How to assign value to a struct with bit-fields?

Question

I have a struct with bit-fields (totally 32 bit width) and I have a 32-bit variable. When I try to assign the variable value to my struct, I got an error:

error: conversion from ‘uint32_t {aka unsigned int}’ to non-scalar type ‘main()::CPUID’ requested.

struct CPUIDregs
    {
       uint32_t EAXBuf;
    };
CPUIDregs CPUIDregsoutput;   


int main () {

 struct CPUID          
    {
          uint32_t   Stepping         : 4;         
          uint32_t   Model            : 4;        
          uint32_t   FamilyID         : 4;        
          uint32_t   Type             : 2;        
          uint32_t   Reserved1        : 2;         
          uint32_t   ExtendedModel    : 4;         
          uint32_t   ExtendedFamilyID : 8;          
          uint32_t   Reserved2        : 4;          
    };

    CPUID CPUIDoutput = CPUIDregsoutput.EAXBuf;

Do you have any idea how to do it in the shortest way? Thanks

P.S. Of course I have more appropriate value of EAX in real code, but I guess it doesn't affect here.

You may want a `union` rather than a `struct` here. Else you can only set each variable in the struct separately. you can not do `CPUID CPUIDoutput = EAX;` you'll have to do `CPUIDoutput.stepping = EAX;` — andre, Jun 01 '12 at 13:46

Shahbaz · Accepted Answer · 2018-08-01T14:14:06.337

14

You should never rely on how the compiler lays out your structure in memory. There are ways to do what you want with a single assignment, but I will neither recommend nor tell you.

The best way to do the assignment would be the following:

static inline void to_id(struct CPUid *id, uint32_t value)
{
    id->Stepping         = value & 0xf;
    id->Model            = value >> 4 & 0xf;
    id->FamilyID         = value >> 8 & 0xf;
    id->Type             = value >> 12 & 0x3;
    id->Reserved1        = value >> 14 & 0x3;
    id->ExtendedModel    = value >> 16 & 0xf;
    id->ExtendedFamilyID = value >> 20 & 0xff;
    id->Reserved2        = value >> 28 & 0xf;
}

And the opposite

static inline uint32_t from_id(struct CPUid *id)
{
    return id->Stepping
         | id->Model << 4
         | id->FamilyID << 8
         | id->Type << 12
         | id->Reserved1 << 14
         | id->ExtendedModel << 16
         | id->ExtendedFamilyID << 20
         | id->Reserved2 << 28;
}

edited Aug 01 '18 at 14:14

answered Jun 01 '12 at 14:27

Shahbaz

44,690
18
114
177

Now, I see only this way of assignment for me. – Irina Jun 01 '12 at 14:48
@user1430759. This is the only safe/portable way to do this. Don't worry about performance as this will pretty much get all optimized by the compiler. Also, try to double check the numbers before copy pasting the code. – Shahbaz Jun 01 '12 at 14:50
And feel free to up-vote and accept if you liked the answer ;) – Shahbaz Jun 01 '12 at 14:50
1

+1 for providing an answer that isn't a hack and doesn't produce UB. – Captain Obvlious Jun 01 '12 at 16:44

score 10 · Answer 2 · answered Jun 08 '16 at 20:12

10

Use a union.

union foo {
    struct {
        uint8_t a : 4;
        uint8_t b : 4;
        uint8_t c : 4;
        uint8_t d : 4;
        uint16_t e;
    };
    uint32_t allfields;
};

int main(void) {
    union foo a;

    a.allfields = 0;
    a.b = 3;

    return 0;
}

answered Jun 08 '16 at 20:12

Ben

101
1
2

Nice one, would there also be something to avoid spelling out a.allfields? So for example just a = 42. Even if I have to use a GCC extension to accomplish that. – Zeyneb Nov 15 '21 at 02:16

score 2 · Answer 3 · answered Jun 04 '12 at 10:32

2

Just if somebody´s interested, I´ve got a better solution for my own question:

*(reinterpret_cast<uint32_t *> (&CPUIDoutput)) = CPUIDregsoutput.EAXBuf;

answered Jun 04 '12 at 10:32

Irina

149
1
1
12

3

@Anna, I hope you are not using this in real code that would be used in real situations. The problem is that the compiler is free to add padding between members of your struct for optimization purposes. So, you cannot rely on your struct and your number to have the same layout. Even without padding, your statement will absolutely be wrong in a big-endian machine. – Shahbaz Dec 09 '12 at 09:51

score 1 · Answer 4 · answered Jun 01 '12 at 13:43

1

These are struct members, so you need to assign directly do them, or make sure the RHS of your assignment is a value of type CPUID. Not sure why you expect to be able to assign to the struct from an integer.

The facts that the struct contains bitfields, and that the sum of the bits happens to be the same as the number of bits in the integer you're trying to assign, mean nothing. They're still not compatible types, for assignment purposes.

If this was too vague, consider showing more/better code.

answered Jun 01 '12 at 13:43

unwind

378,987
63
458
590

I guess in edited code version it is more obviously why I am trying to assign EAX to the whole structure at once. So, anyway if I understood you right, it is no way for me to do it? I can only assign each field separately? – Irina Jun 01 '12 at 14:29

How to assign value to a struct with bit-fields?

4 Answers4

Linked