Write struct to file

Question

When I write a struct to file, how the memory sets up in the file? For instance this struct and function:

struct vector3D
{
    public:
        float x, y, z;

    vector3D(float modelX, float modelY, float modelZ)
    {
        x = modelX;
        y = modelY;
        z = modelZ;
    }

    vector3D()
    {
        x = 0;
        y = 0;
        z = 0;
    }
}


inline void writeVector3D(vector3D vec, FILE *f)
{
    fwrite((void*)(&vec), sizeof(vector3D), 1, f);
}

And this code in main:

vector3D vec(1, 2, 3);
writeVector3D(vec, file);

How does the information sets up in the file? does it like 123? Or struct has different set up?

This writes out the binary representation. A downside to doing a binary output is how you handle changes to the data structure (compared to reading/writing text, such as XML or JSON). — crashmstr
– crashmstr, Commented Jun 25, 2014 at 18:11

Victor Istomin · Accepted Answer · 2014-06-25 18:22:39Z

2

You probably need to read about:

Data structure alignment (http://en.wikipedia.org/wiki/Data_structure_alignment) - for information about how struct members are arranged in memory
Endianness (Endianness) - for information about how single variable arranged in memory
Floating-point representation in memory (can't add third link) - because floating-point variables is much more 'strange' than integer ones.

Data will be written in same order as they are placed in memory, including alignment gaps.

edited Jun 25, 2014 at 18:22

answered Jun 25, 2014 at 18:16

Victor Istomin

1,1668 silver badges14 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Matias Cicero · Accepted Answer · 2014-06-25 18:15:08Z

1

It writes it as a sequential binary stream.

The size of the file will be the size of the struct.

In your case, it would write a total of 12 bytes (4 bytes per float), and it will be structured this way:

First 4 bytes would represent the float 1
Second 4 bytes would represent the float 2
Third 4 bytes would represent the float 3

answered Jun 25, 2014 at 18:15

Matias Cicero

26.5k21 gold badges91 silver badges160 bronze badges

Comments

Community · Accepted Answer · 2017-05-23 12:20:44Z

0

You need to have preprocessor #pragma pack(1) to byte align the structure otherwise its aligned depending on the processor architecture(32-bit or 64-bit). Also check this #pragma pack effect

edited May 23, 2017 at 12:20

CommunityBot

11 silver badge

answered Jun 25, 2014 at 18:22

dvasanth

1,3771 gold badge9 silver badges10 bronze badges

Collectives™ on Stack Overflow

Write struct to file

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related