So, is that the fault of sizeof() or the structure really holds 56 bytes?

I am gonna use this structure to gether the header of a bitmap file. My question is, am I gonna get 54 bytes or 56 bytes by a file stream function such as .read() ?

Look into your compiler's documentation for a way to pack structures.

What do you mean by packing structures?

Are there any gaps between variables in a structure? If so, how can we avoid them?

#include <iostream.h>
#include <memory.h>

int main (int argc, char * argv[])
{
struct
{
short Signture;
long FileSize;
short Reserved1;
short Reserved2;
long ImgDataStartOff;
long InfoHeaderSize;
long Width;
long Height;
short Planes;
short NumOfPix;
long Compression;
long SizeOfImgData;
long HRes;
long VRes;
long NumOfCol;
long NumOfImpCol;
} Header;

memset(&Header, 196, 75); //Fill the structure with '-' characters

Header.Signture =0x4241;
Header.FileSize =0x46454443;
Header.Reserved1 =0x4847;
Header.Reserved2 =0x4A49;
Header.ImgDataStartOff =0x4E4D4C4B;
Header.InfoHeaderSize =0x5251504F;
Header.Width =0x56555453;
Header.Height =0x5A595857;
Header.Planes =0x5C5B;
Header.NumOfPix =0x5E5D;
Header.Compression =0x6261605F;
Header.SizeOfImgData =0x66656463;
Header.HRes =0x6A696867;
Header.VRes =0x6E6D6C6B;
Header.NumOfCol =0x7271706F;
Header.NumOfImpCol =0x76757473;

union
{
char * CPtr;
short * SPtr;
} Ptr;

Ptr.SPtr = &Header.Signture;

cout<<"Size of Header = "<<sizeof(Header)<<endl;

for(int i=0; i<60; i++) cout<<i<<"\t"<<Ptr.CPtr[i]<<endl;

system("PAUSE>nul");
return EXIT_SUCCESS;
}

Size of Header = 56
0 A
1 B
2 ─
3 ─
4 C
5 D
6 E
7 F
8 G
9 H
10 I
11 J
12 K
13 L
14 M
15 N
16 O
17 P
18 Q
19 R
20 S
21 T
22 U
23 V
24 W
25 X
26 Y
27 Z
28 [
29 \
30 ]
31 ^
32 _
33 `
34 a
35 b
36 c
37 d
38 e
39 f
40 g
41 h
42 i
43 j
44 k
45 l
46 m
47 n
48 o
49 p
50 q
51 r
52 s
53 t
54 u
55 v
56 ─
57 ─
58 ─
59 ─