Game Boy Cartridge ROM Checksum code

gbsum.c

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

int main(int argc, char** argv) {
        FILE* fp = fopen(argv[1], "rb");
        if (fp == NULL) {
                printf("Failed to open the file to read: %s\n", argv[1]);
                return -1;
        }
        fseek(fp, 0L, SEEK_END);
        size_t len = ftell(fp);
        rewind(fp);
        uint8_t* buff = malloc(len);
        if (buff == NULL) {
                printf("Failed to allocate %zu bytes of memory", len);
                return -2;
        }
        fread(buff, len, 1, fp);
        fclose(fp);
        size_t sum = 0;
        buff[0x14E] = 0;
        buff[0x14F] = 0;
        for (size_t i = 0; i < len; ++i) {
                sum += buff[i];
        }
        buff[0x14E] = ((uint8_t*)&sum)[1];
        buff[0x14F] = ((uint8_t*)&sum)[0];
        fopen(argv[1], "wb");
        if (fp == NULL) {
                printf("Failed to open the file to write: %s\n", argv[1]);
                return -3;
        }
        fwrite(buff, len, 1, fp);
        fclose(fp);
        return 0;
}

Compile with clang:

clang -O2 gbsum.c -o gbsum