals documentation

A useful GDB initilization script .gdbinit (a local copy here) Detailed explanation here

Hacking GDB

add_com
add_com_alias
add_info
add_info_alias

For example, info functions command is added in function _initialize_symtab gdb/symtab.c as

  add_info ("functions", functions_info,
        _("All function names, or those matching REGEXP."));

Invoking GDB

Running debuggee ("inferior" in GDB parlance) and set up its environments

Break points, watch points, single-step execution

One should set the environmental variable LD_BIND_NOW to 1 if one wants to set a break point in a function in a dynamic link library.

set breakpoint pending on	When setting a break point on a not-yet-loaded dynamic library, this will automatically create a pending break point WITHOUT asking.
b function	Set a break point at first line of function
tbreak function	Set one-time break point at first line of function
b num	Set a break point at line number num
b *0x12345	Set a break point at address 0x12345
b *function	Set a break point at actual starting address of function
b *0x12345 if foo==5	Stop at address 0x12345 if foo==5 See here for more details on conditional break points.
watch foo	Stop if the variable foo changed. The number of watch points is limited (depending on hardware). On x86, only 4 hardware watch points can be set simultaneously.
set can-use-hw-watchpoint 0	Do not use hardware watch points.
watch *0x12345	Stop if the value at address 0x12345 changed
rwatch foo	Stop if the variable foo is read. This feature is hardware dependent.
awatch foo	Stop if the variable foo is read or written. This feature is hardware dependent.
catch syscall	Stop when a system call occurs.
catch syscall [name| number]	Stop when a specified system call occurs.
catch throw	Stop when a C++ exception is thrown.
handle signal stop handle signal nostop handle signal print	Signal handling.
b function if foo==5 commands silent printf "x is %d\n",x c end	Execute the commands "silent", "print...", and "c" after reaching the break point at function
info break	List all break/watch points
clear	Remove all break/watch points
clear function	Remove the break point at function
disable num	Disable the break point num (use "info break" first to get a list of break/watch points)
c	Continue execution after reaching a break point
rc	Reverse execution until reaching a break point
finish	Continue execution until the current function returns
return 1	Return from current function immediately with value 1
s	Execute one line of source code
n	Same as above, but don't jump into function calls
si	Execute one machine instruction
ni	Same as above, but don't jump into function calls
reverse-step	Reverse execute one line of source code. See here for a tutorial.
reverse-next	Same as above, but don't jump into function calls
reverse-stepi	Reverse execute one machine instruction
reverse-nexti	Same as above, but don't jump into function calls
checkpoint	Save a snapshot of debuggee's state
restart checkpoint_id	Restore the debuggee's state to checkpoint_id
delete checkpoint checkpoint_id	Delete checkpoint_id
info checkpoints	Show all checkpoints
(Record and replay the execution)	See here for details and here for a tutorial.

Examining the stack

Examining source files and machine code

define hook-stop
x/i $pc
end

Examining data, memory, registers

define hook-stop
print var
end

Changing data, memory, registers, execution

Examining the symbol table

Dump memory to files

Memory region attributes

Tracing facility

Troubleshooting GDB

What is
[Thread debugging using libthread_db enabled]
Cannot find new threads: generic error
error ?

 $ LD_PRELOAD=/lib/libpthread.so.0 gdb --args ./myapps

 (gdb) set exec-wrapper env 'LD_PRELOAD=/lib/libpthread.so.0'

Target does not support this type of hardware watchpoint error ?

 (gdb) start
 (gdb) rwatch foo