您能否使用指向包含结构的指针来修改嵌套结构的值？

Question

I've got a question regarding the behavior of structs in C.

Take the given structs as an example:

typedef struct {
    char num1;
    char num2;
} structa;

typedef struct {
    structa innerstruct;
    char num3;
    char num4;
} structb;

I assume since structb contains a structa field, rather than a pointer to one, that structb would be laid out in memory as such:

struct {
    char num1; //innerstruct num1
    char num2; //innerstruct num2
    char num3; //num3
    char num4; //num4
};

If so, can you access each field like this?

*(((char *)&structbvar) + 0) = 1; //innerstruct num1
*(((char *)&structbvar) + 1) = 2; //innerstruct num2
*(((char *)&structbvar) + 2) = 3; //num3
*(((char *)&structbvar) + 3) = 4; //num4

or access them like this?

((structa)structbvar).num1 = 1; //innerstruct num1
((structa)structbvar).num2 = 2; //innerstruct num2

Regardless of if that's bad practice or not.

Also is it reliable/portable?

---

Edit, as pointed out by Matt McNabb you can't directly cast structs so it should be:

((structa *)&structbvar)->num1 = 1; //innerstruct num1
((structa *)&structbvar)->num2 = 2; //innerstruct num2

Answer 1

The *(char *) version is well-defined although you will need to check for padding after the end of innerstruct and before num3 .

(structa)structbvar is ill-formed. I think *(structa *)&structbvar would be OK, because of the rule about structs with common initial sequence (although I'll double-check that and update my answer). However that is a bit pointless as you could just write structbvar.innerstruct .

Answer 2

If you use pointer to force access a struct field, the reliability of your code depends on the alignment of your memory. So it's not recommended, unless you force the memory to align as you expected.

Another way to access the field without calling the nested struct name, is using the Anonymous Struct with union. It's C99 Standard and of course portable and reliable, except your compiler doesn't support C99.

You can define a union with Anonymous Strct like this:

typedef union {
    uint32_t wWord;
    struct {
        uint16_t hwLow;
        uint16_t hwHigh;
    };
} test_t;

Then access each field like this:

test_t tTest;
tTest.wWord = 0x00FF0000;

Also:

tTest.hwHigh = 0x00FF;