实现常用C语言字符串处理函数

1.strcpy()

char *my_strcpy(char *s1, const char *s2)
{
    
    char* p1 = s1;
    const char* p2 = s2;
    while (*p1++ = *p2++)
    {
    
    }
    return s1;
}

2.strlen()

unsigned int my_strlen(const char* s)
{
    
    const char* p = s;
    while (*p++)
    {
    
    }
    return p - s - 1;
}

3.strcat()

char* my_strcat(char* s1, const char* s2)
{
    
    char* p1 = s1;
    const char* p2 = s2;
    unsigned int len = strlen(s1);
    p1 += len;
    while (*p1++ = *p2++)
    {
    
    }
    return s1;
}

4.strcmp()

int my_strcmp(const char* s1, const char* s2)
{
    
    const char* p1 = s1;
    const char* p2 = s2;

    while (*p1 || *p2)
    {
    
        if (*p1 - *p2)
        {
    
            return *p1 - *p2 > 0 ? 1 : -1;
        }
        p1++;
        p2++;
    }
    return 0;
}

5.atoi()

int my_atoi(const char* nptr)
{
    
    const char* p = nptr;
    int sign = 1;
    int num = 0;
    while (*p == ' ')
    {
    
        p++;
    }
    sign = (*p == '-') ? -1 : 1;
    if (*p == '+' || *p == '-')
    {
    
        p++;
    }
    while (*p >= '0' && *p <= '9')
    {
    
        num = num * 10 + *p - '0';
        p++;
    }
    return num*sign;
}

6.itoa()

char* my_itoa(int value, char* str, int base)
{
    
    char* p = str;
    int reverse_start = 0;
    if (value < 0)
    {
    
        reverse_start = 1;
        *p++ = '-';
        value = -value;
    }
    else if (value == 0)
    {
    
        *p++ = '0';
    }
    while (value)
    {
    
        *p++ = value % base + '0';
        value /= base;
    }
    *p = '\0';
    int i, j;
    for (i = reverse_start, j = strlen(str) - 1; i < j; i++, j--)
    {
    
        char tmp = str[i];
        str[i] = str[j];
        str[j] = tmp;
    }
    return str;
}

7.memmove()

void* my_memmove(void* dest, void* src, size_t num)
{
    
    char* psrc = (char*)src;
    char* pdest = (char*)dest;
    
    if (pdest <= psrc || pdest > psrc + num)
    {
    
        for (size_t i = 0; i < num; i++)
        {
    
            *pdest++ = *psrc++;
        }
    }
    else
    {
    
        pdest += num;
        psrc += num;
        for (size_t i = 0; i < num; i++)
        {
    
            *(--pdest) = *(--psrc);
        }
    }
    return dest;
}