C语⾔内存函数

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。

• 这个函数在遇到 '\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。

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

memcpy函数的模拟实现:

void * memcpy ( void * dst, const void * src, size_t count)
{
 void * ret = dst;
 assert(dst);
 assert(src);
 /*
 * copy from lower addresses to higher addresses
 */
 while (count--) {
 *(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的。

• 如果源空间和⽬标空间出现重叠，就得使⽤memmove函数处理。

#include 
#include 
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 ", arr2[i]);
 }
return 0;
}

输出的结果：

1 2 1 2 3 4 5 8 9 10

memmove的模拟实现：

void * memmove ( void * dst, const void * src, size_t count)
{
 void * ret = dst;
 if (dst <= src || (char *)dst >= ((char *)src + count)) {
 /*
 * Non-Overlapping Buffers
 * copy from lower addresses to higher addresses
 */
 while (count--) {
 *(char *)dst = *(char *)src;
 dst = (char *)dst + 1;
 src = (char *)src + 1;
 }
 }
 else {
 /*
 * Overlapping Buffers
 * copy from higher addresses to lower addresses
 */
 dst = (char *)dst + count - 1;
 src = (char *)src + count - 1;
 while (count--) {
 *(char *)dst = *(char *)src;
 dst = (char *)dst - 1;
 src = (char *)src - 1;
 }
 }
 return(ret);
}

3. memset 函数的使⽤

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

memset是⽤来设置内存的，将内存中的值以字节为单位设置成想要的内容。

#include 
#include 
int main ()
{
 char str[] = "hello world";
 memset (str,'x',6);
 printf(str);
 return 0;
}

输出的结果：

xxxxxxworld

4. memcmp 函数的使⽤

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

#include 
#include 
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);
 else printf("'%s' is the same as '%s'.\n", buffer1, buffer2);
 return 0;
}