用 3 个字节填充 C 中的结构

Question

Let's say I have a struct and variables in C like this:

typedef struct {
  uint8_t x;
  uint8_t y;
  uint8_t z;
}my_type;

my_type a;
my_type a10[10];

With C99 is it certain that

• sizeof(a) == 3 ? (and not 4 or 8)
• sizeof(a10) == 30 ? (and not 40 or 80)

Answer 1

With C99 is it certain that sizeof(a) == 3?

This type will be used as color for an RGB888 framebuffer. So it's important to NOT have space between the adjacent RGB values.

typedef struct {
  uint8_t x;
  uint8_t y;
  uint8_t z;
}my_type;

No. The size may be 3 or 4 (or theoretically others, but not likely)

---

2 solutions:

• Give up portability and use compiler specif code to pack the structure.

• Change struct to an array of 3 uint8_t and adjust code.

I recommend the last.

Answer 2

As mentioned in the comments, padding could be added by default.
To avoid that you could use __attribute__((packed)) for your struct.
This will tell the compiler to pack your struct as tight as possible.
Would look like this:

typedef struct {
  uint8_t x;
  uint8_t y;
  uint8_t z;
} __attribute__((packed)) my_type;