### Author:  [@wenting-zhao](https://discourse.julialang.org/u/wenting-zhao)
#### Post date: [July 12, 2017, 3:33pm UTC](https://discourse.julialang.org/t/segfault-in-ccall/4809/1 "2017-07-12T15:33:12Z")
Here’s a minimal example:
```julia
julia> mutable struct FILE
end
julia> outfile = ccall((:open, "libc"), Ptr{FILE}, (Cstring, Cstring), "test", "w+")
Ptr{FILE} @0xffffffffffffffff
julia> ccall((:fprintf, "libc"), Cuint, (Ptr{FILE}, Cstring), outfile, "write something")
signal (11): Segmentation fault: 11
while loading no file, in expression starting on line 0
flockfile at /usr/lib/system/libsystem_c.dylib (unknown line)
vfprintf_l at /usr/lib/system/libsystem_c.dylib (unknown line)
fprintf at /usr/lib/system/libsystem_c.dylib (unknown line)
anonymous at ./ (unknown line)
jl_call_fptr_internal at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia_internal.h:339 [inlined]
jl_call_method_internal at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia_internal.h:358 [inlined]
jl_toplevel_eval_flex at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/toplevel.c:589
jl_toplevel_eval_in at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/builtins.c:496
eval at ./boot.jl:235
jlcall_eval_17976 at /Applications/Julia-0.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
eval_user_input at ./REPL.jl:66
jlcall_eval_user_input_20272 at /Applications/Julia-0.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
macro expansion at ./REPL.jl:97 [inlined]
#1 at ./event.jl:73
unknown function (ip: 0x11c63492f)
jl_apply at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia.h:1424 [inlined]
start_task at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/task.c:267
Allocations: 1110321 (Pool: 1109185; Big: 1136); GC: 0
Segmentation fault: 11
```
It’s Julia 0.6 x86\_64-apple-darwin13.4.0 release and this is run on macOS Sierra.
Does anyone know what might cause this and if there is a workaround for it? Thanks!
---
### Author:  [@nalimilan](https://discourse.julialang.org/u/nalimilan)
#### Post date: [July 12, 2017, 3:36pm UTC](https://discourse.julialang.org/t/segfault-in-ccall/4809/2 "2017-07-12T15:36:57Z")
AFAIK `open` takes a `Cint` for the second argument, not a string. The string version only works with the Julia function of the same name. Indeed, you can see that the pointer returned by the first call is not valid, it’s equal to `-1 % UInt`.
---
### Author:  [@wenting-zhao](https://discourse.julialang.org/u/wenting-zhao)
#### Post date: [July 12, 2017, 4:03pm UTC](https://discourse.julialang.org/t/segfault-in-ccall/4809/3 "2017-07-12T16:03:58Z")
Thanks for you reply. Sorry, that was a typo; I meant `fopen`. So here’s my actual error that I need help with:
The function signature for the C function I’m trying to call is
`int Cudd_DumpDot (DdManager *dd, int n, DdNode **f, char const *const *inames, char const *const *onames, FILE *fp)`
And here’s my code:
```julia
function output_dot(mgr::Ptr{Manager}, f::Ptr{Node}, filename::String)
outfile = ccall((:fopen, "libc"), Ptr{FILE}, (Ptr{UInt8}, Ptr{UInt8}), "test", "w+")
ccall((:Cudd_DumpDot, _LIB_CUDD), Void, (Ptr{Manager}, Cuint, Ptr{Ptr{Node}},
Ptr{Ptr{UInt8}}, Ptr{Ptr{UInt8}}, Ptr{FILE}), mgr, 1, pointer_from_objref(f), C_NULL, C_NULL, outfile)
ccall((:fclose, "libc"), Cuint, (Ptr{FILE},), outfile)
end
```
So when I called the function, I got
```julia
signal (11): Segmentation fault: 11
while loading no file, in expression starting on line 0
flockfile at /usr/lib/system/libsystem_c.dylib (unknown line)
vfprintf_l at /usr/lib/system/libsystem_c.dylib (unknown line)
