If I set a watchpoint for a variable local to the current scope, it will be auto deleted when going out of the scope. Is there any way to set it once and keep it auto alive whenever entering the same scope?
Is there anyway to set conditional watchpoint, like watch var1 if var1==0? In my case, the condition does't work. gdb stops whenever var1's value is changed, instead of untill var1 == 0 is true. My gdb is GNU gdb 6.8-debian.
I agree with Dave that a conditional breakpoint is the way to go.
However, to do what you asked, you can use GDB's commands command to set a list of GDB commands to execute whenever a breakpoint is hit. I find this incredibly useful.
I suggest writing your GDB commands into a file so that they are easy to edit and easy to reload with the source command. Or you can specify command files to load on the GDB command line or use .gdbinit to make them load automatically.
An example of a good use of commands:
Suppose that I have a function format that is called by a lot of other functions. I want to break on it, but only after function do_step_3 has been called.
break do_step_3
commands
break format
continue
end
You could use this for your problem with something like:
break func
commands
watch var
continue
end
You can set conditions on watchpoints in the same way that you do with breakpoints. This is in the documentation but admittedly it hardly calls attention to itself.
So watch my_var if my_var > 3 works just fine, as does the condition command.
To recreate the watchpoint if the variable it is watching goes out of scope, have gdb do this automatically using a breakpoint at the start of the function as Zan has described.
You can set a watchpoint that does not go out of scope by setting it to the memory address.
(gdb) p &var1
$1 = (int *) 0x41523c0
(gdb) watch *(int *)0x41523c0
Hardware watchpoint 1: *(int *)0x41523c0
This also works for other data types and pointers.
I'm not sure which language us are using, so the exact answer will vary, but could you change the variable to either be static, global, or dynamically allocated (and don't free it when the function returns?). This way it's raw address won't change, and gdb will be able breakpoint on it.
Instead of watching the value whe it equals a specific value; you should set a conditional break point on the line where you want to check the value of var1. This should effectively have the same effect
e.g.
(gdb) break main.c:123 if (var1 == 0)
Related
GDB allows registering a set of commands to a specific breakpoint via commands NUM syntax. I need to register the set of commands for a specific breakpoint via an external file, by using a syntax something like the following:
commands ./main.c:18
silent
print buffer[0]
cont
end
commands ./io.c:29
silent
printf "Hello world %i\n", myvar1
cont
end
The commands path/to/file:XX syntax is made up by me. Because the NUM in commands NUM syntax requires exactly the breakpoint's runtime ID number (assigned by GDB), I can not use a deterministic syntax for that purpose.
I'm currently registering breakpoints via a text file with such a content:
break ./main.c:18
break ./io.c:29
and then issuing source breakpoints.txt command inside GDB. It seems that there is no way to register commands at the same time while registering a breakpoint:
(gdb) help break
Set breakpoint at specified line or function.
break [PROBE_MODIFIER] [LOCATION] [thread THREADNUM] [if CONDITION]
PROBE_MODIFIER shall be present if the command is to be placed in a
probe point. Accepted values are -probe' (for a generic, automatically guessed probe type), -probe-stap' (for a SystemTap probe) or
`-probe-dtrace' (for a DTrace probe).
LOCATION may be a line number, function name, or "*" and an address.
If a line number is specified, break at start of code for that line.
If a function is specified, break at start of code for that function.
If an address is specified, break at that exact address.
With no LOCATION, uses current execution address of the selected
stack frame. This is useful for breaking on return to a stack frame.
THREADNUM is the number from "info threads".
CONDITION is a boolean expression.
Multiple breakpoints at one place are permitted, and useful if their
conditions are different.
Question
Is there any easy way to set some predetermined commands for a predetermined breakpoint from within a file?
If not, is there any equivalent way to pass the (gdb) info breakpoints output to a file or a program while pipe is not available in GDB (version 5.3)? Currently I'm trying a workaround by using logging feature for that purpose:
set logging file /tmp/breakpoints
set logging on
info breakpoints
set logging off
Is there any easy way to set some predetermined commands for a predetermined breakpoint from within a file?
Yes: if you use commands without NUM, the commands will apply to the last breakpoint set. So you want something like:
break main.c:18
commands
silent
print buffer[0]
cont
end
I've set a breakpoint in gdb, and I'd like to see the exact line of source the breakpoint is on, just to confirm it's correct -- is there a quick way to do this?
