embedded - Confusion in file Verification with CRC16 in C

Question

Here i want to know the how can i implement calculate CRC16 for Any type of file,

Here i have idea about CRC16 and its code logic. here i made one function which take file path as a input and find out CRC value of this. here i pass file name in this function and this functions calculates CRC value of this file. but i want to calculate all types of files like .tar,.tar.gz,.txt,.bin,.scr etc.

so here i open this all files and "rb" mode and take one by one character and find out CRC value its right way? may be i missing something in this. its works fine for .txt and all other files but it will create problems in .tar, .tar.gz type files. Because here i have one file which is 890 MB and its name is file.tar.gz and its take 203 Microseconds and I have other file which is 382 MB and its name is file2.tar its take 6097850.000000 Microseconds its unbelievable for me hows its possible?

My question are these

1 Some thing problem in my CRC code ?

2 Problem in reading file data style, may be i am reading file data in wrong manner for .tar.gz.

Here is my code :

int CRC16(const char* filePath) {
    //Declare variable to store CRC result.
    unsigned short result;
    //Declare loop variables.
    int intOuterLoopIndex, intInnerLoopIndex, nLen;
    result = 0xffff; //initialize result variable to perform CRC checksum calculation.

    //Store message which read from file.
    //char content[2000000];

    //Create file pointer to open and read file.
    FILE *readFile;

    //Use to read character from file.
    char readChar;

    //open a file for Reading
    readFile = fopen(filePath, "rb");

    //Checking file is able to open or exists.
    if (!readFile) {
        fputs("Unable to open file %s", stderr);
    }

    /*
     Here reading file and store into variable.
     */
    int chCnt = 0;
    while ((readChar = getc(readFile)) != EOF) {
        result ^= (short) (readChar);
        for (intInnerLoopIndex = 0; intInnerLoopIndex < 8; intInnerLoopIndex++) {
            if ((result & 0x0001) == 0x0001) {
                result = result >> 1; //Perform bit shifting.
                result = result ^ 0xa001; //Perform XOR operation on result.
            } else {
                result = result >> 1; //Perform bit shifting.
            }
        }

        //content[chCnt] = readChar;
        chCnt++;
    }
    printf("CRC data length in file: %d", chCnt);

    return (result);
}

Answer 1

char readChar; is wrong, it needs to be int readChar;

getc() returns an int, so it can signal EOF to you. EOF is likely the value -1

If you convert the return value to a char, and read a byte with the value 255, 255 will be interpreted as -1, and you stop reading at the first byte that has the value 255.