《C标准库》——之<string.h>

<string.h>里的字符串操作函数是经常要用到的，因此阅读了源码后自己实现了一些：

拷贝函数

void * Mymemcpy(void * sDst, const void * sSrc, size_t count)
{
	assert(NULL != sDst && NULL != sSrc);
	char* saveDst			=	(char *)sDst;
	const char* saveSrc		=	(char *)sSrc;

	for (; 0 < count; ++saveDst, ++saveSrc, --count)
		*saveDst = *saveSrc;
	
	return (void*)sDst;
}

void * Mymemmove(void* sDst, const void* sSrc, size_t count)
{
	assert(NULL != sDst && NULL != sSrc);
	char* saveDst			=	(char *)sDst;
	const char* saveSrc		=	(char *)sSrc;

	if ((saveDst > saveSrc) && (saveDst < saveSrc + count))
		
		for (saveDst += count, saveSrc += count; 0 < count; --count) // copy backwards
			*--saveDst = *--saveSrc;
	else

		for (; 0 < count; ++saveDst, ++saveSrc, --count) // copy forwards
			*saveDst = *saveSrc;

	return (void *)sDst;
}

char * Mystrcpy(char* sDst, const char* sSrc)
{
	assert(NULL != sDst);
	char* saveDst			=	sDst;
	for (; '\0' != (*saveDst++ = *sSrc++);)
		;
	return (char *)sDst;
}

char * Mystrncpy(char* sDst, const char* sSrc, size_t count)
{
	assert(NULL != sDst && NULL != sSrc);
	char* saveDst			=	sDst;

	for (; (0 < count) && ('\0' != *sSrc); --count)
		*saveDst = *sSrc;

	for (; 0 < count; --count)
		*saveDst++ = '\0';

	return (char *)sDst;

}

void * Mymemset(void* sDst, int ch, size_t count)
{
	unsigned char* saveDst	=	(unsigned char *)sDst;
	unsigned char c			=	ch;

	for (; (0 < count) && ('\0' != *saveDst); ++saveDst, --count)
		*saveDst = c;
	
	return (void *)sDst;
}

查找函数

void * Mymemchr(const void* str, const int ch, size_t n)
{
	assert(NULL != str);
	const unsigned char	  c		=	ch;
	const unsigned char * ssave	=	(unsigned char *)str;

	for (; ('\0' != *ssave) && (0 < n); --n, ++ssave)
		if (c == *ssave)
			return (void*)ssave;

	return NULL;
}

char * Mystrchr(const char* str, int ch)
{
	assert(NULL != str);
	const unsigned char	c	=	ch;

	for (; '\0' != *str; ++str)
		if ('\0' == *str)
			return NULL;

	return (char*)str;	
}

char * Mystrrchr(const char* str, int ch)
{
	assert(NULL != str);
	const unsigned char c	=	ch;
	const char * ssave		=	NULL;

	for (; '\0' != *str; ++ str)
		if (c == *str)
			ssave = str;

	return (char *)ssave;
}

char * Mystrstr(const char* str1, const char * str2)
{
	assert(NULL != str1 && NULL != str2);
	if ('\0' == *str2)
		return (char*)str1;

	for (; NULL != (str1 = Mystrchr(str1, *str2)); ++str1)
	{
		const char * ssave1	=	str1;
		const char * ssave2	=	str2;

		for (;;)
		{
			if ('\0' == *++ssave2)
				return (char*)str1;
			else
				if (*++ssave1 != *ssave2)
					break;
		}

	}

	return NULL;
}

连接函数

char * Mystrcat(char * sDst, const char * sSrc)
{
	assert(NULL != sDst && NULL != sSrc);
	char * ssave	=	sDst;

	for (; '\0' != *ssave; ++ssave)
		;

	for (; '\0' != (*ssave = *sSrc); ++ssave, ++sSrc)
		;

	return (char*)sDst;
}

char * Mystrncat(char * sDst, const char * sSrc, size_t count)
{
	assert(NULL != sDst && NULL != sSrc);
	char* ssave	=	sDst;

	for (; '\0' != *ssave; ++ssave)
		;
	
	for (; ('\0' != *sSrc) && (0 < count); --count)
		*ssave++ = *sSrc++;

	*ssave = '\0';

	return (char*)sDst;
}

比较函数

int Mymemcmp(const void* str1, const void * str2, size_t n)
{
	assert(NULL != str1 && NULL != str2);
	const unsigned char * ssave1	=	(unsigned char *)str1;
	const unsigned char * ssave2	=	(unsigned char *)str2;

	for (; 0 < n; ++ssave1, ++ssave2, --n)
	{
		if (*ssave1 != *ssave2)
			return ((*ssave1 < *ssave2) ? -1 : 1);
		else if ('\0' == *ssave1)
			return (0);

	}
	return 0;
}

int Mystrcmp(const char* str1, const char * str2)
{
	assert(NULL != str1 && NULL != str2);
	for (; *str1 == *str2; ++str1, ++str2)
		if('\0' == *str1 )
			return 0;

	return ((*str1 < *str2) ? -1 : 1);
}

int Mystrncmp(const char* str1, const char * str2, size_t n)
{
	assert(NULL != str1 && NULL != str2);
	for (; 0 < n; ++str1, ++str2, --n)
		if (*str1 != *str2)
			return ((*str1 < *str2)? -1 : 1);
		else if ('\0' == *str1)
			return 0;

	return 0;
}

int memcmp(const void* sSrc1, const void* sSrc2, size_t num)
{
    assert (NULL != sSrc1 && NULL != sSrc2);

    const unsigned char* s1 = (const char*)sSrc1;
    const unsigned char* s2 = (const char*)sSrc2;

    for (;0 < num; ++s1, ++s2, --num)
    {
        if (*s1 != *s2)
            return ((*s1 > *s2) ? 1 : -1);
    }
    return 0;
}

计算字符串长度

size_t strlen(const char* sSrc)
{
    assert (NULL != sSrc);

    const char* s = sSrc, *sv = s;

    while (*sv != '\0')
        ++sv;
    return (size_t)(sv - s);
}