strstr函数

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/*
包含文件：string.h
函数名: strstr
函数原型：extern char *strstr(char *str1, char *str2);
功能：从字符串str1中查找是否有字符串str2，如果有，从str1中的str2位置起，返回str1中str2起始位置的指针，如果没有，返回null。
如果s2为空串，则返回s1
返回值：返回该位置的指针，如找不到，返回空指针。
*/

#include <stdio.h>

//Copyright 1986 - 1999 IAR Systems. All rights reservedchar *strstr(const char *s1, const char *s2){int n;if (*s2){//s2不是空字符串while(*s1){//开始遍历s1for(n=0; *(s1+n) == *(s2+n); n++){if (!*(s2+n+1)){//如果n值为s2的长度,说明s2是s1的字串return (char*)s1;}}s1++; //没有找到，指针下移}return NULL;   //s2不是s1的字串}return (char*)s1;//s2为空串，返回s1}void find_str(char const* str, char const* substr) {    char* pos = strstr(str, substr);    if(pos) {        printf("found the string '%s' in '%s' at position: %d\n", substr, str, pos - str);    } else {        printf("the string '%s' was not found in '%s'\n", substr, str);    }} int main(int argc, char* argv[]) {    char* str = "one two three";    find_str(str, "two");    find_str(str, "");    find_str(str, "nine");    find_str(str, "n");      return 0;}

1.Copyright 1990 Software Development Systems, Inc.char *strstr( const char *s1, const char *s2 ){　int len2;　if ( !(len2 = strlen(s2)) )　return (char *)s1;　for ( ; *s1; ++s1 )　{　  　if ( *s1 == *s2 && strncmp( s1, s2, len2 )==0 )  　return (char *)s1;　}　return NULL;}

GCC-4.8.0char *　strstr (const char *s1, const char *s2)　{  　const char *p = s1;    　const size_t len = strlen (s2);　  for (; (p = strchr (p, *s2)) != 0; p++)  　{    　if (strncmp (p, s2, len) == 0)    　return (char *)p;   　}      return (0);　}

char *strstr(char *s1 , char *s2){  if(*s1==0)  {    if(*s2) return(char*)NULL;    return (char*)s1;  }  while(*s1)  {    int i=0;    while(1)    {      if(s2[i]==0) return s1;      if(s2[i]!=s1[i]) break;      i++;    }    s1++;  }  return (char*)NULL;}

//linux源码#ifndef __HAVE_ARCH_STRSTR   /**  * strstr - Find the first substring in a %NUL terminated string  * @s1: The string to be searched  * @s2: The string to search for  */  char *strstr(const char *s1, const char *s2)  {      size_t l1, l2;        l2 = strlen(s2);      if (!l2)          return (char *)s1;      l1 = strlen(s1);      while (l1 >= l2) {          l1--;          if (!memcmp(s1, s2, l2))              return (char *)s1;          s1++;      }      return NULL;  }  EXPORT_SYMBOL(strstr);  #endif