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Examining stack and threads

Overview of the backtrace, thread and frame commands.

Examining the execution state using backtrace, thread and frame commands​

When your program stops, the first thing you need to know is where it stopped and how it got there. These commands can help you investigate:

  • frame info - displays information about the currently selected frame
  • thread info - lists information about all threads
  • thread current - shows the currently selected thread
  • backtrace (bt) - shows the backtrace of the current thread, including:
    • Thread number and PID
    • Instruction address where execution stopped
    • All frames from the current execution frame (frame 0) up through the call stack
  • backtrace all (bt all) - shows backtraces for all active threads

Change current frame and thread​

What is stack frame?

The call stack is divided into contiguous pieces called stack frames. Each frame contains the data associated with one function call, including:

  • the function's arguments
  • its local variables
  • the return address

All stack frames are allocated in a memory region called the call stack.

When a program stops, the frame and thread where execution halted become the current frame and current thread. Most of commands operate on these current elements. For example, var locals prints all local variables in the current frame. If you switch to a different frame, var locals will display the variables from that function's context.

You can change the current thread or frame by specifying its number (check it using frame info or thread info commands):

  • frame switch <number> - changes the current frame
  • thread switch <number> - changes the current thread

Usage example​

Consider this Rust code:

pub fn main() {
    for i in 1..3 {
        std::thread::spawn(move || {
            infinite_inc(i);
        });
    }
    
    std::thread::sleep(std::time::Duration::from_secs(10));
}

fn infinite_inc(step: u32) {
     let mut cntr = 0;
     loop {
        cntr += step;
     }
}

Lets try to stop this program, view current frame and thread, inspect the backtrace, change current thread and frame and observe local variables.