Stack Traces

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

    Calling returns a vector of StackTraces.StackFrame s. For ease of use, the alias 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)
    4. julia> example()
    5. 7-element Array{Base.StackTraces.StackFrame,1}:
    6.  example() at REPL[1]:1
    7.  top-level scope
    8.  eval at boot.jl:317 [inlined]
    9. [...]
    11. julia> @noinline child() = stacktrace()
    12. child (generic function with 1 method)
    14. julia> @noinline parent() = child()
    15. parent (generic function with 1 method)
    17. julia> grandparent() = parent()
    18. grandparent (generic function with 1 method)
    20. julia> grandparent()
    21. 9-element Array{Base.StackTraces.StackFrame,1}:
    22.  child() at REPL[3]:1
    23.  parent() at REPL[4]:1
    24.  grandparent() at REPL[5]:1
    25. [...]

    Note that when calling stacktrace() you’ll typically see a frame with eval at boot.jl. When calling 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)
    4. julia> example()
    5. 7-element Array{Base.StackTraces.StackFrame,1}:
    6.  example() at REPL[1]:1
    7.  top-level scope
    8.  eval at boot.jl:317 [inlined]
    9.  eval(::Module, ::Expr) at REPL.jl:5
    10.  eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
    11.  macro expansion at REPL.jl:116 [inlined]
    12.  (::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92

    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 :

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

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

    1. julia> @noinline bad_function() = undeclared_variable
    2. bad_function (generic function with 1 method)
    4. julia> @noinline example() = try
    5.            bad_function()
    6.        catch
    7.            stacktrace()
    8.        end
    9. example (generic function with 1 method)
    11. julia> example()
    12. 7-element Array{Base.StackTraces.StackFrame,1}:
    13.  example() at REPL[2]:4
    14.  top-level scope
    15.  eval at boot.jl:317 [inlined]
    16. [...]

    You may notice that in the example above the first stack frame points 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 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 . 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)
    4. julia> @noinline example() = try
    5.            bad_function()
    6.        catch
    7.            stacktrace(catch_backtrace())
    8.        end
    9. example (generic function with 1 method)
    11. julia> example()
    12. 8-element Array{Base.StackTraces.StackFrame,1}:
    13.  bad_function() at REPL[1]:1
    14.  example() at REPL[2]:2
    15. [...]

    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)
    4. julia> @noinline parent() = child()
    5. parent (generic function with 1 method)
    7. julia> @noinline function grandparent()
    8.            try
    9.                parent()
    10.            catch err
    11.                println("ERROR: ", err.msg)
    12.                stacktrace(catch_backtrace())
    13.            end
    14.        end
    15. grandparent (generic function with 1 method)
    17. julia> grandparent()
    18. ERROR: Whoops!
    19. 10-element Array{Base.StackTraces.StackFrame,1}:
    20.  error at error.jl:33 [inlined]
    21.  parent() at REPL[2]:1
    22.  grandparent() at REPL[3]:3
    23. [...]

    Julia 1.1

    Exception stacks requires at least Julia 1.1.

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

    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.

    A call to backtrace returns a vector of Union{Ptr{Nothing}, Base.InterpreterIP}, which may then be passed into 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. [...]
    16. julia> stacktrace(trace)
    17. 6-element Array{Base.StackTraces.StackFrame,1}:
    18.  top-level scope
    19.  eval at boot.jl:317 [inlined]
    20.  eval(::Module, ::Expr) at REPL.jl:5
    21.  eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
    22.  macro expansion at REPL.jl:116 [inlined]
    23.  (::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 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
    1. julia> pointer = backtrace()[1];
    3. julia> frame = StackTraces.lookup(pointer)
    4. 1-element Array{Base.StackTraces.StackFrame,1}:
    5.  jl_apply_generic at gf.c:2167
    7. julia> println("The top frame is from $(frame[1].func)!")