Question

I have a c pointer to a structre type called uchar4 which looks like

{
    uchar x; 
    uchar y; 
    uchar z; 
    uchar w;
}

I also have data passed in as uint8*. I’d like to create a uchar* pointing to the data at the uint8* so I’ve tried doing this:

uint8 *data_in;
uchar4 *temp = (uchar4*)data_in;

However, the first 8 bytes always seem to be wrong. Is there another way of doing this?

EDIT:

the input data = 32 43 F6 A8 88 5A 30 8D 31 31 98 A2 E0 37 07 34

the output data = 32 88 31 E0 37 07 34 8D 31 31 98 A2 E0 37 07 34

The first half is always these seemingly random values

Answer 1

Behold, the humble union:

#include <stdio.h>

typedef struct {
    unsigned char x, y, z, w;
} xyzw;

typedef union {
    xyzw x;
    unsigned int i;
} ixyzw;


unsigned int xyzw_to_i(ixyzw *p)
{
    return p->i;
}

int main()
{
    xyzw x = {1, 2, 3, 4};

    printf("sizeof unsigned char %d\n", sizeof(unsigned char));
    printf("sizeof unsigned int  %d\n", sizeof(unsigned int));
    printf("sizeof xyzw          %d\n", sizeof(xyzw));
    printf("sizeof ixyzw         %d\n", sizeof(ixyzw));

    printf("xyzw = { %d, %d, %d, %d }\n", x.x, x.y, x.z, x.w);
    printf("i    = 0x%08x\n", xyzw_to_i((ixyzw *) &x));
    return 0;
}

which on my machine happens to yield:

sizeof unsigned char 1
sizeof unsigned int  4
sizeof xyzw          4
sizeof ixyzw         4
xyzw = { 1, 2, 3, 4 }
i    = 0x04030201

but one can’t count on this behavior across compilers or machines.