CRC 16 program to calculate check sum

Question

I'm currently writing a CRC16 program which computes the CRC of characters using the CRC 16 polynomial X^16 + X^15 + X^2 + 1. the program should read data from the standard input and output the 16 bit CRC in hexadecimal. Nonetheless, when I execute the program I get the wrong value for output.

Here is my code:

#include <stdint.h>

#define CRC16 0x8005

unsigned short crc(unsigned char msg[], int len)
{
    unsigned short out = 0;
    int bits = 0, t_flag;
    int x = 0;

    /* Sanity check: */
    if(msg == NULL)
        return 0;

    while(len > x)
    {
        unsigned short data = msg[x];
        t_flag = out >> 15;

        /* Get next bit: */
        out <<= 1;
        out |= (data >> bits) & 1; // item a) work from the least significant bits

        /* Increment bit counter: */
        bits++;
        if(bits > 7)
        {
            bits = 0;
            data++;
            len--;
        }

        /* Cycle check: */
        if(t_flag)
            out ^= CRC16;

    }

    // item b) "push out" the last 16 bits
    int i;
    for (i = 0; i < 16; ++i) {
        t_flag = out >> 15;
        out <<= 1;
        if(t_flag)
            out ^= CRC16;
    }

    // item c) reverse the bits
    unsigned short crc1 = 0;
    i = 0x8000;
    int j = 0x0001;
    for (; i != 0; i >>=1, j <<= 1) {
        if (i & out) crc1 |= j;
    }

    return crc1;
}
int main (int argc, char *argv[]) {
//if (argv[1] == "trace") {

//printf(argv[1]);
//}
char ARGV;


if(argc < 1) {

    printf("Must have atleast one arguments\n");
        return 1;

}
char buf[256];
int c , r;
int count = -1;

while((c = getchar())!=EOF) {
    buf[count++] = putchar(c);
}

 r = crc(buf, count);

 //printf("%s\n",argv[1] );

 printf("%04hx\n", r);
 //print("%x\n", argv[1]);
 return (0);
//printf(" %4x\n", crc(argv[1], 16));

}

Output:

(I'm reading in 123456789 in my txt file)

./crc1 < testfile.txt
123456789
7bda

It's supposed to be BB3D but I'm getting 7bda. Can someone help me figure out what I'm doing wrong?

Answer 1

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

#define CRC16 0x8005

uint16_t gen_crc16(const uint8_t *data, uint16_t size)
{
    uint16_t out = 0;
    int bits_read = 0, bit_flag;


    // test
    printf("buffer in function %s\n", data);

    /* Sanity check: */
    if(data == NULL)
        return 0;

    while(size > 0)
    {
        bit_flag = out >> 15;

        /* Get next bit: */
        out <<= 1;
        out |= (*data >> bits_read) & 1; // item a) work from the least significant bits

        /* Increment bit counter: */
        bits_read++;
        if(bits_read > 7)
        {
            bits_read = 0;
            data++;
            size--;
        }

        /* Cycle check: */
        if(bit_flag)
            out ^= CRC16;

    }

    // item b) "push out" the last 16 bits
    int i;
    for (i = 0; i < 16; ++i) {
        bit_flag = out >> 15;
        out <<= 1;
        if(bit_flag)
            out ^= CRC16;
    }

    // item c) reverse the bits
    uint16_t crc = 0;
    i = 0x8000;
    int j = 0x0001;
    for (; i != 0; i >>=1, j <<= 1) {
        if (i & out) crc |= j;
    }

    return crc;
}

int main()
{
    char buf[]="123456789";
    int c , r;
    printf ("the buf has %s", buf);
    r = gen_crc16(buf,sizeof(buf)-1);
    printf("%04hx\n", r);

    return (0);
}

Answer 2

mmmmm, why don't you try to use some already made implementations first? is your polynomial correct? X^16+X^2+X^0? maybe is X^16+X^15+X^2+X^0

uint16_t Crc16WordCalc(const uint16_t *DataIn, uint32_t DataLen, uint16_t InitVal)
{
    uint32_t    i;
    uint16_t CRCSum;
    uint8_t tempData;

