tcp/ip通信中ip头部结构iph->check校验计算

通过raw socket修改通信数据后，可通过该函数重新校验计算iph->check值

在http://www.cnblogs.com/dpf-10/p/7899237.html查看实际调用

static inline unsigned short ip_fast_csum(unsigned char* iph,unsigned int ihl){

    unsigned int sum;

__asm__ __volatile__(

    "movl (%1), %0 ;\n"

    "subl $4, %2 ;\n"

    "jbe 2f ;\n"

    "addl 4(%1), %0 ;\n"

    "adcl 8(%1), %0 ;\n"

    "adcl 12(%1), %0 ;\n"

    "1: adcl 16(%1), %0 ;\n"

    "lea 4(%1), %1 ;\n"

    "decl %2 ;\n"

    "jne 1b ;\n"

    "adcl $0, %0 ;\n"

    "movl %0, %2 ;\n"   //保存sum的值到%2

    "shrl $16, %0 ;\n"  //右移16位（读取高16位）到%0

    "addw %w2, %w0 ;\n" //%0的16位加%2的16位

    "adcl $0, %0 ;\n"   //若进位加上进位

    "notl %0 ;\n" //取反

    "2: ;\n"

/* Since the input registers which are loaded with iph and ihl

are modified, we must also specify them as outputs, or gcc

will assume they contain their original values. */

    : "=r" (sum), "=r" (iph), "=r" (ihl)

    : "" (iph), "" (ihl)

    : "memory");

    return (sum);

}