checksum-8位和16位校验和代码示例

// gcc filename.c -o filename  
// ./filename  
  
#include <stdio.h>  
  
unsigned char ip_hdr_8[] =   
{  
    0x45,0x00,  
    0x00,0x3c,  
    0x00,0x00,  
    0x00,0x00,  
    0x40,0x01,  
    0x00,0x00,  // checksum  
    0xc0,0xa8,  
    0x01,0x25,  
    0xda,0x3c,  
    0x06,0x82  
};  
  
unsigned short ip_hdr_16[] =   
{  
    0x4500,  
    0x003c,  
    0x0000,  
    0x0000,  
    0x4001,  
    0x0000,     // checksum  
    0xc0a8,  
    0x0125,  
    0xda3c,  
    0x0682  
};  
  
unsigned int (unsigned int cksum, void *pBuffer, unsigned int size)  
{  
    char num = 0;  
    unsigned char *p = (unsigned char *)pBuffer;  
   
    if ((NULL == pBuffer) || (0 == size))  
    {  
        return cksum;  
    }  
     
    while (size > 1)  
    {  
        cksum += ((unsigned short)p[num] << 8 & 0xff00) | (unsigned short)p[num + 1] & 0x00FF;  
        size  -= 2;  
        num   += 2;  
    }  
     
    if (size > 0)  
    {  
        cksum += ((unsigned short)p[num] << 8) & 0xFFFF;  
        num += 1;  
    }  
   
    while (cksum >> 16)  
    {  
        cksum = (cksum & 0xFFFF) + (cksum >> 16);  
    }  
     
    return ~cksum;  
}  
  
  
unsigned short checksum_16(unsigned short *addr,int len)  
{  
    unsigned short chksum;  
    unsigned int sum = 0;  
  
    while(len > 1)  
    {  
        sum += *addr++;  
        len -= 2;  
    }  
      
    if(len == 1)  
        sum += *(unsigned char*)addr;  
          
    sum = (sum>>16) + (sum & 0xffff);  
    sum += (sum>>16);  
    chksum = ~sum;  
    return (chksum);  
}  
  
void main()  
{  
    unsigned short chksum;  
  
    printf("--------------------------------------------n");  
      
    chksum = checksum_8(0,ip_hdr_8,sizeof(ip_hdr_8));  
    printf("checksum_8 = 0x%04x n",chksum);  
    ip_hdr_8[10] = chksum >> 8;  
    ip_hdr_8[11] = chksum & 0xff;  
      
    chksum = checksum_8(0,ip_hdr_8,20);  
    if(chksum)  
        printf("Checksum_8 is incorrect! n");  
    else  
        printf("Checksum_8 is correct! n");  
  
    printf("--------------------------------------------n");  
      
    chksum = checksum_16(ip_hdr_16,sizeof(ip_hdr_16));  
    printf("checksum_16 = 0x%04x n",chksum);  
    ip_hdr_16[5] = chksum;  
      
    chksum = checksum_16(ip_hdr_16,20);  
    if(chksum)  
        printf("Checksum 16 is incorrect! n");  
    else  
        printf("Checksum 16 is correct! n");  
      
    printf("--------------------------------------------n");  
}