How to pack struct in C for non-aligned data?

[Henry Yiu]

In the following structure, bytesPerSector is not aligned to the sizeof(short), so after I read the raw data into buffer, and try to extract the bytesPerSector portion, I got the wrong number. Is there a easy fix for this? I am using Visual C++ Express on a x86 processor.

struct BS_Structure {
	unsigned char     jumpBoot[3];
	unsigned char     OEMName[8];
	unsigned short    bytesPerSector;
	unsigned char     sectorPerCluster;
} ptr;

unsigned char buffer[512];
unsigned short bytesize;

ReadSDCard (buffer);
ptr = (BS_Structure*) buffer;
bytesize = ptr->bytesPerSector;

[donbock]

bytesPerSector should indeed be properly aligned. What makes you think is isn't? An alignment problem will typically show up with a run-time error message, not improper operation.

Are you sure you don't overflow the bounds of the jumpBoot or OEMName arrays?

[newb16]

If the specification of the structure says something like 'on position 11 and 12 there is a 16-bit value'( yes it does - http://en.wikipedia.org/wiki/Fat16#Boot_Sector ) you need to manually extract them from the raw data, like
unsigned short bytesPerSector = raw[12] << 8 | raw[11];
Hoping that a compiler propery align it is not reliable. Endianness also matters.