I need to debug a complex program (SWUpdate) which spawns (actually fork()s) several sub-processes.
I can "follow" execution on child process using GDB commands:
(gdb) set detach-on-fork off
(gdb) set follow-fork-mode child
but I didn't find a way to set a breakpoint directly on a line which will be executed only in a child process.
Is there some way to set a breakpoint at a certain location regardless of executing thread/process?
Related
I have read how gdb attaches to another running process from this link:
How does a debugger peek into another process' memory?
But what about starting a program directly with it as such:
gdb ./my_program
Does gdb fork and run my_program using it and attach to it like it's explained int he above link (i.e with ptrace on linux), or the process is entirely different?
or the process is entirely different?
No: the process is the same -- fork, parent calls PTRACE_ATTACH, child calls PTRACE_TRACEME. That last point guarantees that GDB can debug the child process from its first instruction.
There is one additional complication: GDB uses $SHELL in order to handle input / output redirection, so there is fork -> exec shell (in the child) -> exec program.
I'm having a tough time figuring this one out; I have a program, iverilog that executes a system() call to invoke another program, ivl. I want to debug the second program, ivl in gdb, but I can't get gdb to set any breakpoints in the child process when I invoke gdb with the parent process. Here's what the programs look like:
//iverilog-main.cpp (Parent process)
int main(){
//...
system("ivl arg1 arg2");
//...
return 0;
}
.
//ivl-main.cpp (child process)
int main(){
//...
//stuff I want to debug
//...
return 0;
}
.
The gdb commands I'm running are: gdb iverilog -x cmds.gdb
# cmds.gdb
set args hello.v
set follow-fork-mode child
set breakpoint pending on
break ivl-main.cpp:main
run
Unfortunately, gdb doesn't break at ivl-main.cpp:main,it just completes without ever breaking; the output I get is:
Starting program: /usr/local/bin/iverilog hello.v
[New process 18117]
process 18117 is executing new program: /bin/dash
[Inferior 2 (process 18117) exited normally]
I'm certain ivl-main.cpp:main is being called because when I run the ivl program in gdb it successfully breaks there.
My thinking is that gdb doesn't recognize ivl-main.cpp as a source file when its running gdb iverilog, and it's not setting that breakpoint when it enters the child process which does contain ivl-main.cpp as a source file. So I think if I set the breakpoint for ivl-main.cpp when gdb enters the child process, it should work. The only way I can think of doing this is to manually break at the system() call and step into the child process, then set the breakpoint. Is there a more elegant approach that would force gdb to break whenever entering a child process?
Normally GDB only debugs one process at a time- if your program forks then you will debug the parent or the child, but not both simultaneously. By default, GDB continues debugging the parent after a fork, but you can change this behavior if you so desire with the following command:
set follow-fork-mode child
Alternately, you can tell GDB to keep both the parent and the child under its control. By default GDB only follows one process, but you can tell it to follow all child processes with this command:
set detach-on-fork off
GDB refers to each debugged process as an "inferior". When debugging multiple processes you can examine and interact each process with the "inferiors" command similar to how you would use "threads" to examine or interact with multiple threads.
See more documentation here:
https://sourceware.org/gdb/onlinedocs/gdb/Forks.html
This answer provides one way to achieve what you want.
In theory, set follow-fork-mode child should work.
In practice, the iverilog is likely itself a shell script that runs (forks) multiple commands, so at every fork you will need to decide whether you want to continue debugging the parent or the child. One wrong decision and you've lost control of the process that will eventually execute your program. This very likely explains why it didn't work for you.
I'm debugging a C++ application which creates trees of forks. Using GDB defaults, the child processes will be detached on the fork and as a result I see only one inferior shown afterwards.
I tried to attach to one of the child processes and despite it not being listed as an inferior for the other GDB process, in the new GDB session I get an error that the process is already being traced (by the first GDB session).
Is this expected behavior? What steps can I take to debug the forked process in a separate GDB session? What steps can I take to debug the problem further?
I have a network software that I need to debug. It forks at multiple places and I need to debug one particular function handling one particular request.
Is there any way to setup a global breakpoint that would be caught even when it is in an inferior process?
I cannot use follow-fork-mode child because this will follow the first request, not the one I need to debug.
One way to do this is to have gdb remain attached to all the processes. Then you would set your breakpoint and run the program as usual; the breakpoint would fire in any sub-process that happened to hit that location. You can use breakpoint conditions to try to reduce the number of hits.
To put gdb into multi-inferior mode, I use this:
set detach-on-fork off
set non-stop on
set pagination off
Depending on your version of gdb, you might also need set target-async on.
This mode can be a bit peculiar to work in. For example, when one thread stops, the other keep going. Also, breakpoint stops are reported, but not always obvious; and I think gdb doesn't immediately switch to the stopping thread (this may have changed in gdb git, I forget).
Without getting into to to much detail, I'm working on a program that consists of several separate processes all running on embedded QNX RTOS. They don't have a parent-child relationship, they are all spawned using spawnlp(P_NOWAIT, ...) and they all communicate with each other using the IPC mechanism provided by the OS.
When I'm debugging with GDB and I hit a breakpoint in the process I'm working in, all of my threads are paused, which is great. But is there a way to also have it pause execution of my other processes? Right now what's happening is all the other processes keep on truckin' while my process is paused and so all the IPC queues get full etc. etc.
Thanks in advance,
HF
You can associate a list of GDB commands with each breakpoint. So when you hit a breakpoint in process A, you can for example send a SIGTRAP to process B, which should drop it into the debugger:
(gdb) b main
Breakpoint 1 at 0x804834a: file testA.c, line 40.
(gdb) command
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just "end".
>shell kill -s TRAP `pidof testB`
>end
(gdb)
More info at Breakpoint Command Lists