struct_sizes.c

#include <stdio.h>
#include <stdint.h>

// set struct alignment in memory to 1 byte
#pragma pack(push, 1)

typedef struct _FAT32_BOOTSECTOR
{
    uint8_t jmp[3];
    int8_t OemName[8];
    uint16_t BytesPerSector;
    uint8_t SectorsPerCluster;
    uint16_t ReservedSectors;
    uint8_t NumberOfFATs;
    uint16_t RootEntries;
    uint16_t TotalSectors;
    uint8_t Media;
    uint16_t SectorsPerFAT;
    uint16_t SectorsPerTrack;
    uint16_t HeadsPerCylinder;
    uint32_t HiddenSectors;
    uint32_t BigTotalSectors;
    uint32_t BigSectorsPerFAT;
    uint16_t Flags;
    uint16_t Version;
    uint32_t RootCluster;
    uint16_t InfoSector;
    uint16_t BootBackupStart;
    uint8_t Reserved[12];
    uint8_t DriveNumber;
    uint8_t Unused;
    uint8_t ExtBootSignature;
    uint32_t SerialNumber;
    int8_t VolumeLabel[11];
    int8_t FileSystem[8];
    uint8_t BootCode[422];
} FAT32_BOOTSECTOR;

#pragma pack(pop)

typedef struct _FAT32_BOOTSECTOR_2
{
    uint8_t jmp[3];
    int8_t OemName[8];
    uint16_t BytesPerSector;
    uint8_t SectorsPerCluster;
    uint16_t ReservedSectors;
    uint8_t NumberOfFATs;
    uint16_t RootEntries;
    uint16_t TotalSectors;
    uint8_t Media;
    uint16_t SectorsPerFAT;
    uint16_t SectorsPerTrack;
    uint16_t HeadsPerCylinder;
    uint32_t HiddenSectors;
    uint32_t BigTotalSectors;
    uint32_t BigSectorsPerFAT;
    uint16_t Flags;
    uint16_t Version;
    uint32_t RootCluster;
    uint16_t InfoSector;
    uint16_t BootBackupStart;
    uint8_t Reserved[12];
    uint8_t DriveNumber;
    uint8_t Unused;
    uint8_t ExtBootSignature;
    uint32_t SerialNumber;
    int8_t VolumeLabel[11];
    int8_t FileSystem[8];
    uint8_t BootCode[422];
} FAT32_BOOTSECTOR_2;

int main(int argc, char *argv[])
{
    printf("FAT32_BOOTSECTOR (1-byte aligned) size: %lu\n", sizeof(FAT32_BOOTSECTOR));
    printf("FAT32_BOOTSECTOR_2 size: %lu\n", sizeof(FAT32_BOOTSECTOR_2));

    return 0;
}