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:
6-element Array{Base.StackTraces.StackFrame,1}:
top-level scope
eval at boot.jl:317 [inlined]
eval(::Module, ::Expr) at REPL.jl:5
eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
macro expansion at REPL.jl:116 [inlined]
(::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92Calling 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.)
julia> example() = stacktrace()
example (generic function with 1 method)
julia> example()
7-element Array{Base.StackTraces.StackFrame,1}:
example() at REPL[1]:1
top-level scope
eval at boot.jl:317 [inlined]
[...]
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()
9-element Array{Base.StackTraces.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()
7-element Array{Base.StackTraces.StackFrame,1}:
example() at REPL[1]:1
top-level scope
eval at boot.jl:317 [inlined]
eval(::Module, ::Expr) at REPL.jl:5
eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
macro expansion at REPL.jl:116 [inlined]
(::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92Extracting 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:
julia> frame = stacktrace()[3]
eval(::Module, ::Expr) at REPL.jl:5
julia> frame.func
:eval
julia> frame.file
Symbol("~/julia/usr/share/julia/stdlib/v0.7/REPL/src/REPL.jl")
julia> frame.line
5
julia> top_frame.linfo
MethodInstance for eval(::Module, ::Expr)
julia> top_frame.inlined
false
julia> top_frame.from_c
falsejulia> top_frame.pointer
0x00007f92d6293171This 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()
7-element Array{Base.StackTraces.StackFrame,1}:
example() at REPL[2]:4
top-level scope
eval at boot.jl:317 [inlined]
[...]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 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:
julia> @noinline bad_function() = undeclared_variable
bad_function (generic function with 1 method)
julia> @noinline example() = try
bad_function()
catch
stacktrace(catch_backtrace())
end
example (generic function with 1 method)
julia> example()
8-element Array{Base.StackTraces.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)
stacktrace(catch_backtrace())
end
end
grandparent (generic function with 1 method)
julia> grandparent()
ERROR: Whoops!
10-element Array{Base.StackTraces.StackFrame,1}:
error at error.jl:33 [inlined]
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 Union{Ptr{Nothing}, Base.InterpreterIP}, which may then be passed into stacktrace for translation:
julia> trace = backtrace()
18-element Array{Union{Ptr{Nothing}, Base.InterpreterIP},1}:
Ptr{Nothing} @0x00007fd8734c6209
Ptr{Nothing} @0x00007fd87362b342
Ptr{Nothing} @0x00007fd87362c136
Ptr{Nothing} @0x00007fd87362c986
Ptr{Nothing} @0x00007fd87362d089
Base.InterpreterIP(CodeInfo(:(begin
Core.SSAValue(0) = backtrace()
trace = Core.SSAValue(0)
return Core.SSAValue(0)
end)), 0x0000000000000000)
Ptr{Nothing} @0x00007fd87362e4cf
[...]
julia> stacktrace(trace)
6-element Array{Base.StackTraces.StackFrame,1}:
top-level scope
eval at boot.jl:317 [inlined]
eval(::Module, ::Expr) at REPL.jl:5
eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
macro expansion at REPL.jl:116 [inlined]
(::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92Notice 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:
julia> stacktrace(trace, true)
21-element Array{Base.StackTraces.StackFrame,1}:
jl_apply_generic at gf.c:2167
do_call at interpreter.c:324
eval_value at interpreter.c:416
eval_body at interpreter.c:559
jl_interpret_toplevel_thunk_callback at interpreter.c:798
top-level scope
jl_interpret_toplevel_thunk at interpreter.c:807
jl_toplevel_eval_flex at toplevel.c:856
jl_toplevel_eval_in at builtins.c:624
eval at boot.jl:317 [inlined]
eval(::Module, ::Expr) at REPL.jl:5
jl_apply_generic at gf.c:2167
eval_user_input(::Any, ::REPL.REPLBackend) at REPL.jl:85
jl_apply_generic at gf.c:2167
macro expansion at REPL.jl:116 [inlined]
(::getfield(REPL, Symbol("##28#29")){REPL.REPLBackend})() at event.jl:92
jl_fptr_trampoline at gf.c:1838
jl_apply_generic at gf.c:2167
jl_apply at julia.h:1540 [inlined]
start_task at task.c:268
ip:0xffffffffffffffffIndividual pointers returned by backtrace can be translated into StackTraces.StackFrame s by passing them into StackTraces.lookup:
julia> pointer = backtrace()[1];
julia> frame = StackTraces.lookup(pointer)
1-element Array{Base.StackTraces.StackFrame,1}:
jl_apply_generic at gf.c:2167
julia> println("The top frame is from $(frame[1].func)!")
The top frame is from jl_apply_generic!