    CRCSum = InitVal;
    for(i = 0; i < DataLen; i++)
    {
        tempData = (uint8_t)((DataIn[i] >> 8) & 0xFF);
        CRCSum = (CRCSum<<8)^CRC16Lut[((CRCSum>>8)&0xFF) ^ tempData];

        tempData = (uint8_t)(DataIn[i] & 0xFF);
        CRCSum = (CRCSum<<8)^CRC16Lut[((CRCSum>>8)&0xFF) ^ tempData];
    }

    return CRCSum; 
}


// LOOK UP TABLE for CRC16 generation
// Polynomial X^16+X^15+X^2+1
 0x0000, 0x8005, 0x800F, 0x000A, 0x801B, 0x001E, 0x0014, 0x8011,
 0x8033, 0x0036, 0x003C, 0x8039, 0x0028, 0x802D, 0x8027, 0x0022,
 0x8063, 0x0066, 0x006C, 0x8069, 0x0078, 0x807D, 0x8077, 0x0072,
 0x0050, 0x8055, 0x805F, 0x005A, 0x804B, 0x004E, 0x0044, 0x8041,
 0x80C3, 0x00C6, 0x00CC, 0x80C9, 0x00D8, 0x80DD, 0x80D7, 0x00D2,
 0x00F0, 0x80F5, 0x80FF, 0x00FA, 0x80EB, 0x00EE, 0x00E4, 0x80E1,
 0x00A0, 0x80A5, 0x80AF, 0x00AA, 0x80BB, 0x00BE, 0x00B4, 0x80B1,
 0x8093, 0x0096, 0x009C, 0x8099, 0x0088, 0x808D, 0x8087, 0x0082,
 0x8183, 0x0186, 0x018C, 0x8189, 0x0198, 0x819D, 0x8197, 0x0192,
 0x01B0, 0x81B5, 0x81BF, 0x01BA, 0x81AB, 0x01AE, 0x01A4, 0x81A1,
 0x01E0, 0x81E5, 0x81EF, 0x01EA, 0x81FB, 0x01FE, 0x01F4, 0x81F1,
 0x81D3, 0x01D6, 0x01DC, 0x81D9, 0x01C8, 0x81CD, 0x81C7, 0x01C2,
 0x0140, 0x8145, 0x814F, 0x014A, 0x815B, 0x015E, 0x0154, 0x8151,
 0x8173, 0x0176, 0x017C, 0x8179, 0x0168, 0x816D, 0x8167, 0x0162,
 0x8123, 0x0126, 0x012C, 0x8129, 0x0138, 0x813D, 0x8137, 0x0132,
 0x0110, 0x8115, 0x811F, 0x011A, 0x810B, 0x010E, 0x0104, 0x8101,
 0x8303, 0x0306, 0x030C, 0x8309, 0x0318, 0x831D, 0x8317, 0x0312,
 0x0330, 0x8335, 0x833F, 0x033A, 0x832B, 0x032E, 0x0324, 0x8321,
 0x0360, 0x8365, 0x836F, 0x036A, 0x837B, 0x037E, 0x0374, 0x8371,
 0x8353, 0x0356, 0x035C, 0x8359, 0x0348, 0x834D, 0x8347, 0x0342,
 0x03C0, 0x83C5, 0x83CF, 0x03CA, 0x83DB, 0x03DE, 0x03D4, 0x83D1,
 0x83F3, 0x03F6, 0x03FC, 0x83F9, 0x03E8, 0x83ED, 0x83E7, 0x03E2,
 0x83A3, 0x03A6, 0x03AC, 0x83A9, 0x03B8, 0x83BD, 0x83B7, 0x03B2,
 0x0390, 0x8395, 0x839F, 0x039A, 0x838B, 0x038E, 0x0384, 0x8381,
 0x0280, 0x8285, 0x828F, 0x028A, 0x829B, 0x029E, 0x0294, 0x8291,
 0x82B3, 0x02B6, 0x02BC, 0x82B9, 0x02A8, 0x82AD, 0x82A7, 0x02A2,
 0x82E3, 0x02E6, 0x02EC, 0x82E9, 0x02F8, 0x82FD, 0x82F7, 0x02F2,
 0x02D0, 0x82D5, 0x82DF, 0x02DA, 0x82CB, 0x02CE, 0x02C4, 0x82C1,
 0x8243, 0x0246, 0x024C, 0x8249, 0x0258, 0x825D, 0x8257, 0x0252,
 0x0270, 0x8275, 0x827F, 0x027A, 0x826B, 0x026E, 0x0264, 0x8261,
 0x0220, 0x8225, 0x822F, 0x022A, 0x823B, 0x023E, 0x0234, 0x8231,
 0x8213, 0x0216, 0x021C, 0x8219, 0x0208, 0x820D, 0x8207, 0x0202