checksum.c

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

typedef uint32_t DWORD;
typedef uint16_t WORD;

int main(int argc, char *argv[]) {

    char *filename = "firmware.bin";
    FILE *input_file;
    if (argc > 1)
        filename = argv[1];
    fprintf(stderr, "Opening file %s\n", filename);
    input_file = fopen(filename, "r");

    DWORD dLength, dCheckSum, dExpectedCheckSum, dAddress, i, dImageBuf[512*1024];
    WORD wSignature;

    fread(&wSignature,1,2,input_file); // read signature bytes
    if (wSignature != 0x5943) // check ‘CY’ signature byte
    {
        printf("Invalid image");
        return 1;
    }
    fread(&i, 2, 1, input_file); // skip 2 dummy bytes

    dCheckSum = 0;
    while (1)
    {
        fread(&dLength,4,1,input_file); // read dLength
        fread(&dAddress,4,1,input_file); // read dAddress
        if (dLength==0) break; // done
        // read sections
        fread(dImageBuf, 4, dLength, input_file);
        for (i=0; i<dLength; i++) dCheckSum += dImageBuf[i];
    }
    printf("checksum:          %#x\n", dCheckSum);
    // read pre-computed checksum data
    fread(&dExpectedCheckSum, 4, 1, input_file);
    printf("expected checksum: %#x\n", dExpectedCheckSum);
    if (dCheckSum != dExpectedCheckSum)
    {
        printf("Fail to boot due to checksum error (calculated %#x != file %#x)\n", dCheckSum, dExpectedCheckSum);
        return 1;
    }
    return 0;
}

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

int main(int argc, char **argv) {
    unsigned long ulong;
    unsigned short ushort;
    uint32_t uint32Magic;
    uint16_t uInt16Magic;

    printf("the size of unsigned long %ld\n", sizeof(ulong) );
    printf("the size of unsigned short %ld\n", sizeof(ushort) );
    printf("the size of uint32_t %ld\n", sizeof(uint32Magic) );
    printf("the size of uint16_t %ld\n", sizeof(uInt16Magic) );

    return 0;
}

yields:

the size of unsigned long 8
the size of unsigned short 2
the size of uint32_t 4
the size of uint16_t 2

https://stackoverflow.com/questions/35844586/can-i-assume-the-size-of-long-int-is-always-4-bytes/35844670

Lols, so don't I feel silly now. Thanks for your help @miek!