C的段错误或者什么都不是

   还是照旧,先贴两段代码说明问题.

#include <stdio.h>

typedef struct{
    int a;
    int b;
    char c;
    int d;
}*p_Test;

int main(){
    printf("%#X\n",&(((p_Test)0)->d));

    printf("%#X\n",&(*((int *)0)));
    return 0;
}

对应汇编:

.file "test1.c"

.section .rodata

.LC0:

.string "%#X\n"

.text

.globl main

.type main, @function

main:

pushl %ebp

movl %esp, %ebp

andl $-16, %esp

subl $16, %esp

movl $.LC0, %eax

movl $12, 4(%esp)

movl %eax, (%esp)

call printf

movl $.LC0, %eax

movl $0, 4(%esp)

movl %eax, (%esp)

call printf

movl $0, %eax

leave

ret

.size main, .-main

.ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"

.section .note.GNU-stack,"",@progbits

另外一段为:

#include <stdio.h>

typedef struct{
    int a;
    int b;
    char c;
    int d;
}*p_Test;

int main(){
    printf("%#X\n",(((p_Test)0)->d));

    printf("%#X\n",(*((int *)0)));
    return 0;
}

对应汇编为:

.file "test1.c"

.section .rodata

.LC0:

.string "%#X\n"

.text

.globl main

.type main, @function

main:

pushl %ebp

movl %esp, %ebp

andl $-16, %esp

subl $16, %esp

movl $0, %eax

movl 12(x), %edx

movl $.LC0, %eax

movl %edx, 4(%esp)

movl %eax, (%esp)

call printf

movl $0, %eax

movl (%eax), %edx

movl $.LC0, %eax

movl %edx, 4(%esp)

movl %eax, (%esp)

call printf

movl $0, %eax

leave

ret

.size main, .-main

.ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"

.section .note.GNU-stack,"",@progbits

今天老师讲了Linux源码中include/linux/kernel.h中的container_of,代码是这样的:

/**
* 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) ({             \
         const typeof( ((type *)0)->member ) *__mptr = (ptr);     \
         (type *)( (char *)__mptr - offsetof(type,member) );})

开始不太理解,他以0作为指针地址去访问一个结构,难道不会发生段错误吗?后来自己又写了个测试,发现如果用任意数作为的指针不是进行实质性的内存访问的话,那么编译器是允许的,怎么说?看上面对照的汇编代码就知道了,编译器在编译成汇编代码的时候就已经把值计算出来了,而不是在运行时作为内存地址偏移进行操作,所以...哎,我也说不清楚,反正这些细节,自己长点记性...按老师那句话,领会精神!