memset memcmp memcpy memmove 自己实现
memset
#include <stdio.h>
#include <memory.h>
#include <assert.h>
void* my_memset(void* dest, int c, size_t cnt){
assert(NULL != dest);
char *a = (char*)dest;
while(cnt-- > 0){
*a++ = c;
}
return dest;
}
int main(){
int a[10];
for(int i = 0; i < sizeof(a) / sizeof(int); ++i){
printf("%d ", *(a+i));
}
printf("\n");
my_memset(a,0,sizeof(a));
for(int i = 0; i < sizeof(a) / sizeof(int); ++i){
printf("%d ", *(a+i));
}
printf("\n");
}
memcmp
#include <stdio.h>
#include <memory.h>
#include <assert.h>
int my_memcmp(const void* s1, const void* s2, size_t cnt){
assert(NULL != s1 && NULL != s2);
const char *t1 = s1;
const char *t2 = s2;
int res = 0;
while(cnt-- > 0){
if(*t1 > *t2){
res =1;
break;
}
else if(*t1 < *t2){
res = -1;
break;
}
else{
t1++;
t2++;
}
}
return res;
}
int main(){
char* s1 = "abcdaaa";
char *s2 = "abcdaa";
int res = my_memcmp(s1,s2,18);
if(0 == res)
printf("s1 == s2\n");
else if(res > 0)
printf("s1 > s2\n");
else
printf("s1 < s2\n");
int i1[] = {255,2,3,4,5};
int i2[] = {511,22,23,5};
int res1 = my_memcmp(i1,i2,1);
if(0 == res1)
printf("i1 == i2\n");
else if(res1 > 0)
printf("i1 > i2\n");
else
printf("i1 < i2\n");
}
memcpy
如果copy元的首地址 + copy的字节数的结果 >= copy先的首地址的话,就会出现覆盖,得不到正确的结果。
下面的实现结果太笨了,倒过来复制是最简单的,请看后面的memmove的实现方式。
include <stdio.h>
#include <memory.h>
#include <assert.h>
#include <malloc.h>
void* my_memcpy(void* dest, const void* src, size_t cnt){
assert(NULL != dest && NULL != src);
char *tmp = dest;
const char *st = src;
//判断元的首地址+要copy的字节数是否大于copy先的首地址
size_t bit = st + cnt - tmp;
char *tt = NULL;
char *tm = NULL;
//为了free使用
char *ta;
//如果copy的首地址大于元的首地址,并且元的首地址+要copy的字节数是否大于copy先的首地址,覆盖就会发生,所以要把将被覆盖的一段内存保存下来,先开辟空间(空间大小是bit),对tt的赋值,在后面的while里。
if(tmp > st && bit > 0){
tt = (char*)malloc(bit);
}
//由于tt被用于赋值(初始化),所以tt已经不是首地址了;当copy到被覆盖的字节的时候,要从首地址拿值,所以从tm中拿值,不从st中拿值;ta一直指向开辟空间的首地址,最后用于释放这个开辟的空间。
ta = tm = tt;
while(cnt-- > 0){
//tt不为NULL,就是说明了,将要发生覆盖,所以把要被覆盖的字节存放到tt中,但要注意不发生覆盖的字节不需要存放进去,所以加了bit-- > 0的条件
if(NULL != tt && bit-- > 0){
*tt++ = *tmp;
}
//NULL != tt说明了,是覆盖patten,并且到了要被覆盖的字节,所以不从st中取值,从tm中取值。
if(st >= (char*)dest && NULL != tt){
*tmp++ = *tm++;
st++;
}
//说明不是覆盖的patten,无脑复制就可以了。
else{
*tmp++ = *st++;
}
}
free(ta);
return dest;
}
int main(){
char s1[20] = {‘a‘,‘b‘,‘c‘,‘d‘};
char *s2 = "xyzdef";
//char *s3 = memcpy(s1+1,s1,3);
char *s3 = my_memcpy(s1+1,s1,3);
printf("s1 = [%s]\n", s1);
printf("s3 = [%s]\n", s3);
char s11[20] = {‘a‘,‘b‘,‘c‘,‘d‘};
char *s22 = "xyzdef";
char *s33 = memcpy(s11+1,s11,3);
printf("s11 = [%s]\n", s11);
printf("s33 = [%s]\n", s33);
int i1[10] = {1,2};
int i2[10] = {11,22,33,3};
int i3[10] = {1};
int *pi3 = i3;
pi3 = (int*)my_memcpy(i2+2,i2,sizeof(int) * 3);
for(int i = 0; i < sizeof(i2) / sizeof(int); ++i){
printf("i1[%d] = %d ",i, i2[i]);
}
printf("\n");
for(int i = 0; i < sizeof(i2) / sizeof(int); ++i){
printf("i3[%d] = %d ",i, pi3[i]);
}
printf("\n");
}
memmove
#include <stdio.h>
#include <memory.h>
#include <assert.h>
void* my_memmove(void* dest, const void* src, size_t cnt){
assert(NULL != dest && NULL != src);
char* tmp = dest;
const char* st = src;
//判断出是覆盖的patten,所以从后往前覆盖
if(tmp > st && st + cnt > tmp){
while(cnt-- > 0){
*(tmp + cnt) = *(st + cnt);
}
}
//判断出不是覆盖的patten,所以无脑从前往后覆盖
else {
while(cnt-- > 0){
*tmp++ = *st++;
}
}
return dest;
}
int main(){
char s1[20] = {‘a‘,‘b‘,‘c‘,‘d‘};
char *s2 = "xyzdef";
char *s3 = my_memmove(s1,s1+1,3);
printf("s1 = [%s]\n", s1);
printf("s3 = [%s]\n", s3);
char s11[20] = {‘a‘,‘b‘,‘c‘,‘d‘};
char *s22 = "xyzdef";
char *s33 = memmove(s11,s11+1,3);
printf("s11 = [%s]\n", s11);
printf("s33 = [%s]\n", s33);
int i1[10] = {1,2};
int i2[10] = {11,22,33,3};
int i3[10] = {1};
int *pi3 = i3;
pi3 = (int*)my_memmove(i2+2,i2,sizeof(int) * 3);
for(int i = 0; i < sizeof(i2) / sizeof(int); ++i){
printf("i1[%d] = %d ",i, i2[i]);
}
printf("\n");
for(int i = 0; i < sizeof(i2) / sizeof(int); ++i){
printf("i3[%d] = %d ",i, pi3[i]);
}
printf("\n");
}
原文地址:https://www.cnblogs.com/xiaoshiwang/p/9185344.html
时间: 2024-11-05 20:39:26