1.strlen

• size_t strlen(const char *s);
• 返回字符个数（不包含结尾\0）

#include<stdio.h>
#include<string.h>int mystr(const char* s)
{int cnt = 0;int index = 0;while(s[index] != '\0'){index++;cnt++;}return cnt;
}
int main()
{char s[] = "hello";printf("strlen=%lu\n",mystr(s));return 0;
}

2.strcmp

• int strcmp(const char *s1, const char *s2);
• 比较两个字符串，返回：
• 0：s1==s2
• 1：s1 > s2
• -1：s1 < s2

#include<stdio.h>
#include<string.h>int mycmp(const char *s1, const char *s2)
{int index = 0;while(*s1 == *s2 && *s1 != '\0'){s1++;   //s1与s2是地址，++相当于指向下一个元素的地址，*s1指s1所指向地址的数值s2++;}return *s1-*s2;   //返回的是最后不相同的字符的ASCII码差值/*while(s1[index] == s2[index] && s1[index] != '\0'){index++;}return s1[index] - s2[index];*/
}
int main()
{char s1[] = "abc";char s2[] = "abc";printf("%d\n",mycmp(s1,s2));return 0;
}

3.strcpy

• char* strcpy(char* restrict dst, const char* restrict src);
• 把src的字符串拷贝到dst
• restrict表明src和dst不重叠（C99）
• 返回dst

char* mycpy(char* dst, const char* src)
{
/*int index = 0;while(src[index]){dst[index] = src[index];index++;}dst[index] = '\0';return dst;*///指针版本char* ret = dst;while(*src != '\0'){*dst = *src;dst++;src++;}*dst = '\0';return ret; 
}
int main()
{char s1[] = "abc";char s2[] = "abc";mycpy(s1,s2);return 0;
}

4.strchr/strrchr:在字符串中找字符

• char* strchr(const char* s, int c);
• char* strrchr(const char* s, int c);
• 返回NULL，表示没有找到

int main()
{char s[] = "hello";char* p = strchr(s,'l');*p = '\0';printf("%s\n",p);  //llo//寻找第二个lp = strchr(p+1,'l');printf("%s\n",p);  //lo//把llo复制到另外的字符串中char* t = (char*)malloc(strlen(p) + 1);strcpy(t,p);printf("%s\n",t);  //llofree(t);//把l前面的字符串复制到另外的字符串char* q = (char*)malloc(strlen(s)+1);strcpy(q,s);printf("%s\n",q);  //hefree(q);return 0;
}

5.strstr/strcasestr:字符串中找字符串

• char* strstr(const char* s1, const char* s2 );
• char* strcasestr(const char* s1, const char* s2); //会忽略大小写

int main()
{char *s1 = "hello";char *s2 = "lo";char *p;p = strstr(s1, s2);if (p)printf("%s", p);  //loelseprintf("NotFound!");return 0;
}