欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  IT编程

memset memcmp memcpy memmove 自己实现

程序员文章站 2022-07-07 20:57:38
memset memcmp memcpy memmove 自己实现 memset c include include include include void my_memcpy(void dest, const void src, size_t cnt){ assert(NULL != dest ......

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");
}