Hello,
I'm trying to learn how to call C libraries from Julia. So I wrote a simple
C library that does nothing in particular and I am having trouble talking
to that library from Julia. Here is my library:
---------------------------------------------
% cat foo.c
#include <stdio.h>
#include <stdlib.h>
int N = 7;
int Nmax = 9;
struct particle {
double x;
double y;
double z;
};
struct particle* particles = NULL;
void particles_add(struct particle pt){
while (Nmax <= N){
Nmax += 128;
particles = realloc(particles,sizeof(struct particle)*Nmax);
}
particles[N] = pt;
N++;
}
---------------------------------------------
% make
gcc -Wall -std=c99 -Wpointer-arith -c -fPIC foo.c
gcc -Wall -std=c99 -Wpointer-arith -shared -o libfoo.so foo.o
---------------------------------------------
So, there are two things I would like to do from Julia: I'd like to read
the variable "N" and I'd like to create a "particle" struct and pass it to
the "particles_add()" function. I used the documentation on this page:
http://julia.readthedocs.org/en/latest/manual/calling-c-and-fortran-code/
Based on what I read, this is what I came up with:
---------------------------------------------
% cat foo.jl
const lib = "./libfoo.so"
N() = unsafe_load(cglobal(("N" ,lib), Cint), 0)
Nmax() = unsafe_load(cglobal(("Nmax",lib), Cint), 0)
immutable Particle
x ::Cdouble
y ::Cdouble
z ::Cdouble
end
function add(pt::Particle)
ccall(("particles_add", lib), Void, (Particle,), pt)
end
function add(;x=0,y=0,z=0)
add(Particle(x,y,z))
end
---------------------------------------------
However, nothing here works: I cannot read "N" (I get gibberish) and I
cannot call "particles_add()" (I get a segfault):
---------------------------------------------
% julia
...
julia> include("foo.jl")
add (generic function with 2 methods)
julia> N()
32570
julia> Nmax()
7
julia> add(x=0)
signal (11): Segmentation fault
particles_add at ./libfoo.so (unknown line)
julia_add_20314 at (unknown line)
jlcall_add_20314 at (unknown line)
unknown function (ip: -490800184)
unknown function (ip: -490804016)
unknown function (ip: -490736582)
jl_f_top_eval at /usr/bin/../lib/x86_64-linux-gnu/julia/libjulia.so
(unknown line)
eval_user_input at REPL.jl:53
jlcall_eval_user_input_20278 at (unknown line)
jl_apply_generic at /usr/bin/../lib/x86_64-linux-gnu/julia/libjulia.so
(unknown line)
anonymous at task.jl:95
jl_handle_stack_switch at
/usr/bin/../lib/x86_64-linux-gnu/julia/libjulia.so (unknown line)
julia_trampoline at /usr/bin/../lib/x86_64-linux-gnu/julia/libjulia.so
(unknown line)
unknown function (ip: 4199613)
__libc_start_main at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
unknown function (ip: 4199667)
unknown function (ip: 0)
zsh: segmentation fault (core dumped) julia
---------------------------------------------
So, at this point I am really stuck. Clearly I have missed a lot of
fundamental issues in either my C code, or my Julia code, or both. Can
anyone figure out what I'm doing wrong?
Cheers,
Daniel.
