# Weird segmentation fault when ccalling a function

**URL:** <https://discourse.julialang.org/t/weird-segmentation-fault-when-ccalling-a-function/4959>\
**Category:** General Usage\
**Created:** [July 19, 2017, 10:28pm UTC](https://discourse.julialang.org/t/weird-segmentation-fault-when-ccalling-a-function/4959 "2017-07-19T22:28:11Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![giordano](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/giordano/32/2166_2.png) [@giordano](https://discourse.julialang.org/u/giordano)\
**Post date:** [July 19, 2017, 10:28pm UTC](https://discourse.julialang.org/t/weird-segmentation-fault-when-ccalling-a-function/4959/1 "2017-07-19T22:28:11Z")

</div>

An [issue](https://github.com/giordano/Cuba.jl/issues/12) reporting a segmentation fault has been recently filed for `Cuba.jl`. The minimal way to reproduce the crash is the following:

```julia
using Cuba

integrand = (x,f) -> f[] = cos(x[])

integral = Vector{Cdouble}(1)
error = Vector{Cdouble}(1)
prob = Vector{Cdouble}(1)
neval = Ref{Int64}(0)
fail = Ref{Cint}(0)
nregions = Ref{Cint}(0)

ccall((:llVegas, Cuba.libcuba), Cdouble,
      (Cint, # ndim
       Cint, # ncomp
       Ptr{Void}, # integrand
       Any, # userdata
       Int64, # nvec
       Cdouble, # reltol
       Cdouble, # abstol
       Cint, # flags
       Cint, # seed
       Int64, # minevals
       Int64, # maxevals
       Int64, # nstart
       Int64, # nincrease
       Int64, # nbatch
       Cint, # gridno
       Ptr{Cchar}, # statefile
       Ptr{Void}, # spin
       Ptr{Int64}, # neval
       Ptr{Cint}, # fail
       Ptr{Cdouble}, # integral
       Ptr{Cdouble}, # error
       Ptr{Cdouble}),# prob
      Cint(1), Cint(1), Cuba.integrand_ptr(integrand), integrand, Int64(1),
      1e-4, 1e-12, Cint(0), Cint(0), Int64(0),
      Int64(255), # ← this is maxevals
      Int64(1000), Int64(500), Int64(1000), Cint(0), "", C_NULL,
      neval, fail, integral, error, prob)

```

If `maxevals <= 255`, the `ccall` would case a segault, otherwise it works. What makes this issue even weirder is that this isn’t reproducible on all platform, but only on platforms where these tests fail:

- [AppVeyor](https://ci.appveyor.com/project/giordano/cuba-jl) (fails on Win64 + Julia 0.5 and Julia 0.6; Win32 + Julia 0.7)
- [Travis](https://travis-ci.org/giordano/Cuba.jl) (fails on GNU/Linux + Julia 0.5 and 0.6; macOS + Julia 0.5 and 0.7)

Any clue about what may cause the segmentation fault?

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [July 19, 2017, 11:10pm UTC](https://discourse.julialang.org/t/weird-segmentation-fault-when-ccalling-a-function/4959/2 "2017-07-19T23:10:22Z")

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This seems to be an error in the library. It’s reading uninitialized memory at

```julia
24 t->rng.sobol.prev[dim] ^= t->rng.sobol.v[dim][zerobit];

```

where `dim = 0`, `zerobit = 9` (`seq = 511`)  
The array seems to be initalized based on `maxeval` and only `9` elements are initialized (so `[9]` is not)

```julia
(rr) p t->rng.sobol.v[0]  
$26 = {256, 128, 64, 32, 16, 8, 4, 2, 1, 140722933424048, -1913574705073793792, 94744365070272, 0, 
  1, 0, 140722933424048, 140075351664505, 0, 140075353160889, 0, 140722933426688, 0, 
  140722933423344, 1, 140722933423376, 140722933422080, 1, 140722933423528, 140722933423504, 
  140722933423528}

```

I don’t know enough about the library to investigate further.

---

<div class="post-metadata">

**Author:** ![giordano](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/giordano/32/2166_2.png) [@giordano](https://discourse.julialang.org/u/giordano)\
**Post date:** [July 19, 2017, 11:21pm UTC](https://discourse.julialang.org/t/weird-segmentation-fault-when-ccalling-a-function/4959/3 "2017-07-19T23:21:46Z")

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Thanks for looking into this! Actually, I excluded an error in the Cuba library because I can’t reproduce the segmentation fault in a C program calling the library (for example I set `MAXEVAL` to 100 in [this test file](https://github.com/giordano/Cuba.jl/blob/9d0354a19a3968d8f02c72d1291808cce8798908/test/benchmark.c)) 😕

If I’ll manage to produce a reproducible recipe in C, I’ll report the issue to the author. Thanks!

> [@yuyichao](#):
>
> I don’t know enough about the library to investigate further.

Fair enough 😉