When generating a shared C library using MinGW 4.4.0 on Windows and then generating an import library using implib, the resultant binary seems to have an improper default calling convention. This is evident when the C functions return structures. For example, given two files, test.d and test.c with the contents as follows:

//test.d
module test.d;

import tango.io.Stdout;

extern (C)
{
    struct COLOR
    {
        float r,g,b,a;
    }
    COLOR make_color(float r, float g, float b, float a);
}

void main()
{
    float[5] arr1 = 0;
    for(int ii = 0; ii < 10; ii++)
    {
        make_color(1, 1, 1, 1);
        Stdout.formatln("{},", arr1);
    }
}

//test.c
typedef struct COLOR
{
    float r,g,b,a;
} COLOR;

COLOR make_color(float r, float g, float b, float a)
{
    COLOR col;
    col.r = r;
    col.g = g;
    col.b = b;
    col.a = a;
    return col;
}

The command line instructions to reproduce this bug are then as follows:

gcc -shared -o test.dll test.c
implib /s test.lib test.dll
dmd test.d test.lib -L+tango.lib
test.exe

Outputs:
[0.00, 0.00, 0.00, 0.00, 0.00],
[0.00, 0.00, 0.00, 0.00, 0.00],
[0.00, 0.00, 0.00, 0.00, 0.00],
[1.00, 0.00, 0.00, 0.00, 0.00],
[1.00, 1.00, 0.00, 0.00, 0.00],
[1.00, 1.00, 1.00, 0.00, 0.00],
[1.00, 1.00, 1.00, 1.00, 0.00],
[0.17e-38, 1.00, 1.00, 1.00, 1.00],
[0.70e-44, 0.17e-38, 1.00, 1.00, 1.00]

As can be seen, the arr1 array gets slowly stomped over. This does not happen if I compile the C file with dmc, nor does it happen if I specify the linkage for the make_color function to be Windows (for both the C file in the D file). The issue here is with the default linkage type which seems to be incompatible in this case.