Each struct we define will be unpacked unless otherwise specified. We can define a structure in C as follows:

typedef struct{
  unsigned char a;
  char b;
  int c;
  unsigned int d;
  char e[10];
} myStruct;

We can use it and populate it with values directly from our Go code without any issues arising:

func main() {
    v := C.myStruct{
        a: C.uchar('A'),
        b: C.char('Z'),
        c: C.int(100),
        d: C.uint(10),
        e: [10]C.char{'h', 'e', 'l', 'l', 'o'},
    }
    log.Printf("%#v", v)
}

This small test will give us the following output:

main._Ctype_struct___0{
    a:0x41, 
    b:90, 
    c:100, 
    d:0xa, 
    e:[10]main._Ctype_char{104, 101, 108, 108, 111, 0, 0, 0, 0, 0},
     _:[2]uint8{0x0, 0x0},
}

This tells us that there is an extra blank field that is used for padding because the last field is 10 bytes, which is 2 bytes short of being a multiple of 4 (that is, 12 bytes).