自实现字符串操作函数汇总

题目：字符串常用操作，包括：字符串长度、比较、拷贝、连接、子串查找、子串截取（持续增加....）
分析：主要考察指针操作，小心边界溢出、越界等问题
代码：

/******************************************************************************

  Copyright (C), 2001-2011, HW Tech. Co., Ltd.

 ******************************************************************************
  File Name :
  Version :
  Author :
  Created : 2012/03/12
  Last Modified :
  Description :
  Function List :

  History :
  1.Date : 2012/03/12
    Author :
    Modification: Created file

******************************************************************************/
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

unsigned int strlenth(char *s) /* 获取字符串长度 */
{
    unsigned int lenth = 0;

    if (NULL == s)
        return lenth;

    // 代码在这里实现
    char* tmp = s;
    while (*tmp != '\0')
    {
        lenth ++;
        tmp++;
    }

    return lenth;
}

void strcopy(char **target, char *source) /* 字符串拷贝 */
{
    /* 代码在这里实现 */
    if ( (NULL == source) || (NULL == target))
        return;

    char* dest;
    char* src = source;
    int len = strlenth(source);
    dest = (char*)malloc(len+1);
    if (NULL == dest)
        return;
    memset( dest,'\0',(len+1) );
     *target = dest;

    while (*src != '\0')
    {
        *dest = *src;
        src++;
        dest++;
    }

    return;
}

int strcompare(char *s, char *t) /* 字符串比较，s>t，则返回1；s=t，则返回0；s<t，则返回-1 */
{
    /* 代码在这里实现 */
    if ( (NULL == s) || (NULL == t))
        return 0;

    char* str1 = s;
    char* str2 = t;

    while ( (*str1 != '\0') && (*str2 != '\0'))
    {
        if (*str1 > *str2)
            return 1;
        else if (*str1 < *str2)
            return -1;
        else
        {
            str1++;
            str2++;
        }
    }
    if ( (*str1 == '\0') && (*str2 == '\0') )
        return 0;
    else if ( (*str1 == '\0') && (*str2 != '\0' ) )
        return -1;
    else if ( (*str1 != '\0') && (*str2 == '\0') )
        return 1;

    return 0;
}

void strcombine(char **x, char *s, char *t) /* 字符串连接，将字符串t接到s后面，x为连接后的新串 */
{
    /* 代码在这里实现 */
    if ( (NULL == s) || (NULL == t) || (NULL == x))
        return ;

    int len_s = strlenth(s);
    int len_t = strlenth(t);
    char* dest;
    dest = (char*)malloc(len_s + len_t + 1);
    if (NULL == dest)
        return ;
    memset(dest,'\0',(len_s + len_t));

    *x = dest;

    char *tmp = s;
    while (*tmp != '\0')
    {
        *dest = *tmp;
        tmp++;
        dest++;
    }
    tmp = t;
    while (*tmp != '\0')
    {
        *dest = *tmp;
        tmp++;
        dest++;
    }
    *dest = '\0';
}

void strcatch(char *s, unsigned int index, unsigned int lenth, char **t) /* 字符串截取，从第index个字符开始，截取lenth长度的字符串，并输出到字符串t */
{
    /* 代码在这里实现 */
    unsigned int len = strlenth(s);
    if ( (NULL == s) || (len<=0) || (index<0) || (index>=len) || (NULL == t) || (lenth<=0) || (index+lenth>len) )
        return ;

    char *dest;
    dest = (char*)malloc(index+1);
    if (NULL == dest)
        return ;

    *t = dest;
    unsigned int count = 0;
    char *tmp = s;
    while ( (*tmp != '\0') && (count < index) )
    {
        count++;
        tmp++;
    }
    while ( (*tmp != '\0') && (lenth > 0) )
    {
        *dest = *tmp;
        dest++;
        tmp++;
        lenth--;
    }
    *dest = '\0';
}


bool strsubstr(char *s, char *sub) /* 字符串子串查找，如果子串sub在s中存在，则返回1，否则返回0 */
{
    bool result = 0;

    /* 代码在这里实现 */
    if ( (NULL == s) || (NULL == sub) || (strlenth(s)<strlenth(sub)) )
        return 0;
    char *tmp = s;
    char *tmp_str;

    while (*tmp != '\0')
    {
        tmp_str = sub;
        while ( (*tmp != '\0') && (*tmp_str != '\0'))
        {
            if (*tmp == *tmp_str)
            {
                tmp++;
                tmp_str++;
            }
            else
            {
                break;
            }
        }
        if ( ( (*tmp == '\0') && (*tmp_str == '\0') ) || (*tmp!='\0') && (*tmp_str == '\0'))
        {
            result =1;
            return result;
        }
        else
        {
            tmp++;
        }
    }
    return result;
}