C语言的字符函数和字符串函数详解

int my_strlen(const char *str)
{
	int count = 0;
	while(*str)
	{
		conut++;
		str++;
	}
	return count;
}

//不能创建临时变量计数器，下面用递归的方式
int my_strlen(const char *str)
{
	if(*str == '\0')
	return 0;
	else
	return 1+my_strlen(str1);
}

//指针-指针
int my_strlen(const char *str)
{
	char *p = str;
	while(*p != '\0')
	{
		p++;
	}
	return p-str;
}	

//1.参数顺序
//2.函数的功能，停止条件
//3.assert
//4.const修饰指针
//5.函数返回值
//6.题目出自《高质量C/C++编程》最后的试题部分
char *my_strcpy(char *dest, const char *src)
{
	char *ret = dest//记录目标空间的起始地址
	asset(dest && src)//dest，src不能为NULL
	while((*dest++ = *src++))
	{
		;
	}
	return ret;
}

char *my_strcat(char *dest, const char *src)
{
	char *ret = dest;
	assert(dest && src);
	//找到目标空间的'\0'
	while(*dest)
	{
		dest++;
	}
	//追加内容
	while((*dest++ = *src++))
	{
		;
	}
	return ret;
}

int my_strcmp(char *str1, const char *str2)
{
	assert(str1 && str2);
	while(*str1 == *str2)
	{
		if(*str1 == '\0')
		return 0;
		str1++;
		str2++;
	}
	return *str1-*str2;
}

#include <stdio.h>
#include <string.h>
int main ()
{
  char str1[20];
  char str2[20];
  strcpy (str1,"To be ");
  strcpy (str2,"or not to be");
  strncat (str1, str2, 6);
  puts (str1);
  return 0;
}

* strncmp example */
#include <stdio.h>
#include <string.h>
int main ()
{
  char str[][5] = { "R2D2" , "C3PO" , "R2A6" };
  int n;
  puts ("Looking for R2 astromech droids...");
  for (n=0 ; n<3 ; n++)
    if (strncmp (str[n],"R2xx",2) == 0)
    {
      printf ("found %s\n",str[n]);
    }
  return 0;
}

#include <stdio.h>
#include <string.h>
int main ()
{
  char str[] ="This is a simple string";
  char * pch;
  pch = strstr (str,"simple");
  if (pch != NULL)
    strncpy (pch,"sample",6);
  puts (str);
  return 0;
}

char *my_strstr(const str, const substr)
{
	const char *s1 = str;
	const char *s2 = substr;
	const char *cur = str;
	assert(str && substr);
	if(*substr == '\0')
		return str;
	while(*cur)
	{
		s1 = cur;
		s2 = substr;
		while(*s1 && *s2 && *s1==*s2)
		{
			s1++;
			s2++;
		}
		if(*s1=='\0')
			return cur;
		cur++;
	}
	return NULL
}		

#include <stdio.h>
#include <string.h>
int main ()
{
  char str[] ="- This, a sample string.";
  char * pch;
  printf ("Splitting string \"%s\" into tokens:\n",str);
  pch = strtok (str," ,.-");
  while (pch != NULL)
  {
    printf ("%s\n",pch);
    pch = strtok (NULL, " ,.-");
  }
  return 0;
}

#include <stdio.h>
#include <string.h>
#include <errno.h>
int main ()
{
  FILE * pFile;
  pFile = fopen ("unexist.ent","r");
  if (pFile == NULL)
    printf ("Error opening file unexist.ent: %s\n",strerror(errno));
  return 0;
}

int tolower (int c);
int toupper (int c);

#include <stdio.h>
#include <ctype.h>
int main ()
{
  int i=0;
  char str[]="Test String.\n";
  char c;
  while (str[i])
 {
    c=str[i];
    if (isupper(c)) 
        c=tolower(c);
    putchar (c);
    i++;
 }
  return 0;
}

void *my_memcpy(void *dest, void *src, size_t num)
{
	assert(dest && src);
	void *ret = dest;
	while(num--)
	{
		*(char *)dest= *(char *)src;
		dest=(char *)dest + 1;
		src=(char *)src + 1:
	}
	return ret;
}

void *my_memmove(void *dest, const void *src, size_t num)
{
	void *ret = dest;
	assert(dest && src);
	if(dest < src)
	{
		while(num--)
		{
			*(char *)dest = *(char *)src;
			dest = *(char *)dest + 1;
			src = *(char *)src +1;
		}
	else
	{
		while(num--)
		{
			*((char *)dest+num) = *((char *)src+num);
		}
	}
	return ret;
}	

#include <stdio.h>
#include <string.h>
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;
}