Stack Traces

The StackTraces module provides simple stack traces that are both human readable and easy to use programmatically.

Viewing a stack trace

The primary function used to obtain a stack trace is stacktrace:

  1. 6-element Array{Base.StackTraces.StackFrame,1}:
  2. top-level scope
  3. eval at boot.jl:317 [inlined]
  4. eval(::Module, ::Expr) at REPL.jl:5
  5. eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
  6. macro expansion at REPL.jl:116 [inlined]
  7. (::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92

Calling stacktrace() returns a vector of StackTraces.StackFrame s. For ease of use, the alias StackTraces.StackTrace can be used in place of Vector{StackFrame}. (Examples with [...] indicate that output may vary depending on how the code is run.)

  1. julia> example() = stacktrace()
  2. example (generic function with 1 method)
  3. julia> example()
  4. 7-element Array{Base.StackTraces.StackFrame,1}:
  5. example() at REPL[1]:1
  6. top-level scope
  7. eval at boot.jl:317 [inlined]
  8. [...]
  9. julia> @noinline child() = stacktrace()
  10. child (generic function with 1 method)
  11. julia> @noinline parent() = child()
  12. parent (generic function with 1 method)
  13. julia> grandparent() = parent()
  14. grandparent (generic function with 1 method)
  15. julia> grandparent()
  16. 9-element Array{Base.StackTraces.StackFrame,1}:
  17. child() at REPL[3]:1
  18. parent() at REPL[4]:1
  19. grandparent() at REPL[5]:1
  20. [...]

Note that when calling stacktrace() you’ll typically see a frame with eval at boot.jl. When calling stacktrace() from the REPL you’ll also have a few extra frames in the stack from REPL.jl, usually looking something like this:

  1. julia> example() = stacktrace()
  2. example (generic function with 1 method)
  3. julia> example()
  4. 7-element Array{Base.StackTraces.StackFrame,1}:
  5. example() at REPL[1]:1
  6. top-level scope
  7. eval at boot.jl:317 [inlined]
  8. eval(::Module, ::Expr) at REPL.jl:5
  9. eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
  10. macro expansion at REPL.jl:116 [inlined]
  11. (::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92

Extracting useful information

Each StackTraces.StackFrame contains the function name, file name, line number, lambda info, a flag indicating whether the frame has been inlined, a flag indicating whether it is a C function (by default C functions do not appear in the stack trace), and an integer representation of the pointer returned by backtrace:

  1. julia> frame = stacktrace()[3]
  2. eval(::Module, ::Expr) at REPL.jl:5
  3. julia> frame.func
  4. :eval
  5. julia> frame.file
  6. Symbol("~/julia/usr/share/julia/stdlib/v0.7/REPL/src/REPL.jl")
  7. julia> frame.line
  8. 5
  9. julia> frame.linfo
  10. MethodInstance for eval(::Module, ::Expr)
  11. julia> frame.inlined
  12. false
  13. julia> frame.from_c
  14. false
  15. julia> frame.pointer
  16. 0x00007f92d6293171

This makes stack trace information available programmatically for logging, error handling, and more.

Error handling

While having easy access to information about the current state of the callstack can be helpful in many places, the most obvious application is in error handling and debugging.

  1. julia> @noinline bad_function() = undeclared_variable
  2. bad_function (generic function with 1 method)
  3. julia> @noinline example() = try
  4. bad_function()
  5. catch
  6. stacktrace()
  7. end
  8. example (generic function with 1 method)
  9. julia> example()
  10. 7-element Array{Base.StackTraces.StackFrame,1}:
  11. example() at REPL[2]:4
  12. top-level scope
  13. eval at boot.jl:317 [inlined]
  14. [...]

You may notice that in the example above the first stack frame points at line 4, where stacktrace is called, rather than line 2, where bad_function is called, and bad_function‘s frame is missing entirely. This is understandable, given that stacktrace is called from the context of the catch. While in this example it’s fairly easy to find the actual source of the error, in complex cases tracking down the source of the error becomes nontrivial.

This can be remedied by passing the result of catch_backtrace to stacktrace. Instead of returning callstack information for the current context, catch_backtrace returns stack information for the context of the most recent exception:

  1. julia> @noinline bad_function() = undeclared_variable
  2. bad_function (generic function with 1 method)
  3. julia> @noinline example() = try
  4. bad_function()
  5. catch
  6. stacktrace(catch_backtrace())
  7. end
  8. example (generic function with 1 method)
  9. julia> example()
  10. 8-element Array{Base.StackTraces.StackFrame,1}:
  11. bad_function() at REPL[1]:1
  12. example() at REPL[2]:2
  13. [...]

Notice that the stack trace now indicates the appropriate line number and the missing frame.

  1. julia> @noinline child() = error("Whoops!")
  2. child (generic function with 1 method)
  3. julia> @noinline parent() = child()
  4. parent (generic function with 1 method)
  5. julia> @noinline function grandparent()
  6. try
  7. parent()
  8. catch err
  9. println("ERROR: ", err.msg)
  10. stacktrace(catch_backtrace())
  11. end
  12. end
  13. grandparent (generic function with 1 method)
  14. julia> grandparent()
  15. ERROR: Whoops!
  16. 10-element Array{Base.StackTraces.StackFrame,1}:
  17. error at error.jl:33 [inlined]
  18. child() at REPL[1]:1
  19. parent() at REPL[2]:1
  20. grandparent() at REPL[3]:3
  21. [...]

