memset memcmp memcpy memmove 自己实现

#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");
}

#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");

}

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");

}

#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");
}