[camcamcam]

hi there,

in C a char var uses 1 byte in memory, and a int var uses 4 bytes in memory.
I have a structure with a char field and with a int field, so the sizeof function should return 5, but it returns 8.

#include <stdio.h>
#include <stdlib.h>

struct fileHeader {
       char tipo;
       int asdf;
};

int main()
{
    
    printf("%d", sizeof(struct fileHeader));
    
    printf("\n%d", sizeof(char));
    printf("\n%d", sizeof(int));
    
    /*
    char word[]="\x6d\x6f\x64\x65\x6d"; 
	   
    printf("%s\n", word);
    */
    
    system("pause");
    return 0;
}

There are 3 bytes that should not appear, what is going on?.

Thanks.


ps: i am using dev-c++.

[Skydiver]

By default the compiler puts padding to make sure that the structure elements are double word aligned. The CPU works faster when data is aligned at these borders.

If you swap the order of the two members, you may discover that the size may be smaller.

[Oler1s]

sizeof(int) is not necessarily 4. This is implementation defined.

structs can be padded, which is also implementation defined.

[Skydiver]

Depending on your compiler, you can override the default packing borders. For example, with the MSVC compiler #pragma pack is used to change the packing: http://msdn.microsof...(v=vs.100).aspx

[camcamcam]

that explains why the structure uses more space than the members on it, so when i write a structure into a file, i put "garbage" in the file.

Well i won't write structures into a file anymore, probably that is why i can't make a Bitmap fileheader correctly.

thanks a lot for the answers.

[Skydiver]

From my quick searches, it looks like #pragma pack is also supported by GCC, so you don't have to abandon bitblting structures to and from files if you don't really want to. You just need to be aware of the packing issues.

[camcamcam]

thank you very much, i am reading about it, and i found the way to use #pragma pack to delete the padding between the members of a structure and now my structure has exactly the size of the members!.

here is a detailed information:
http://www.xbdev.net...s/bmp/index.php
http://tedlogan.com/techblog2.html

thanks.

[Salem_c]

Structure packing should be regarded as a temporary fix.

1. http://en.wikipedia....wiki/Endianness
There is no pragma to fix endian issues. If your machine endian is different to the file format, then you're going to have to rearrange the bytes yourself.

2. Performance.
The reason structs have holes to begin with is to provide optimal access to data members. The logical inference of that is if the struct is packed, then member data access is sub-optimal. On architectures which don't allow unaligned memory accesses, the compiler has to generate extra code every time you access an unaligned member in the structure. If you're just doing I/O, then you're OK. But if this is in your performance sensitive code, you might want to think about what you're doing.

3. Data types
If you're trying to match up some external file format, then you should avoid raw types like 'int' or 'short', since these vary in size from one machine to another. Use the stdint.h types, or define your own typedefs if you want C89 portability as well.