segfault when printing to screen from within multithreaded loop

[samuelpowell]

Nightly binary on OS X 10.11.5

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|__/                   |  x86_64-apple-darwin13.4.0

julia> Base.Threads.nthreads()
4

julia> Base.Threads.@threads for n = 1:4
           println()
       end






signal (11): Segmentation fault: 11
while loading no file, in expression starting on line 0
uv__write at /Users/osx/buildbot/slave/package_osx10_9-x64/build/deps/srccache/libuv-ecbd6eddfac4940ab8db57c73166a7378563ebd3/src/unix/stream.c:890
uv_write2 at /Users/osx/buildbot/slave/package_osx10_9-x64/build/deps/srccache/libuv-ecbd6eddfac4940ab8db57c73166a7378563ebd3/src/unix/stream.c:1450
uv_write at /Users/osx/buildbot/slave/package_osx10_9-x64/build/deps/srccache/libuv-ecbd6eddfac4940ab8db57c73166a7378563ebd3/src/unix/stream.c:1475
jl_uv_write at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/jl_uv.c:411
uv_write at ./stream.jl:799
unsafe_write at ./stream.jl:820
unsafe_write at ./io.jl:141
write at ./io.jl:180
unknown function (ip: 0x102c09971)
jl_call_method_internal at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia_internal.h:175
print; at ./char.jl:0
unknown function (ip: 0x314448adf)
jl_call_method_internal at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia_internal.h:175
jl_apply at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia.h:1394
jl_threading_run at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/threading.c:691
macro expansion; at ./threadingconstructs.jl:46
unknown function (ip: 0x31444888f)
jl_call_method_internal at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia_internal.h:175
jl_toplevel_eval_in_warn at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/builtins.c:573
eval at ./boot.jl:234
unknown function (ip: 0x102bc2def)
jl_call_method_internal at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia_internal.h:175
eval_user_input at ./REPL.jl:62
unknown function (ip: 0x314446d16)
jl_call_method_internal at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia_internal.h:175
macro expansion; at ./REPL.jl:92
unknown function (ip: 0x31443d20f)
jl_call_method_internal at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia_internal.h:175
jl_apply at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia.h:1394
Allocations: 1683687 (Pool: 1682825; Big: 862); GC: 0
Segmentation fault: 11

[andreasnoack]

If you just want to have something printed from within the threading loop for, say, debugging purposes then you can bypass Libuv with

Threads.@threads for i = 1:10
    ccall(:jl_,Void,(Any,), "this is thread number $(Threads.threadid())")
end

[samuelpowell]

@andreasnoack exactly so, thank you.

[ViralBShah]

@ranjanan Worth noting this in the threading documentation. This is a natural thing everyone will want to try, and our printing and i/o is not thread safe.

[yuyichao]

Dup of #14494

[sbromberger]

Just to add to this: sometimes there's unintended I/O; for example, with depwarn. This code will cause segfaults in threads on Julia 0.6 even though there's no explicit output:

julia> Threads.@threads for i = 1:10
       z = abs([1:10;])
       end
WARNING: WARNING: WARNING: WARNING: WARNING:
signal (11): Segmentation fault: 11
while loading no file, in expression starting on line 0
uv__write at /Users/bromberger1/dev/julia/julia6/deps/srccache/libuv-52d72a52cc7ccd570929990f010ed16e2ec604c8/src/unix/stream.c:890
uv_write2 at /Users/bromberger1/dev/julia/julia6/deps/srccache/libuv-52d72a52cc7ccd570929990f010ed16e2ec604c8/src/unix/stream.c:1450
uv_write at /Users/bromberger1/dev/julia/julia6/deps/srccache/libuv-52d72a52cc7ccd570929990f010ed16e2ec604c8/src/unix/stream.c:1475
jl_uv_write at /Users/bromberger1/dev/julia/julia6/src/jl_uv.c:414
uv_write at ./stream.jl:798
unsafe_write at ./stream.jl:832
print at ./strings/io.jl:122 [inlined]
#with_output_color#509 at ./util.jl:404
jl_apply at /Users/bromberger1/dev/julia/julia6/src/./julia.h:1424 [inlined]
jl_f__apply at /Users/bromberger1/dev/julia/julia6/src/builtins.c:426
#with_output_color at ./<missing>:0
jl_apply at /Users/bromberger1/dev/julia/julia6/src/./julia.h:1424 [inlined]
jl_f__apply at /Users/bromberger1/dev/julia/julia6/src/builtins.c:426
abs{T <: Number}(x::AbstractArray{T}) is deprecated, use abs.(x) instead.#print_with_color#510 at ./util.jl:417
jl_apply at /Users/bromberger1/dev/julia/julia6/src/./julia.h:1424 [inlined]
jl_f__apply at /Users/bromberger1/dev/julia/julia6/src/builtins.c:426

(it also had the effect of pegging CPUs at 100% until ^C was issued.)