Stack trace

As an example, the following Python program contains an error.

def a():
    i = 0
    j = b(i)
    return j

def b(z):
    k = 5
    if z == 0:
        c()
    return k + z

def c():
    error()

a()

Running the program under the standard Python interpreter produces the following error message.

Traceback (most recent call last):
  File "tb.py", line 15, in <module>
    a()
  File "tb.py", line 3, in a
    j = b(i)
  File "tb.py", line 9, in b
    c()
  File "tb.py", line 13, in c
    error()
NameError: name 'error' is not defined

The stack trace shows where the error occurs, namely in the c function. It also shows that the c function was called by b, which was called by a, which was in turn called by the code on line 15 (the last line) of the program. The activation records for each of these three functions would be arranged in a stack such that the a function would occupy the bottom of the stack and the c function would occupy the top of the stack.

In Java, stack traces can be dumped manually with Thread.dumpStack()[5] Take the following input:

public class Main {
  public static void main(String args[]) {
    demo();
  }
  static void demo() {
    demo1();
  }
  static void demo1() {
    demo2();
  }
  static void demo2() {
    demo3();
  }
  static void demo3() {
    Thread.dumpStack();
  }
}

The exception lists functions in descending order, so the most-inner call is first.

java.lang.Exception: Stack trace
        at java.lang.Thread.dumpStack(Thread.java:1336)
        at Main.demo3(Main.java:15)
        at Main.demo2(Main.java:12)
        at Main.demo1(Main.java:9)
        at Main.demo(Main.java:6)
        at Main.main(Main.java:3)

Both C and C++ (pre-C++23) do not have native support for obtaining stack traces, but libraries such as glibc and boost provide this functionality.[6][7] In these languages, some compiler optimizations may interfere with the call stack information that can be recovered at runtime. For instance, inlining can cause missing stack frames, tail call optimizations can replace one stack frame with another, and frame pointer elimination can prevent call stack analysis tools from correctly interpreting the contents of the call stack.[6]

For example, glibc's backtrace() function returns an output with the program function and memory address.

./a.out() [0x40067f]
./a.out() [0x4006fe]
./a.out() [0x40070a]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x7f7e60738f45]
./a.out() [0x400599]

As of C++23, stack traces can be dumped manually by printing the value returned by static member function std::stacktrace::current():[8]

std::cout << std::stacktrace::current() << '\n';

Rust has two types of errors. Functions that use the panic macro are "unrecoverable" and the current thread will become poisoned experiencing stack unwinding. Functions that return a std::result::Result are "recoverable" and can be handled gracefully.[9] However, recoverable errors cannot generate a stack trace as they are manually added and not a result of a runtime error.

As of June 2021, Rust has experimental support for stack traces on unrecoverable errors. Rust supports printing to stderr when a thread panics, but it must be enabled by setting the RUST_BACKTRACE environment variable.[10]

When enabled, such backtraces look similar to below, with the most recent call first.

thread 'main' panicked at 'execute_to_panic', main.rs:3
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
   1: std::panicking::default_hook::{{closure}}
   2: std::panicking::default_hook
   3: std::panicking::rust_panic_with_hook
   4: std::panicking::begin_panic
   5: futures::task_impl::with
   6: futures::task_impl::park
...