Thread: Reading a “short” variable into a structure

How are you attempting to read the short variable? What data are you reading by your method?

Without those bits of information, there is no way to answer your question.

My guess is that you have a padding problem - for performance reasons, compilers often add padding between struct members. Your code is assuming there is no such padding.

1. Verify that sizeof(struct A) is indeed 4

2. Verify that this gives you the same result.

Code:

struct A H;
u_char *p;  // points to a sequence of bytes 12 34 08 00
memcpy(&H,p,sizeof(H));
if (H.Short3 == Match)

If p is a mis-aligned address, then dereferencing members with alignment requirements can produce weird effects.

3. Verify the endian of your machine.

Code:

struct A H;
H.Char1 = 0x12;
H.Char2 = 0x34;
H.Short3 = Match;
unsigned char *p = (unsigned char *)&H;
for ( i = 0 ; i < 4 ; i++ ) printf("%02x ", p[i]);

If your machine is opposite endian to your data stream, then you'll see
12 34 00 08

It looks like order problem

I would read 2 bytes into short using code:

Code:

H->Short3 = (u_short)high;
H->Short3 <<= 8;
H->Short3 |= (u_short)low;

looks like in your method the values of high and low are swapped

so the byte order is defined by rfc describing corresponding header you are reading - use it.