fprintf at /usr/lib/system/libsystem_c.dylib (unknown line)
Cudd_DumpDot at /Users/zhao/.julia/v0.6/CUDD/deps/cudd-3.0.0/cudd/.libs/libcudd-3.0.0.0.dylib (unknown line)
output_dot at /Users/zhao/.julia/v0.6/CUDD/src/ADD.jl:87
unknown function (ip: 0x119239d33)
do_call at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/interpreter.c:75
eval at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/interpreter.c:242
jl_interpret_toplevel_expr at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/interpreter.c:34
jl_toplevel_eval_flex at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/toplevel.c:577
jl_toplevel_eval_in at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/builtins.c:496
eval at ./boot.jl:235
jlcall_eval_17976 at /Applications/Julia-0.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
eval_user_input at ./REPL.jl:66
jlcall_eval_user_input_20272 at /Applications/Julia-0.6.app/Contents/Resources/julia/lib/julia/sys.dylib (unknown line)
macro expansion at ./REPL.jl:97 [inlined]
#1 at ./event.jl:73
unknown function (ip: 0x11922c92f)
jl_apply at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/./julia.h:1424 [inlined]
start_task at /Users/osx/buildbot/slave/package_osx10_9-x64/build/src/task.c:267
Allocations: 1025905 (Pool: 1024796; Big: 1109); GC: 0
Segmentation fault: 11
```
So I was wondering if there is any place that I didn’t write properly?
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [July 12, 2017, 4:24pm UTC](https://discourse.julialang.org/t/segfault-in-ccall/4809/4 "2017-07-12T16:24:30Z")
> [@wenting-zhao](#):
>
> outfile = ccall((:fopen, “libc”), Ptr{FILE}, (Ptr{UInt8}, Ptr{UInt8}), “test”, “w+”)
First, as Keno mentioned in the github issue, have you checked the return value for error?
Also in general you should use `Cstring` instead of `Ptr{UInt8}` for, well, C strings and you don’t need the `"libc"`. These shouldn’t be the issue here though.
> [@wenting-zhao](#):
>
> ccall((:Cudd\_DumpDot, \_LIB\_CUDD), Void, (Ptr{Manager}, Cuint, Ptr{Ptr{Node}},
> Ptr{Ptr{UInt8}}, Ptr{Ptr{UInt8}}, Ptr{FILE}), mgr, 1, pointer\_from\_objref(f), C\_NULL, C\_NULL, outfile)
In general, unless you are always manually managing the C objects, you must not pass bare pointers to `ccall`. Instead you should define the appropriate `cconvert` and `unsafe_convert` (you can find how to use them in the doc).
Also, you should never use `pointer_from_objref` as ccall arguments especially not on an isbit object, semantically the return value is invalid as soon as the function returns. Use `Ref{Ptr{...}}` as argument type if you really need a pointer to the pointer.
It’s impossible to tell if other parameters are passed correctly (other than the possible issues mentioned above) but the handling of `FILE*` shouldn’t have any problems. The following works for me.
```julia
julia> ccall(:fopen, Ptr{Void}, (Cstring, Cstring), "test", "w+")
Ptr{Void} @0x0000000002407460
julia> fptr = ans
Ptr{Void} @0x0000000002407460
julia> ccall(:fprintf, Cint, (Ptr{Void}, Cstring), fptr, "write something")
15
julia> ccall(:fclose, Cint, (Ptr{Void},), fptr)
0
shell> cat test
write something
```
---
### Author:  [@wenting-zhao](https://discourse.julialang.org/u/wenting-zhao)
#### Post date: [July 12, 2017, 5:02pm UTC](https://discourse.julialang.org/t/segfault-in-ccall/4809/5 "2017-07-12T17:02:38Z")
If I use `Ref{Ptr{...}}` as the argument type, then what do I pass as an argument for `f`?
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [July 12, 2017, 5:26pm UTC](https://discourse.julialang.org/t/segfault-in-ccall/4809/6 "2017-07-12T17:26:25Z")
`f`