# CRC24Q implementation

Discussion in 'C' started by imported_DDC, Sep 6, 2011.

1. ### imported_DDCNew Member

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I am trying to implement the algorithm of a CRC check, which basically created a value, based on an input message.
So, consider I have a hex message 3F214365876616AB15387D5D59, and I want to obtain the CRC24Q value of the message.
The algorithm that I found to do this is the following:
Code:
```    typedef     unsigned long crc24;
crc24 crc_check(unsigned char *input) {
unsigned char *octets;
crc24 crc = 0xb704ce; // CRC24_INIT;
int i;
int len = strlen(input);
octets = input;

while (len--) {
crc ^= ((*octets++) << 16);

for (i = 0; i < 8; i++) {
crc <<= 1;
if (crc & 0x1000000)
crc ^= CRC24_POLY;
}
}
return crc & 0xFFFFFF;
}```
where *input=3F214365876616AB15387D5D59.
The problem is that ((*octets++) << 16) will shift by 16 bits the ascii value of the hex character and not the character itself.
So, I made a function to convert the hex numbers to characters.
I know the implementation looks weird, and I wouldn't be surprised if it were wrong.
This is the convert function:
Code:
```    char* convert(unsigned char* message) {
unsigned char* input;
input = message;
int p;

char *xxxx[20];
xxxx[0]="";

for (p = 0; p < length(message) - 1; p = p + 2) {
char* pp[20];
pp[0] = input[0];
char *c[20];
*input++;
c[0]= input[0];
*input++;
strcat(pp,c);
char cc;
char tt[2];
cc = (char ) strtol(pp, &pp, 16);
tt[0]=cc;
strcat(xxxx,tt);

}
return xxxx;
}```
SO:
Code:
```    unsigned char *msg_hex="3F214365876616AB15387D5D59";
crc_sum = crc_check(convert((msg_hex)));
printf("CRC-sum: %x\n", crc_sum);```
Thank you very much for any suggestions.