Monday, October 31, 2022

[SOLVED] Reading Raw Bytes into a Padded Struct (C++)

Issue

I have an 18 byte struct in C++. I want to read 18 bytes of data from a file straight into this struct. However, my C++ compiler pads the struct to be 20 bytes (4 byte aligned). This is relatively easy to get around for just my compiler alone but I would prefer to use a method that is more reliable cross-platform/cross-compiler.

This is the struct:

struct Test {
    uint8_t a;
    uint8_t b;
    uint8_t c;
    uint16_t d;
    uint16_t e;
    uint8_t f;
    uint16_t g; 
    uint16_t h; 
    uint16_t i; 
    uint16_t j; 
    uint8_t  k;  
    uint8_t  l;
};

Currently I use an ifstream and read it to the struct like so:

Test test;
// file is a binary ifstream
file.read((char*)test.a, 18);

On my system this skips past the two bytes of padding at the beginning of the struct. I could add bytes to the front of the struct to guarantee it to be 32 bytes which would be a valid alignment on most systems, however I don't know if that would actually work with how structs need their elements to be naturally aligned.

Any help on this would be great but I could always end up copying the bytes manually into the attributes 😔.


Solution

You have several options, and as usual, you should choose whatever best fits your needs:

  1. as stated before, don't read/write directly from/to memory, instead write each field separately (kind of how Java people would).
    This is the, I think, most portable, but WAY slower than the later methods.

  2. reorder the struct to match normal alignment (good practice anyway)
    in your example:

struct Test {
    uint8_t a;
    uint8_t b;
    uint8_t c;
    uint8_t f; // moved
    uint16_t d;
    uint16_t e;
//  uint8_t f;
    uint16_t g; 
    uint16_t h; 
    uint16_t i; 
    uint16_t j; 
    uint8_t  k;  
    uint8_t  l;
    uint16_t spare;
};

Note: it still have 2 byte padding, but not in the middle :)

  1. use #pragma pack(push, 1) on the struct to tell the compiler to NOT align the bytes

Note: you may need to place multiple #pragma to support different compilers

#pragma pack(push, 1)
struct Test {
    uint8_t a;
    uint8_t b;
    uint8_t c;
    uint16_t d;
    uint16_t e;
    uint8_t f;
    uint16_t g; 
    uint16_t h; 
    uint16_t i; 
    uint16_t j; 
    uint8_t  k;  
    uint8_t  l;
    uint16_t spare;
};
#pragma pack(pop)

I'd like to add that proper alignment helps the CPU process faster, therefore, you don't want to force pack = 1 on all structs... only those intended to be transmitted or received via communication channel.



Answered By - Tomer W
Answer Checked By - Mary Flores (WPSolving Volunteer)
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