Exception stacks and catch_stack

Julia 1.1

Exception stacks requires at least Julia 1.1.

While handling an exception further exceptions may be thrown. It can be useful to inspect all these exceptions to identify the root cause of a problem. The julia runtime supports this by pushing each exception onto an internal exception stack as it occurs. When the code exits a catch normally, any exceptions which were pushed onto the stack in the associated try are considered to be successfully handled and are removed from the stack.

The stack of current exceptions can be accessed using the experimental Base.catch_stack function. For example,

  1. julia> try
  2. error("(A) The root cause")
  3. catch
  4. try
  5. error("(B) An exception while handling the exception")
  6. catch
  7. for (exc, bt) in Base.catch_stack()
  8. showerror(stdout, exc, bt)
  9. println(stdout)
  10. end
  11. end
  12. end
  13. (A) The root cause
  14. Stacktrace:
  15. [1] error(::String) at error.jl:33
  16. [2] top-level scope at REPL[7]:2
  17. [3] eval(::Module, ::Any) at boot.jl:319
  18. [4] eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
  19. [5] macro expansion at REPL.jl:117 [inlined]
  20. [6] (::getfield(REPL, Symbol("##26#27")){REPL.REPLBackend})() at task.jl:259
  21. (B) An exception while handling the exception
  22. Stacktrace:
  23. [1] error(::String) at error.jl:33
  24. [2] top-level scope at REPL[7]:5
  25. [3] eval(::Module, ::Any) at boot.jl:319
  26. [4] eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
  27. [5] macro expansion at REPL.jl:117 [inlined]
  28. [6] (::getfield(REPL, Symbol("##26#27")){REPL.REPLBackend})() at task.jl:259

In this example the root cause exception (A) is first on the stack, with a further exception (B) following it. After exiting both catch blocks normally (i.e., without throwing a further exception) all exceptions are removed from the stack and are no longer accessible.

The exception stack is stored on the Task where the exceptions occurred. When a task fails with uncaught exceptions, catch_stack(task) may be used to inspect the exception stack for that task.

Comparison with) backtrace

A call to backtrace returns a vector of Union{Ptr{Nothing}, Base.InterpreterIP}, which may then be passed into stacktrace for translation:

  1. julia> trace = backtrace()
  2. 18-element Array{Union{Ptr{Nothing}, Base.InterpreterIP},1}:
  3. Ptr{Nothing} @0x00007fd8734c6209
  4. Ptr{Nothing} @0x00007fd87362b342
  5. Ptr{Nothing} @0x00007fd87362c136
  6. Ptr{Nothing} @0x00007fd87362c986
  7. Ptr{Nothing} @0x00007fd87362d089
  8. Base.InterpreterIP(CodeInfo(:(begin
  9. Core.SSAValue(0) = backtrace()
  10. trace = Core.SSAValue(0)
  11. return Core.SSAValue(0)
  12. end)), 0x0000000000000000)
  13. Ptr{Nothing} @0x00007fd87362e4cf
  14. [...]
  15. julia> stacktrace(trace)
  16. 6-element Array{Base.StackTraces.StackFrame,1}:
  17. top-level scope
  18. eval at boot.jl:317 [inlined]
  19. eval(::Module, ::Expr) at REPL.jl:5
  20. eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
  21. macro expansion at REPL.jl:116 [inlined]
  22. (::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92

Notice that the vector returned by backtrace had 18 elements, while the vector returned by stacktrace only has 6. This is because, by default, stacktrace removes any lower-level C functions from the stack. If you want to include stack frames from C calls, you can do it like this:

  1. julia> stacktrace(trace, true)
  2. 21-element Array{Base.StackTraces.StackFrame,1}:
  3. jl_apply_generic at gf.c:2167
  4. do_call at interpreter.c:324
  5. eval_value at interpreter.c:416
  6. eval_body at interpreter.c:559
  7. jl_interpret_toplevel_thunk_callback at interpreter.c:798
  8. top-level scope
  9. jl_interpret_toplevel_thunk at interpreter.c:807
  10. jl_toplevel_eval_flex at toplevel.c:856
  11. jl_toplevel_eval_in at builtins.c:624
  12. eval at boot.jl:317 [inlined]
  13. eval(::Module, ::Expr) at REPL.jl:5
  14. jl_apply_generic at gf.c:2167
  15. eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
  16. jl_apply_generic at gf.c:2167
  17. macro expansion at REPL.jl:116 [inlined]
  18. (::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92
  19. jl_fptr_trampoline at gf.c:1838
  20. jl_apply_generic at gf.c:2167
  21. jl_apply at julia.h:1540 [inlined]
  22. start_task at task.c:268
  23. ip:0xffffffffffffffff

Individual pointers returned by backtrace can be translated into StackTraces.StackFrame s by passing them into StackTraces.lookup:

  1. julia> pointer = backtrace()[1];
  2. julia> frame = StackTraces.lookup(pointer)
  3. 1-element Array{Base.StackTraces.StackFrame,1}:
  4. jl_apply_generic at gf.c:2167
  5. julia> println("The top frame is from $(frame[1].func)!")
  6. The top frame is from jl_apply_generic!