I would like to do only flash the code on a remote gdb target if it has changed since last time gdb was run. I envisage something along the lines of the following in gdb script;
target extended-remote /dev/<device>
<Attach to Target>
file <Target Program>
if ![compare-sections -r]
load
start
...however, I cannot see how to make a conditional on a command output.
Can anyone help? I think I probably missed something, but I've no idea what....
The compare-sections command doesn't return a value that can be used in an if statement, but the following may do what you want.
First, define a convenience function named $cmdeval which will execute a gdb command and return its output as a string:
import gdb
class CmdEval(gdb.Function):
"""$cmdeval(str) - evaluate argument string as a gdb command
and return the result as a string. Trailing newlines are removed.
"""
def __init__(self):
super(CmdEval, self).__init__("cmdeval")
def invoke(self, gdbcmd):
return gdb.execute(gdbcmd.string(), from_tty=False, to_string=True).rstrip('\n')
CmdEval()
You can put this in a file named cmdeval.py and type (gdb) source cmdeval.py to load it into gdb.
Next, since compare-sections outputs "MIS-MATCHED" for any section that has been changed, you can look for that string using the $_regex convenience function, which is included in more recent versions of gdb:
(gdb) if $_regex($cmdeval("compare-sections -r"),".*MIS-MATCHED.*")
>echo need to load again\n
>end
Hi I need to write a lldb breakpoint command that evaluates a value and prints out a value.
In gdb I could do it like this:
if ($value==2)
printf "Value is 2\n"
end
But in lldb the 'if-statement' is invalid it seems:
failed with error: 'if' is not a valid command.
error: Unrecognized command 'if'.
Can anyone tell me how to write this comparison inside my breakpoint command? Thanks!
You can use the expression parser to achieve this effect in some cases, and you can use the lldb Python interpreter for whatever complex work you want to do in response to a breakpoint hit. Given the fairly deep level of Python support, we felt if you don't know Python, you time would be better spent learning a little bit of that so you could really script lldb, rather than learning whatever little micro-language we would come up with.
Anyway, so using the interpreter, you could for instance do:
expr if ($value == 2) { (int) printf("Value is 2\n"); }
And using the python interpreter you can write a callback like:
def myCallback (frame, breakpoint_location, dict):
value = frame.FindValue("$value", lldb.eValueTypeConstResult)
if (value.unsigned == 10):
print "Value is 10"
put that in a file called myModule.py, do:
(lldb) command script import myModule.py
and then assign the command to your breakpoint with:
(lldb) breakpoint command add -F myModule.myCallback <BREAKPOINT_NUMBER>
That python example was a little more complex than normal because you were looking up lldb's equivalent of gdb's "convenience variable". If you were looking up a local, you could use frame.FindVariable.
More details on this at:
http://lldb.llvm.org/python-reference.html
I noticed that in GDB, when issuing commands with long output like info variables, the output is displayed one page at time, pressing enter to go down and q to quit.
Is it possible to replace the default pager with another one, such as less, so that I can navigate up and down, quitting, searching, etc?
Is it possible to replace the default pager with another one
No: GDB doesn't call into external program to display the output, it simply pauses the output every screenfull (and you can make it not pause by set height 0).
In addtion to running inside emacs, you could also use screen or tmux (learning them will generally help you in a lot of other situations), or ask GDB to log output (set logging on) and then search in gdb.txt with any $PAGER you want.
Starting with version 9.1, GDB has a pipe command, so you can send a command's output to the pager of your choice. From the documentation:
pipe [command] | shell_command
Executes command and sends its output to shell_command. Note that no space is needed around |. If no command is provided, the last command executed is repeated.
run gdb inside of emacs and you should be able to use emacs' paging commands.
run emacs
type M-x gdb return (M stands for meta - alt key or option on Macs)
The Emacs message bar will now display the message:
Run gdb (like this): gdb
More information is available here: http://tedlab.mit.edu/~dr/gdbintro.html
HTH
you can put the following user-defined commands in ~/.gdbinit, then
% cat ~/.gdbinit
python import os
define less1
python os.popen("less","w").write(gdb.execute("$arg0",to_string=True))
end
define less2
python os.popen("less","w").write(gdb.execute("$arg0 $arg1",to_string=True))
end
...
% gdb
(gdb) less2 info var
...
(gdb) less1 disass
...
It is a bit old thread, but I think it is worth to add. #yichun gave a very nice idea here, but to be more practical it can be extended to any number of arguments:
define less
python import os
python os.popen("less","w").write(gdb.execute(' '.join(["$arg{0}".format(i) for i in range(0, argc)]), to_string=True))
end
Then it can also woth adding exceptions handling and waiting for processes to terminate to avoid keyboard glitches and we have something like that:
% cat ~/.gdbinit
define less
python argc = $argc
python
import os
f = None
try:
f = os.popen("less","w")
f.write(gdb.execute(' '.join(["$arg{0}".format(i) for i in range(0, argc)]), to_string=True))
except Exception as e:
if f:
f.write(str(e))
else:
print (str(e))
finally:
if f:
f.close()
end
end
In gdb 8.1.1 this code in .gdbinit adds the required functionality:
python
import os
class Less(gdb.Command):
def __init__(self):
super().__init__("less", gdb.COMMAND_USER, gdb.COMPLETE_COMMAND)
def invoke(self, argstr, from_tty):
with os.popen("less","w") as pipe:
try:
pipe.write(gdb.execute(argstr, to_string=True))
except Exception as e:
pipe.write(str(e))
Less()
end
Usage
(gdb) less info breakpoints
(gdb) less backtrace
Information: Commands In Python.
I wonder if there is any way to print errno symbolic name instead of just a number in GDB. For example, instead of
errno = 13
I would like to see something like
EACCES
Assuming you have a recent GDB with embedded Python, you can use the Python interpreter to do what you want.
The following (untested) code should be about right:
(gdb) python import errno
(gdb) python print errno.errorcode[13]
You should be able to define a python command, e.g. perrno, that will cut down on typing. Documentation here.
I found it's very annoying to get the program name when defining a GDB script. I can't find any corresponding "info" command, and we can't use argv[0] either, because of multi-process/thread and frame-choosing.
So, what should I do?
If you are using a recent gdb (7 and above) you can play around with the Python support which is quite extensive (and probably where you want to go when defining gdb-scripts in general). I'm no expert at it but for a test-program /tmp/xyz I could use:
(gdb) python print gdb.current_progspace().filename
/tmp/xyz
See http://sourceware.org/gdb/current/onlinedocs/gdb/Python.html for more info on the Python support.
In "normal" gdb you could get the process name with "info proc" and "info target", but I guess you wanted it not just printed but being able to use it further in scripts? Don't know how to get the value out of the python runtime into a gdb variable other than the extremely ugly "using log files and sourcing it and hope for the best". This is how this could be done:
define set-program-name
set logging file tmp.gdb
set logging overwrite on
set logging redirect on
set logging on
python print "set $programname = \"%s\"" % gdb.current_progspace().filename
set logging off
set logging redirect off
set logging overwrite off
source tmp.gdb
end
and in your own function doing:
define your-own-func
set-program-name
printf "The program name is %s\n", $programname
end
I would suggest "going all in" on the python support, and scrap gdb-scripting. I believe it is worth the effort.