Stack Traces

# 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()`:

    julia> stacktrace()
    4-element Array{StackFrame,1}:
     eval(::Module, ::Any) at boot.jl:236
     eval_user_input(::Any, ::Base.REPL.REPLBackend) at REPL.jl:66
     macro expansion at REPL.jl:97 [inlined]
     (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at event.jl:73

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

    julia> example() = stacktrace()
    example (generic function with 1 method)

    julia> example()
    5-element Array{StackFrame,1}:
     example() at REPL[1]:1
     eval(::Module, ::Any) at boot.jl:236
    [...]

    julia> @noinline child() = stacktrace()
    child (generic function with 1 method)

    julia> @noinline parent() = child()
    parent (generic function with 1 method)

    julia> grandparent() = parent()
    grandparent (generic function with 1 method)

    julia> grandparent()
    7-element Array{StackFrame,1}:
     child() at REPL[3]:1
     parent() at REPL[4]:1
     grandparent() at REPL[5]:1
    [...]

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:

    julia> example() = stacktrace()
    example (generic function with 1 method)

    julia> example()
    5-element Array{StackFrame,1}:
     example() at REPL[1]:1
     eval(::Module, ::Any) at boot.jl:236
     eval_user_input(::Any, ::Base.REPL.REPLBackend) at REPL.jl:66
     macro expansion at REPL.jl:97 [inlined]
     (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at event.jl:73

## Extracting useful information

Each `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()`:

    julia> top_frame = stacktrace()[1]
    eval(::Module, ::Any) at boot.jl:236

    julia> top_frame.func
    :eval

    julia> top_frame.file
    Symbol("./boot.jl")

    julia> top_frame.line
    236

    julia> top_frame.linfo
    Nullable{Core.MethodInstance}(MethodInstance for eval(::Module, ::Any))

    julia> top_frame.inlined
    false

    julia> top_frame.from_c
    false

    julia> top_frame.pointer
    0x00007f390d152a59

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.

    julia> @noinline bad_function() = undeclared_variable
    bad_function (generic function with 1 method)

    julia> @noinline example() = try
               bad_function()
           catch
               stacktrace()
           end
    example (generic function with 1 method)

    julia> example()
    5-element Array{StackFrame,1}:
     example() at REPL[2]:4
     eval(::Module, ::Any) at boot.jl:236
    [...]

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 `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 calling `catch_stacktrace()` instead of `stacktrace()`. Instead of returning callstack information for the current context, `catch_stacktrace()` returns stack information for the context of the most recent exception:

    julia> @noinline bad_function() = undeclared_variable
    bad_function (generic function with 1 method)

    julia> @noinline example() = try
               bad_function()
           catch
               catch_stacktrace()
           end
    example (generic function with 1 method)

    julia> example()
    6-element Array{StackFrame,1}:
     bad_function() at REPL[1]:1
     example() at REPL[2]:2
    [...]

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

    julia> @noinline child() = error("Whoops!")
    child (generic function with 1 method)

    julia> @noinline parent() = child()
    parent (generic function with 1 method)

    julia> @noinline function grandparent()
               try
                   parent()
               catch err
                   println("ERROR: ", err.msg)
                   catch_stacktrace()
               end
           end
    grandparent (generic function with 1 method)

    julia> grandparent()
    ERROR: Whoops!
    7-element Array{StackFrame,1}:
     child() at REPL[1]:1
     parent() at REPL[2]:1
     grandparent() at REPL[3]:3
    [...]

## Comparison with `backtrace()`

A call to `backtrace()` returns a vector of `Ptr{Void}`, which may then be passed into `stacktrace()` for translation:

    julia> trace = backtrace()
    21-element Array{Ptr{Void},1}:
     Ptr{Void} @0x00007f10049d5b2f
     Ptr{Void} @0x00007f0ffeb4d29c
     Ptr{Void} @0x00007f0ffeb4d2a9
     Ptr{Void} @0x00007f1004993fe7
     Ptr{Void} @0x00007f10049a92be
     Ptr{Void} @0x00007f10049a823a
     Ptr{Void} @0x00007f10049a9fb0
     Ptr{Void} @0x00007f10049aa718
     Ptr{Void} @0x00007f10049c0d5e
     Ptr{Void} @0x00007f10049a3286
     Ptr{Void} @0x00007f0ffe9ba3ba
     Ptr{Void} @0x00007f0ffe9ba3d0
     Ptr{Void} @0x00007f1004993fe7
     Ptr{Void} @0x00007f0ded34583d
     Ptr{Void} @0x00007f0ded345a87
     Ptr{Void} @0x00007f1004993fe7
     Ptr{Void} @0x00007f0ded34308f
     Ptr{Void} @0x00007f0ded343320
     Ptr{Void} @0x00007f1004993fe7
     Ptr{Void} @0x00007f10049aeb67
     Ptr{Void} @0x0000000000000000

    julia> stacktrace(trace)
    5-element Array{StackFrame,1}:
     backtrace() at error.jl:46
     eval(::Module, ::Any) at boot.jl:236
     eval_user_input(::Any, ::Base.REPL.REPLBackend) at REPL.jl:66
     macro expansion at REPL.jl:97 [inlined]
     (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at event.jl:73

Notice that the vector returned by `backtrace()` had 21 pointers, while the vector returned by `stacktrace()` only has 5. 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:

    julia> stacktrace(trace, true)
    27-element Array{StackFrame,1}:
     jl_backtrace_from_here at stackwalk.c:103
     backtrace() at error.jl:46
     backtrace() at sys.so:?
     jl_call_method_internal at julia_internal.h:248 [inlined]
     jl_apply_generic at gf.c:2215
     do_call at interpreter.c:75
     eval at interpreter.c:215
     eval_body at interpreter.c:519
     jl_interpret_toplevel_thunk at interpreter.c:664
     jl_toplevel_eval_flex at toplevel.c:592
     jl_toplevel_eval_in at builtins.c:614
     eval(::Module, ::Any) at boot.jl:236
     eval(::Module, ::Any) at sys.so:?
     jl_call_method_internal at julia_internal.h:248 [inlined]
     jl_apply_generic at gf.c:2215
     eval_user_input(::Any, ::Base.REPL.REPLBackend) at REPL.jl:66
     ip:0x7f1c707f1846
     jl_call_method_internal at julia_internal.h:248 [inlined]
     jl_apply_generic at gf.c:2215
     macro expansion at REPL.jl:97 [inlined]
     (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at event.jl:73
     ip:0x7f1c707ea1ef
     jl_call_method_internal at julia_internal.h:248 [inlined]
     jl_apply_generic at gf.c:2215
     jl_apply at julia.h:1411 [inlined]
     start_task at task.c:261
     ip:0xffffffffffffffff

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

    julia> pointer = backtrace()[1];

    julia> frame = StackTraces.lookup(pointer)
    1-element Array{StackFrame,1}:
     jl_backtrace_from_here at stackwalk.c:103

    julia> println("The top frame is from $(frame[1].func)!")
    The top frame is from jl_backtrace_from_here!