How do I attach a gdb to a running thread?

Just run a program with s few threads, run gdb and before running attach PROCESS_PID run strace in another console. You must see ptrace (PTRACE_ATTACH) for each thread. Show activity on this post. ptrace PTRACE_ATTACH sends SIGSTOP to the process which suspends the whole process i.e. all threads.

Table of Contents

What is gdb nonstop mode?

For some multi-threaded targets, GDB supports a further mode of operation, called non-stop mode, in which you can examine stopped program threads in the debugger while other threads continue to execute freely.

How do you debug multithreaded?

Debug the multithreaded app

In the source code editor, look for one of the following code snippets: C#
Left-click in the left gutter of the Thread.
On the Debug menu, select Start Debugging (F5).
In the source code editor, locate the line that contains the breakpoint.

What does scheduler locking do?

On some OSes, you can lock the OS scheduler and thus allow only a single thread to run. Set the scheduler locking mode. If it is off , then there is no locking and any thread may run at any time. If on , then only the current thread may run when the inferior is resumed.

What is backtrace in gdb?

A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.

How do I change a thread in gdb?

GDB provides these facilities for debugging multi-thread programs:

automatic notification of new threads.
`thread threadno ‘ , a command to switch among threads.
`info threads’ , a command to inquire about existing threads.
`thread apply [ threadno ] [ all ] args ‘ , a command to apply a command to a list of threads.

Why is it difficult to debug multi threaded programs?

Parallel processing using many threads can greatly improve program performance, but it may also make debugging more difficult because you’re tracking many threads. Multithreading can introduce new types of potential bugs.

What are threads in GDB?

The GDB thread debugging facility allows you to observe all threads while your program runs–but whenever GDB takes control, one thread in particular is always the focus of debugging. This thread is called the current thread. Debugging commands show program information from the perspective of the current thread.

What is the use of wait and lock instruction?

A Lock is a data structure used by threads to implement mutual exclusion without busy-wait spin-blocking. A thread blocks itself by pausing its execution path and switching to another thread.

How do I backtrace for all threads?

To display the backtrace for several or all of the threads, use the command thread apply (see thread apply). For example, if you type thread apply all backtrace , gdb will display the backtrace for all the threads; this is handy when you debug a core dump of a multi-threaded program.

What does optimized out mean in gdb?

It means you compiled with e.g. gcc -O3 and the gcc optimiser found that some of your variables were redundant in some way that allowed them to be optimised away. In this particular case you appear to have three variables a, b, c with the same value and presumably they can all be aliassed to a single variable.

What is thread debugging?

A thread is a sequence of instructions to which the operating system grants processor time. Every process that is running in the operating system consists of at least one thread. Processes that have more than one thread are called multithreaded.

How do I end a gdb session?

To exit GDB, use the quit command (abbreviated q ), or type an end-of-file character (usually C-d ). If you do not supply expression , GDB will terminate normally; otherwise it will terminate using the result of expression as the error code.

How do I skip a loop in gdb?

I can tell GDB to return from a function immediately with return , and call a function with call myFunction . But how do I get it break out of the current loop?…So to summarize the steps would be:

Set a breakpoint at the last line of the loop.
Continue.
When breakpoint hits, set the loop condition variable to false.

Why is race conditions and deadlocks so hard to debug?

Race conditions are, by their nature, hard to debug because they cause erratic and apparently random behavior which can vary across systems and with different inputs.

How do I test a concurrent code?

One good way to test this is to make sure you have access to a multi-cpu machine, then run your test for as long a time/with as many iterations as possible. For example if you have a multithreaded producer consumer algorithm, fill the queue with a good sized list and use 2x as many threads as you might in production.

Can two threads acquire the same lock?

Locks only provide mutual exclusion with other threads that acquire the same lock. All accesses to a data variable must be guarded by the same lock. You might guard an entire collection of variables behind a single lock, but all modules must agree on which lock they will all acquire and release.

What is thread lock?

The LOCK THREAD statement ensures that no other thread executes. This condition remains in effect until the thread is unlocked (with UNLOCK THREAD) or the thread terminates.

How do I attach a gdb to a running thread?