常见字符串函数的模拟实现（一）(c语言)

1.三种方式模拟实现strlen函数
2.模拟实现strcpy
3.模拟实现strcat
4.模拟实现strcmp
5.模拟实现strstr
6.模拟实现memcpy
7.模拟实现memmove

//模拟实现strlen（求字符串长度）// 1>指针int my_strlen(char const*s){    char*p = s;    while (*p != 0)    {        p++;    }    return p - s;}int my_strlen(char s[]){    int len = 0;    while (*s++ != 0)    {        len++;    }    return len;}// 2>函数递归int my_strlen(char*s){    if (*s == '\0')        return 0;    else        return 1 + my_strlen(s + 1);}// 3>计数int my_strlen(char s[]){    int i = 0;    int count = 0;    while (s[i++] != '\0')        count++;    return count;}

//模拟实现strcpy （拷贝字符串）char* my_strcpy(char*dest, char const*src){    char* ret = dest;    assert(src!=NULL);    assert(dest != NULL);    while (*src)    {        *dest++ = *src++;    }    *dest = '\0';    return ret;}

//模拟实现strcat（字符串链接）char* my_strcat(char*dest, const char* src){    char* ret = dest;    assert(*dest != 'NULL');    assert(*src != 'NULL');    while (*dest != '\0')        dest++;    while (*dest++ = *src++)//将src链接到dest上    {        ;    }    return ret;}

//模拟实现strcmp（字符串比较）int my_strcmp(const char*src1, const char*src2){    //判断两个源字符串都不为空    assert(*src1 != '\0');    assert(*src2 != '\0');                      while (!(*src1 - *src2) && src2)    {        *src1++;        *src2++;    }    return*src1 - *src2;}

//模拟实现strstr(查找子串)char* my_strstr(const char*src, const char*sub){    assert(src);    assert(sub);    char*p = src;    char*s1 = p;    char*s2 = sub;    while (*s1)    {        s1 = p;        s2 = sub;        while ((*s1 != '\0') && (*s2 != '\0'))        {            if (*s1++ == *s2++)            {                ;            }            else            {                p++;                break;            }            if (*s2 == '\0')            {                return p;            }        }    }}

//模拟实现memcpy(不考虑内存重叠时的内存拷贝)void my_memcpy(void*dest, const void*src, int count){    char *psrc, *pdest;    psrc = (char *)src;    pdest = (char *)dest;    while (count--)     {        *pdest++ = *psrc++;    }}

//模拟实现memmove(复制内存)void* my_memmove(void*dest, const void*src, int count){    char*psrc;    char*pdest;    if ((NULL ==dest) || (NULL == src))    {        return NULL;    }    if ((src<dest) && (char*)src + count>(char*)dest)    {        psrc = (char*)src + count - 1;        pdest = (char*)dest + count - 1;        while (count--)        {            *pdest-- = *psrc--;        }    }    else    {        psrc = (char*)src;        pdest = (char*)dest;        while (count--)        {            *pdest++ = *psrc++;        }    }    return dest;}

以上代码均已运行测试过，如有错误，忘指出。