结构体中带有指针，使用memset()之后，使用指针的问题

[w_shun]

struct{
int a;
unsigned char *p;
}

定义一个结构体变量，st_test;若使用memset(&st_test,0,sizeof(st_test)),然后 *st_test.p=1;*(st_test+1)=2，会出现什么问题，memset()之后，p是空指针么？

[shadowlsj]

貌似我上不了圖.

unhandled exception at 0x004113d4 in study.exe: 0xc0000005:access violation wrinting location 0x00000000.

給空地址賦值。。。。會死很慘的。

第二個都不知道指哪去了，所以編譯都過不了。

[zhutianjin4355255]

效果跟直接用指针差不多，不管memset多少，指针都为空，自己要重新分配内存

[qq675927952]

memset()之后 p=0; 因为没有对p申请空间，p没有指向任何可以指向的地方，所以*st_test.p=1 就会出错。。。 但是 st_test.p ＝1 则 不会出错

[walkersfaint]

没有空间，直接给值，崩溃到天荒地老

[xbt746]

mark一下
坐等高人來回答

[Csuxiaowu]

#include <iostream>
#include <cstring>
using namespace std;
struct T
{
    int a;
    unsigned char *p;
};
int main()
{
    T test;
    if(test.p==NULL)
    cout<<"yes"<<endl;
    else
    cout<<"no"<<endl;
    memset(&test,0,sizeof(test));
    if(test.p==NULL)
    cout<<"yes"<<endl;
    else
    cout<<"no"<<endl;
}

[猫已经找不回了]

[Quote=引用楼主 w_shun 的回复:]
struct{
int a;
unsigned char *p;
}

定义一个结构体变量，st_test;若使用memset(&st_test,0,sizeof(st_test)),然后 *st_test.p=1;*(st_test+1)=2，会出现什么问题，memset()之后，p是空指针么？
[/Quote]

一开始初始化，就把p指针置为0了，然后*st_test.p=1;这个肯定崩溃了，p是个空指针。

typedef struct sT{
  int a;
  unsigned char *p;
}st;



int main()
{
    st st_test;
    memset(&st_test,0,sizeof(st_test));
    printf("p:%p",st_test.p);
    *st_test.p=1;
    return 0;
}

[shadowlsj]

感覺會崩潰的死去活來，下午虐下機子，直接把現象上圖。

[zrcvic]

memset 之后，p 是一个内部表示为 全 ‘0’ bit 的指针。根据 C 语言标准，不一定是一个 NULL 指针。

不过在一般的平台上，内部表示为全‘0’的指针就是 NULL 指针。

[FrankHB1989]

一般是，不过不一定。
http://www.lysator.liu.se/c/c-faq/c-1.html
1.4: How should NULL be #defined on a machine which uses a nonzero bit pattern as the internal representation of a null pointer?

Programmers should never need to know the internal representation(s) of null pointers, because they are normally taken care of by the compiler. If a machine uses a nonzero bit pattern for null pointers, it is the compiler's responsibility to generate it when the programmer requests, by writing "0" or "NULL," a null pointer. Therefore, #defining NULL as 0 on a machine for which internal null pointers are nonzero is as valid as on any other, because the compiler must (and can) still generate the machine's correct null pointers in response to unadorned 0's seen in pointer contexts.
另外，ISO C99 6.3.2.3 Pointers：
3 An integer constant expression with the value 0, or such an expression cast to type
void *, is called a null pointer constant.55) If a null pointer constant is converted to a
pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal
to a pointer to any object or function.

[luciferisnotsatan]

直接崩溃

win下应该报访问违规
linux 报段错误