sizeof and unsigned char array error? (c/c++)

Question

i have written this structure:

struct bmpheader {
    unsigned char magic[2];
    unsigned int fsize;
    unsigned int unused;
    unsigned int pixdata_offset;
    unsigned int headersize;
    unsigned int width;
    unsigned int height;
    unsigned short planes_color;
    unsigned short bpp;
    unsigned int compression;
    unsigned int sizeofpix;
    unsigned int resolutionx;
    unsigned int resolutiony;
    unsigned int colors;
    unsigned int imp_colors;
};

And i have a problem with sizeof function. When im counting on my fingers, its every time 54 bytes for me. sizeof function gives me every time 56. Why? The problem is with unsigned char array, when i remove magic[2] array, sizeof is 52? sizeof(header.magic) is shown as 2. As far i know unsigned char is 1-byte type?

Thanks in advance for responses.

marcin

The compiler is inserting padding into your struct layout to ensure better alignment of the fields. This padding will be between the two magic chars and the first int to make sure that the ints all fall on 4 byte boundaries. — David Heffernan
– David Heffernan, Commented Feb 12, 2012 at 20:01

Lindydancer · Accepted Answer · 2012-02-12 20:05:36Z

1

Because an int (on your machine) must be, or is more efficient when, placed on a 4-byte address. The compiler will insert two bytes of padding after the char array.

edited Feb 12, 2012 at 20:05

answered Feb 12, 2012 at 19:59

Lindydancer

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3 Comments

David Heffernan Over a year ago

Must is too strong. Compilers often choose to do that for performance reasons rather than necessity.

marxin Over a year ago

Ah. Great. Good to know. Thank you :P Now its working ;p And, i have a little question again (and very easy i think). I have unsigned char magic[2]; How to write there "BM" at once, not by magic[0] = 'B', magic[1] = 'M'? I tried with magic = "BM"; or magic = {'B', 'M'};, both didnt work...

David Heffernan Over a year ago

You cannot assign to an array. You can however use initializer syntax. That's going to mean writing a struct initializer.

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