How to create 24 bit unsigned integer in C

Question

I am working on an embedded application where RAM is extremely tight. For this purpose I need to create a 24 bit unsigned integer data type. I am doing this using a struct:

typedef struct
{
    uint32_t v : 24;
} uint24_t;

However when I interrogate the size of a variable of this type, it returns "4", i.e.:

    uint24_t x;
    x.v = 0;
    printf("Size = %u", sizeof(x));

Is there a way I can force this variable to have 3 bytes?

Initially I thought it was because it is forcing datatypes to be word aligned, but I can for example do this:

typedef struct
{
    uint8_t blah[3];
} mytype;

And in that case the size comes out at 3.

Answer 1

A comment by João Baptista on this site says that you can use #pragma pack. Another option is to use __attribute__((packed)):

#ifndef __GNUC__
# define __attribute__(x)
#endif
struct uint24_t { unsigned long v:24; };
typedef struct uint24_t __attribute__((packed)) uint24_t;

This should work on GCC and Clang.

Note, however, that this will probably screw up alignment unless your processor supports unaligned access.