函数的模拟实现

#include <stdio.h>
void my_strncpy(char* arr2, char* arr1, size_t num)
{
  int i = 0;
  for (i = 0; i < num; i++)
  {
    *(arr2 + i) = *(arr1 + i);
  }
}
int main()
{
  char arr1[] = "hello liangzai";
  char arr2[10] = { 0 };
  //strncpy(arr2,arr1,sizeof(arr2));
  my_strncpy(arr2, arr1, sizeof(arr2));
  return 0;
}

char * mystrncpy(char * dst, const char * src, size_t n)
{
  int i;
  for (i = 0; src[i] && i < n; i++)
  {
    dst[i] = src[i];
  }
   
  if (i < n)
  {
    dst[i] = 0;
  }
  return dst;
}

#include <stdio.h>
#include <string.h>
char* my_strncat(char* arr1, const char* arr2, size_t num)
{
  int sz = strlen(arr1);
  int i = 0;
  for (i = 0; i < num; i++)
  {
    *(arr1 + sz + i) = *(arr2 + i);
  }
  return arr1;
}
int main()
{
  char arr2[] = "shawanyi ";
  char arr1[20] = "hello ";
  //strncat(arr1,arr2,10);
  my_strncat(arr1, arr2, 10);
  return 0;
}

char * mystrncat(char * dst, const char * src, size_t n)
{
  char * tmp = dst;
   
  while (*dst)
  {
    dst++;//记录到dst最后一个位置
  }
   
  int i;
  for (i = 0; src[i] && i < n; i++)
  {
    dst[i] = src[i];
  }
   
  dst[i] = 0;
  return tmp;
}

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <ctype.h>
int my_atoi(const char* arr1)
{
  assert(arr1);
  int sum = 0;
  if (*(arr1) == '\0')
  {
    return 0;
  }
  while(isspace(*arr1))
  {
    arr1++;
  }
  int flat = 1;
  if (*arr1 == '+')
  {
    arr1++;
  }
  else if (*arr1 == '-')
  {
    flat = -1;
    arr1++;
  }
  int ret = 0;
  while (isdigit(*arr1))
  {
    ret = ret * 10 + (*arr1 - '0');
    if (ret > INT_MAX)
    {
      return INT_MAX * flat;
    }
    else if (ret < INT_MIN)
    {
      return INT_MIN * flat;
    }
    arr1++;
  }
  if (isdigit(*arr1) == 0)
  {
    return ret * flat;
  }
  return ret*flat;
}
int main()
{
  char arr1[] = "-123666a44";
  //int n = atoi(arr1);
  int sz = 0;
  sz = my_atoi(arr1);
  return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
//VALID 表示结果合法
//INVALID 表示结果非法
enum State
{
 VALID,//0
 INVALID//1
};
//默认结果可能非法，当正确转换后再改为合法
enum State state = INVALID;
int my_atoi(char *str)
{
 int flag = 1;
 long long ret = 0;
 assert(str);
 state = INVALID;
  //跳过空白字符
 while(isspace(*str))
 {
  str++;
 }
 if(*str == '\0')
 {
  return 0;
 }
  //跳过正负号
 if(*str == '+')
 {
  str++;
 }
 else if(*str == '-')
 {
  flag = -1;
  str++;
 }
  //开始转换数字字符直到非数字字符
 while(isdigit(*str))
 {
  ret = ret * 10 + flag * (*str-'0');
  if((ret > INT_MAX) || (ret < INT_MIN))
  {
   return 0;
  }
  str++;
 }
  //正常停止
 if(*str == '\0')
 {
  state = VALID;
  return (int)ret;
 }
 else
 {
    //遇到非数字字符
  return (int)ret;
 }