Thread Information

In this episode, our guest contributor is Satabdi Das, Software Development Engineer at AWS. Satabdi is an expert at developing and debugging complex software systems in C++. She is also a speaker at C++ community events. Over to her!

In this tutorial, I am going to show you a few GDB commands for debugging multi-threaded programs. Running GDB on C++ programs with multiple threads is difficult, but these commands are going to give you some visibility into the state of your program and help you debug multi-threaded programs.

When I run a multi-threaded program on GDB, I’d like to see the thread information. I am going to use the following commands to get the information about the existing threads:

 info threads

And the output is going to look like the following:

The star to the left of thread number 2 indicates that this is the current thread. I haven’t specified any arguments to this command. So it displays information about all the threads. You can specify the list of threads that you want to see.

For example, I can get information for threads 1 and 2 using the command info threads 1 2 

 

Switching between threads

Sometimes I want to switch between threads and check what a specific thread is doing. I am going to use the command thread 1 to switch to thread number 1.

Let’s execute the next line on GDB.

 

Note that gdb switched to a different thread. This can be annoying when you want to trace the execution for a particular thread. You can use the command set scheduler-locking on

And now GDB is going to run only the current thread.

 

Setting a Breakpoint

Next, I am going to set a thread-specific breakpoint. I am going to set a breakpoint on line 16 for the current thread using the command  b 16 thread 4.

Since I have set the breakpoint only for this thread, GDB is not switching between threads.

Applying a command to a group of threads

The last command I am going to show you is to apply a certain command to a group of threads. For example, I want to print the backtrace of all the threads and I am going to use the command thread apply all bt

 

That’s all I wanted to share with you in this tutorial. Hope you’ve learnt something new about GDB today.

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