### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [May 11, 2023, 1:30pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/1 "2023-05-11T13:30:58Z")
Ok I realize that `dlopen` may not be a new-to-Julia issue, but FFI is important for us to be able to expand usage of Julia in my company. I have read through the manual section on [using C and Fortran](https://docs.julialang.org/en/v1/manual/calling-c-and-fortran-code/) (which is rather good, actually). I think what I am running into here is likely a filesystem path problem.
Here is my demo c code (OS is WSL Ubuntu):
```plaintext
#include
// gcc my_exp.c -fPIC -shared -lm -o libmy_exp.so
// gcc -c my_exp.c -o my_exp.o
double my_exp(double x) {
return exp(x);
}
```
After compiling it as indicated in the comments of the code, I change to the directory that the shared library is in, and start Julia:
```julia
julia> @ccall "./libmyexp".my_exp(3.0::Float64)::Float64
20.085536923187668
julia> using Libdl
julia> h = dlopen("./libmyexp")
Ptr{Nothing} @0x000000000152a330
julia> h = dlopen("./libmyexp.so")
Ptr{Nothing} @0x000000000152a330
```
Am I missing a path or something? Also, is it the “Julian way” to use `dlopen` and `dlsym` when the `@ccall` macro seems to be how you end up calling it anyhow?
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [May 11, 2023, 1:37pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/2 "2023-05-11T13:37:30Z")
What’s the issue here, exactly? Is it unexpected that `@ccall` already works without using `dlopen` beforehand?
---
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [May 11, 2023, 1:41pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/3 "2023-05-11T13:41:23Z")
Oh, perhaps I should have also put this, niether of the `dlsym` handles seems to work:
```julia
julia> @ccall h.my_exp(3.0::Float64)::Float64
ERROR: TypeError: in ccall, expected Symbol, got a value of type Ptr{Nothing}
Stacktrace:
[1] top-level scope
@ ./REPL[5]:1
```
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [May 11, 2023, 1:43pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/4 "2023-05-11T13:43:13Z")
Ah, I see - the macro only directly supports strings and symbols as literals, IIRC. To use the handle, you can use the `ccall` function.
---
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [May 11, 2023, 1:45pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/5 "2023-05-11T13:45:24Z")
@Sukera, ok thanks, that can work - just calling it like that.
I guess I had been used to using `dlopen` and `dlsym` in other FFI cases in Python …
I also tried to make the handle with an absolute path, and that didn’t work either:
```julia
julia> h = dlopen("/home/me/Projects/Julia/libmyexp.so")
Ptr{Nothing} @0x000000000152a330
```
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [May 11, 2023, 1:46pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/6 "2023-05-11T13:46:42Z")
`dlopen` still gives you a handle - that’s just represented by a pointer. Is there another error?
---
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [May 11, 2023, 1:47pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/7 "2023-05-11T13:47:35Z")
```julia
julia> ccall(("my_exp", h), Float64, (Float64,), 3.0)
ERROR: TypeError: in ccall, expected Symbol, got a value of type Ptr{Nothing}
Stacktrace:
[1] top-level scope
@ ./REPL[11]:1
```
(deleted something that I found out wasn’t valid)
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [May 11, 2023, 2:14pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/8 "2023-05-11T14:14:45Z")
Apologies, I got the order mixed up - you need to get the symbol from the dynamic library with `dlsym` first, of course:
```julia
julia> using Libdl
julia> h = dlopen("./libmy_exp.so")
Ptr{Nothing} @0x000055d013c2ce90
julia> hfunc = dlsym(h, :my_exp)
Ptr{Nothing} @0x00007f9b44833109
julia> ccall(hfunc, Float64, (Float64,), 3.0)
20.085536923187668
julia> @ccall $hfunc(3.0::Float64)::Float64
20.085536923187668
```
See also the [docs](https://docs.julialang.org/en/v1/base/c/#Base.@ccall) for `@ccall`.
---
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [May 11, 2023, 2:24pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/9 "2023-05-11T14:24:32Z")
Thanks! Apparently I missed the `$` in front of my function handle?
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [May 11, 2023, 2:25pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/10 "2023-05-11T14:25:46Z")
If you had a function handle, yes - what you’ve shown so far was only the handle for the library 🙂 Note the `hfunc = dlsym(...)`.
---
### Author:  [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [May 11, 2023, 2:27pm UTC](https://discourse.julialang.org/t/problems-with-dlopen/98679/11 "2023-05-11T14:27:05Z")
Heh, yeah it has the dollar sign right there in the documents … I just went on as if it weren’t there. Thanks.