memcpy字符变成了乱码

[hzfushi]

struct A
{
    char *str;
};

typedef struct A TypeA

void fun(void *p)
{
   TypeA b;
   b.str = "Turn";
   memcpy((unsigned char*)p,¶m,sizeof(TypeA));
}

我在使用上面的代码时，执行完后，p指向的存储单元的字符变成了乱码，请问是什么原因。

[HelloDan]

void * memcpy ( void * destination, const void * source, size_t num );



<cstring>

Copy block of memory

Copies the values of num bytes from the location pointed by source directly to the memory block pointed by destination.

The underlying type of the objects pointed by both the source and destination pointers are irrelevant for this function; The result is a binary copy of the data.

The function does not check for any terminating null character in source - it always copies exactly num bytes.

To avoid overflows, the size of the arrays pointed by both the destination and source parameters, shall be at least num bytes, and should not overlap (for overlapping memory blocks, memmove is a safer approach).

Parameters

destination
Pointer to the destination array where the content is to be copied, type-casted to a pointer of type void*.
source
Pointer to the source of data to be copied, type-casted to a pointer of type void*.
num
Number of bytes to copy.



Return Value
destination is returned.

明显是你用得不对。

[goodname]

结构体A里面的str是一个指针变量，在32为系统上只是4个字节而已。你的sizeof(TypeA)就是4.
你拷贝的并不是str字符串的内容。而是将指针的值拷贝走了。

[mengjfu]

p指向那啊？
还是用数组或动态分配吧

[HelloDan]

num
Number of bytes to copy.


¶m 这个哪来？

sizeof(TypeA)---> strlen(b.str);

[tanadar]

#include <stdio.h>
#include <string.h>

struct A
{
    char *str;
};

typedef struct A TypeA;

void fun(void *p)
{
   TypeA b;
   unsigned char *temp = (unsigned char*)p;
   b.str = "Turn";
   memcpy(temp, b.str, strlen(b.str));
   temp[strlen(b.str)] = 0;
}

main()
{
	unsigned char dst[32];
	fun(dst);
	printf("%s\n", dst);
}

[goodname]

memcpy(temp, b.str, strlen(b.str)+1);//多拷贝一个字节就行。

[hai040]

fun改不了p
void fun(void** pp)

[wanggang_0717]

guanzhu

[nosxcy]

memcpy((unsigned char*)p,¶m,sizeof(TypeA));
你把param的地址copy 到p指向的内存了,而没把param的内容copy过去,

PS:顶6楼的

[ware_soft]

看不懂。

[Fleeboy]

顶9楼的

[tanadar]

[Quote=引用 6 楼 goodname 的回复:]
memcpy(temp, b.str, strlen(b.str)+1);//多拷贝一个字节就行。

[/Quote]吹毛求疵，咋不说用strcpy呢？这个更直接。

[breezes2008]

memcpy((unsigned char*)p,¶m,sizeof(TypeA));//浅拷贝，只是拷贝指针的值。

[goodname]

[Quote=引用 12 楼 tanadar 的回复:]
引用 6 楼 goodname 的回复:
memcpy(temp, b.str, strlen(b.str)+1);//多拷贝一个字节就行。

吹毛求疵，咋不说用strcpy呢？这个更直接。
[/Quote]

楼主给出的代码中使用的是memcpy，人家也许就想用这个方法来实现呢？我这么说只是想尽量贴近他的原文，而不是改变。

[todaylxp]

1.p很有可能没有初始化
2.拷贝的长度不对，应该是strlen(b.str)
所以，未被拷贝所覆盖的内存保留了初始值,显示是乱码

[文刀劉Michael]

回帖是好事