ccall fails to pass/return struct{char, char}

[maleadt]

Messing around with the ccall test suite, I came across this test failure on x86_64:

test.c (compile using clang -g -fPIC -shared -Wall test.c -o test.so):

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>

typedef struct {
    char x;
    char y;
} struct10;

struct10 test(struct10 a)
{
    fprintf(stderr,"%" PRId8 ",%" PRId8 "\n", a.x, a.y);
    a.x += 1;
    a.y -= 2;

    return a;
}

test.jl:

using Base.Test

type Struct10
    x::Char
    y::Char
end

let
    a = Struct10('a', 'b')

    x = ccall((:test, "./test.so"), Struct10, (Struct10,), a)

    @test x.x == a.x+1
    @test x.y == a.y-2
end

This yields:

$ julia test.jl
97,0
Test Failed
  Expression: x.x == a.x + 1
   Evaluated: '﹢' == 'b'

The struct is passed incorrectly to the C function, printing 97,0 instead of 97,98. Analyzing the ccall signature, julia seems to bitcast the struct to a i64 while clang generates a coerced i16 parameter (however, having a quick look in the debugger I couldn't spot the 'b' anywhere in memory). The return value is also botched.

This is on a haswell x86_64, with julia from git master built using LLVM 3.7, but it also fails on 0.4 from the Arch repositories.

[tomaklutfu]

Does using Cchar in Struct10 make difference? Char is 4 bytes and Cchar 1 byte.

[maleadt]

Yes, works fine when using Cchar instead. I didn't expect that, as passing Char directly does work properly. Looking at the IR, I assume ccall converts the types automatically, while the struct just gets bitcasted.

[tomaklutfu]

I think type Char in julia and char in C different bittypes. For equivalence, Cchar is the correct one to use in Julia. You can check the size of Struct10 with sizeof. If Char used the size is 8 bytes, else Cchar used the size is 2 bytes (equivalently i64 and i16)

[yuyichao]

Yes, you should use Cchar. See the doc for a complete list. Passing Char in ccall directly probably works because of the calling convention.

[maleadt]

OK thanks. I had the impression some conversions happened automatically, because of how often this actually leads to functional code. Sorry for the noise!

[tomaklutfu]

Conversion happens if ccall is instructed to but you tell ccall that argument type is Struct10 and type of 'a' is Struct10 so no conversion is needed.