1、手工写了一个程序验证void *指针加减运算移动几个字节：
//本程序验证空类型指针减1移动几个字节 #include <stdio.h> int main(int argc, char *argv[]) {int a=10,b=20;int *pa=&a;void * pa1=(void *)pa;printf("int pa->a,%p\n",pa);printf("int pa+1=%p\n",(pa+1));printf("void pa1->a,%p\n",pa1);printf("void pa1+1=%p\n",(pa1+1));return 0; }
输出：
fly@noi:~$ ./p1 int pa->a,0x7ffde5e8db10 int pa+1=0x7ffde5e8db14 void pa1->a,0x7ffde5e8db10 void pa1+1=0x7ffde5e8db11
由上可知，当一个int指针被强转为void型指针后，加1，由以前移动4个字节变为了移动1个字节。
结论：void *指针加减1，移动1个字节，这个在一些计算地址的宏和函数里会用到。
例如：container_of宏：
/*** container_of - cast a member of a structure out to the containing structure* @ptr: the pointer to the member.* @type: the type of the container struct this is embedded in.* @member: the name of the member within the struct.**/ #define container_of(ptr, type, member) ({ \void *__mptr = (void *)(ptr); \BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \!__same_type(*(ptr), void), \"pointer type mismatch in container_of()"); \((type *)(__mptr - offsetof(type, member))); //__mptr指针为void *指针保证了减法运算的结果。
