C语言内存函数

目录

1. memcpy 使用和模拟实现

2. memmove 使用和模拟实现

3. memset 函数的使用

4. memcmp 函数的使用

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1. memcpy 使用和模拟实现

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

• 函数memcpy从source的位置开始向后复制num个字节的数据到destination指向的内存位置
• 函数在遇到 '\0' 的时候并不会停下来
• 如果source和destination有任何的重叠，复制的结果都是未定义的（对于重叠的内存交给memmove来处理）

#include <stdio.h>
#include <string.h>int main()
{int arr1[] = { 1,2,3,4,5,6,7,8,9,10 };int arr2[10] = { 0 };memcpy(arr2, arr1, 20);//20字节int i = 0;for (i = 0; i < 10; i++){printf("%d ", arr2[i]);}return 0;
}

模拟实现:

void * memcpy ( void * dst, const void * src, size_t count)
{void * ret = dst;assert(dst);assert(src);
//copy from lower addresses to higher addresseswhile (count--) //以1字节复制{*(char *)dst = *(char *)src;dst = (char *)dst + 1;src = (char *)src + 1;}return(ret);
}

2. memmove 使用和模拟实现

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

和 memcpy 函数的差别就是 memmove 函数处理的源内存块和目标内存块是可以重叠，如果源空间和目标空间出现重叠，可以使用 memmove 函数处理

#include <stdio.h>
#include <string.h>int main()
{int arr1[] = { 1,2,3,4,5,6,7,8,9,10 };memmove(arr1+2, arr1, 20);int i = 0;for (i = 0; i < 10; i++){printf("%d ", arr1[i]);//1 2 1 2 3 4 5 8 9 10}return 0;
}

模拟实现:

void * memmove ( void * dst, const void * src, size_t count)
{void * ret = dst;if (dst <= src || (char *)dst >= ((char *)src + count))//从前向后复制{//Non-Overlapping Buffers//copy from lower addresses to higher addresseswhile (count--){*(char *)dst = *(char *)src;dst = (char *)dst + 1;src = (char *)src + 1;}}else //从后向前复制{// Overlapping Buffers//copy from higher addresses to lower addresses//方式一dst = (char *)dst + count - 1;src = (char *)src + count - 1;while (count--) {*(char *)dst = *(char *)src;dst = (char *)dst - 1;src = (char *)src - 1;//方式二// while (count--) //*((char*)+count)=*((char *)src+count);}
}    return(ret);
}

3. memset 函数的使用

void * memset ( void * ptr, int value, size_t num );

memset 函数是将内存中的值以字节为单位设置成想要的内容

#include <stdio.h>
#include <string.h>int main ()
{char str[] = "hello world";memset (str,'x',6);printf(str);//xxxxxworldreturn 0;
}

4. memcmp 函数的使用

int memcmp ( const void * ptr1, const void * ptr2, size_t num );

比较从ptr1和ptr2指针指向的位置开始向后的num个字节

#include <stdio.h>
#include <string.h>
int main()
{char buffer1[] = "DWgaOtP12df0";char buffer2[] = "DWGAOTP12DF0";int n;n = memcmp(buffer1, buffer2, sizeof(buffer1));if (n > 0)printf("'%s' is greater than '%s'.\n", buffer1, buffer2);else if (n < 0)printf("'%s' is less than '%s'.\n", buffer1, buffer2);elseprintf("'%s' is the same as '%s'.\n", buffer1, buffer2);//'DWgaOtP12df0' is greater than 'DWGAOTP12DF0'.return 0;
}

谢谢观看