The "info b" command gives me information about the breakpoints, but it doesn't display source:
(gdb) info b
Num Type Disp Enb Address What
1 breakpoint keep y 0x00000000006c3ba4 in MyClass::foo(bar*)
at /home/user1/src/MyClass.cpp:1021
I can type "list MyClass.cpp:1021" to see the lines around this breakpoint, but I'm wondering if there's a shorter way. Googling and reading the gdb manual didn't turn up anything.
I know that if I'm executing the program and have hit the breakpoint, I can just type "list", but I'm asking specifically about the case where I am not at the breakpoint (the program may not even be running).
You can use the list command to show sources. list takes a "linespec", which is gdb terminology for the kinds of arguments accepted by break. So, you can either pass it whatever argument you used to make the breakpoint in the first place (e.g., list function) or you can pass it the file and line shown by info b (e.g., list mysource.c:75).
I think the closest one can get to this is by turning the history on (set history save on) and then press CTRL-R to do a reverse search for the former list command.
More specifically, change your workflow when setting a breakpoint. After each command like b main GDB shows the source file like path/to/main.cpp, line 12. Immediately use this information in a quick list main.cpp:12. To show this location later press CTRL-R and type "main".
https://sourceware.org/gdb/onlinedocs/gdb/Command-History.html
I have read the following SO question:
Do specific action when certain breakpoint hits in gdb
Here, we use 'command' to decide what to do when the SPECIFIED Breakboint Gets Hit.
My Question is:
Suppose I put Breakpoints on ALL the Functions matching a given pattern:
gdb$rbreak func_
=> 100 Breakpoints (say)
When I execute this Code, I want to do the SAME Action - on hitting Each of these functions.
Hence, I cannot define something like:
command break_point_number
// since I don't know how many breakpoints will be there
Can somebody please suggest me:
How can I do a specific action-set when ANY Breakpoint gets Hit in GDB?
Thanks.
With a new enough version of gdb you can use a range:
(gdb) rbreak whatever
... gdb creates breakpoints N, N+1, ..., M
(gdb) commands N-M
> stuff
> end
I forget exactly when this feature went in.
With an older version of gdb, I'm not sure it can easily be done.
It can be done with difficulty: use set logging to write output to a file, then "info break", then "shell" to run scripts to edit the file into gdb commands, then "source". This is very painful.
Is there any way to label/name breakpoints in GDB, in order to more easily identify them in, for instance, info b? And if so, how?
No, there is no way to do this.
http://users.ece.utexas.edu/~adnan/gdb-refcard.pdf
While this doesn't help with the output of info b, for other purposes where you need to reference the breakpoint later in commands you can store the breakpoint number of the last breakpoint from $bpnum in another convenience variable, e.g.:
b foo.c:123
set $im_a_breakpoint = $bpnum
# ... set some more breakpoints and do some other stuff ...
disable $im_a_breakpoint
As a further example, I'm using this pattern right now for a task: I'm using normal gdb breakpoints in code emitted by a JIT, which requires setting the breakpoint on the address after the code has actually been emitted as gdb modifies the code. For this purpose I actually disable the initial set-up breakpoint from within its own commands block:
set $cur_stop_point = 0x41aaa
b basic_jit_cache::copy_block if ((uint32_t)this->code_ptr()) > ($gencode + $cur_stop_point)
set $cur_stop_point_setup_bp = $bpnum
commands
b *($gencode + $cur_stop_point)
disable $cur_stop_point_setup_bp
cont
end
I am trying to set up a watchpoint to monitor a variable in a package consisting of many C++ files.
There are many files
abc.cpp
qwe.cpp
..
xyz.cpp and so on
I want to monitor a variable 'temp' in some function qwerty() in the file abc.cpp
How do I set the watchpoint ?
I tried
watch abc.cpp::temp
watch abc.cpp:temp
watch temp
but I see the errors No symbols 'abc.cpp::temp','abc.cpp:temp','temp' not in current context
Also a info watchpoints tells me that no watchpoints are set. Note that I can set the breakpoints successfully for the same variable
I always set a breakpoint in the function, then set the watchpoint when I hit it, so that I'm in the context, then delete the breakpoint as appropriate.
Do you want to make conditional breakpoints? If then you can use the commands below.
(gdb) break abc::qwerty()
(gdb) condition 1 temp=1 // If you want to break when the value of temp = 1.