# Erratic segmentation fault - from a package using \`ccall\`

**URL:** <https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049>\
**Category:** General Usage\
**Tags:** ccall\
**Created:** [December 18, 2020, 3:09pm UTC](https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049 "2020-12-18T15:09:25Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![stla](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stla/32/20382_2.png) [@stla](https://discourse.julialang.org/u/stla)\
**Post date:** [December 18, 2020, 3:09pm UTC](https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049/1 "2020-12-18T15:09:25Z")

</div>

Hello,

I started a new project: [Junuran](https://github.com/stla/Junuran.jl), a Julia wrapper of the [UNU.RAN C library](https://statmath.wu.ac.at/software/unuran/index.html).

So far the package only allows to sample from a continuous multivariate distribution when the user gives the density of the distribution up to a normalization constant. Example:

```julia
(@v1.5) pkg> add https://github.com/stla/Junuran.jl.git
julia> using Junuran
julia> function pdf(x) return exp(-sum(x.^2)) end
julia> gen = urgen_vnrou(2, pdf, [0.0;0.0])
julia> ursample(gen, 3)
3-element Array{Array{Float64,1},1}:
 [-0.12824691343670097, -0.5747649978925373]
 [0.10665928715550613, 0.552712255711348]
 [-1.2676651522023954, 0.5847703116666545]

```

Very nice. Now I sample again 3 values. Sometimes this works fine. But more often this fails with a segfault:

```julia
julia> ursample(gen, 3)
signal (11): Erreur de segmentation
in expression starting at none:0
jl_gc_pool_alloc at /buildworker/worker/package_linux64/build/src/gc.c:1155
jl_gc_alloc_ at /buildworker/worker/package_linux64/build/src/julia_internal.h:277 [inlined]
jl_new_uninitialized_datatype at /buildworker/worker/package_linux64/build/src/datatype.c:90
inst_datatype_inner at /buildworker/worker/package_linux64/build/src/jltypes.c:1265
jl_apply_tuple_type_v_ at /buildworker/worker/package_linux64/build/src/jltypes.c:1385 [inlined]
jl_apply_tuple_type_v at /buildworker/worker/package_linux64/build/src/jltypes.c:1395
arg_type_tuple at /buildworker/worker/package_linux64/build/src/gf.c:1851
jl_lookup_generic_ at /buildworker/worker/package_linux64/build/src/gf.c:2361 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2394
print_response at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:252
print_response at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:223
do_respond at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:822
jfptr_do_respond_30126.clone_1 at /opt/julias/julia-1.5.3/lib/julia/sys.so (unknown line)
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2214 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2398
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1690 [inlined]
do_apply at /buildworker/worker/package_linux64/build/src/builtins.c:655
jl_f__apply_latest at /buildworker/worker/package_linux64/build/src/builtins.c:705
#invokelatest#1 at ./essentials.jl:710 [inlined]
invokelatest at ./essentials.jl:709 [inlined]
run_interface at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/LineEdit.jl:2355
jfptr_run_interface_28976.clone_1 at /opt/julias/julia-1.5.3/lib/julia/sys.so (unknown line)
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2214 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2398
run_frontend at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:1144
#38 at ./task.jl:356
jfptr_YY.38_31644.clone_1 at /opt/julias/julia-1.5.3/lib/julia/sys.so (unknown line)
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2214 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2398
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1690 [inlined]
start_task at /buildworker/worker/package_linux64/build/src/task.c:705
unknown function (ip: (nil))
Allocations: 7551719 (Pool: 7548890; Big: 2829); GC: 8
Erreur de segmentation (core dumped)

```

Could someone take a look at my code to see if I’m doing something wrong? Thanks.

---

<div class="post-metadata">

**Author:** ![stla](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stla/32/20382_2.png) [@stla](https://discourse.julialang.org/u/stla)\
**Post date:** [December 18, 2020, 4:11pm UTC](https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049/2 "2020-12-18T16:11:16Z")

</div>

Remark: that seems to work fine when I save a script in a file and I run it with `include`.

---

<div class="post-metadata">

**Author:** ![thofma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/thofma/32/1691_2.png) [@thofma](https://discourse.julialang.org/u/thofma)\
**Post date:** [December 18, 2020, 5:19pm UTC](https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049/3 "2020-12-18T17:19:08Z")

</div>

Can you try putting the blocks  
[https://github.com/stla/Junuran.jl/blob/f58184f43e3b937c92ab2fca0503ae6b1b775b0e/src/Junuran.jl#L175-L180](https://github.com/stla/Junuran.jl/blob/f58184f43e3b937c92ab2fca0503ae6b1b775b0e/src/Junuran.jl#L175-L180)  
and  
[https://github.com/stla/Junuran.jl/blob/f58184f43e3b937c92ab2fca0503ae6b1b775b0e/src/Junuran.jl#L183-L191](https://github.com/stla/Junuran.jl/blob/f58184f43e3b937c92ab2fca0503ae6b1b775b0e/src/Junuran.jl#L183-L191)  
inside a

```julia
GC.@preserve urgen begin
         blabla
       end

```

block?

---

<div class="post-metadata">

**Author:** ![stla](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stla/32/20382_2.png) [@stla](https://discourse.julialang.org/u/stla)\
**Post date:** [December 18, 2020, 5:34pm UTC](https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049/4 "2020-12-18T17:34:26Z")

</div>

Hello @thofma

I’ve just tried but the segfault still occurs.

---

<div class="post-metadata">

**Author:** ![thofma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/thofma/32/1691_2.png) [@thofma](https://discourse.julialang.org/u/thofma)\
**Post date:** [December 18, 2020, 5:47pm UTC](https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049/5 "2020-12-18T17:47:05Z")

</div>

Sorry, I can’t spot what else could be wrong (but I think the `GC.@preserve` are necessary anyway).

---

<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [December 18, 2020, 7:41pm UTC](https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049/6 "2020-12-18T19:41:34Z")

</div>

Might be necessary to preserve the `out` vector as well.

---

<div class="post-metadata">

**Author:** ![stla](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stla/32/20382_2.png) [@stla](https://discourse.julialang.org/u/stla)\
**Post date:** [December 18, 2020, 7:59pm UTC](https://discourse.julialang.org/t/erratic-segmentation-fault-from-a-package-using-ccall/52049/7 "2020-12-18T19:59:08Z")

</div>

If I save this script:

```julia
using Junuran
function pdf(x)    
    return(exp(-sum(x .^ 2)))
end
center = [0.0; 0.0] 
gen = urgen_vnrou(2, pdf, center)

```

and I run `include("thescript.jl")`, then I can run multiple times `ursample(gen, 3)` without encounterig any issue.