C常用库函数实现

Posted 2021-12-29 Frey_Liu

void *memcpy(void* dest, const void* src, size_t n)

    assert(dest != NULL && src != NULL && n > 0);

    if ((src < dest) && ((char*)src + n > dest)) 
        char* d = (char*)dest+n-1;
        char* s = (char*)src+n-1;

        while (n--) 
            *d-- = *s--;
        
     else 
        char* d = (char*)dest;
        char* s = (char*)src;

        while (n--) 
            *d++ = *s++;
        
    
    return dest;

// unsafe function
void *memcpy(void *dest, const void *src, size_t count)

    if(dest == NULL || src == NULL || count <= 0)  return NULL;
    char *d = (char *)dest;
    char *s = (char *)src;
    while(count--)
    
        *d = *s;
		d++;
		s++;
    
    return dest;


void *memmove(void *dest, const void *src, size_t count)

    if(dest == NULL || src == NULL || count <= 0)  return NULL;

    if(dest < src)
    
        char *d = (char *)dest;
        char *s = (char *)src;
        while (count--)
        
            *d++ = *s++;
        
    
    else
    
        char *d = (char *)dest + count;
        char *s = (char *)src + count;
        while (count--)
        
            *--d = *--s;
        
    
    return dest;


void* memset(void* src, int c, size_t n)

    assert((src != NULL) && (n > 0));

    char* psrc = (char*)src;

    while (n--)
	
        *psrc++ = (char)c;
    
    return src;


// unsafe function
char* strcpy(char *dest, char *src)

    if(dest == NULL || src == NULL)
        return NULL;

    char *res = dest;
    while(*src != '\\0')
    
        *dest = *src;
        dest++;
        src++;
    
    *dest = '\\0';
    return res;


// n <= strlen(dest)
char* strncpy(char* dest, const char* src, size_t n)

    assert(dest != NULL && src != NULL && n > 0);
	char *pdst = dest;

    if (src < dest && src + n > dest)  // strlen(src) > strlen(dest), copy first n of src to dest
        pdst = dest + n - 1;
        src = src + n -1;

        while (n) 
            *pdst-- = *src--;
			n--;
        
     else 
		while (*src && n) 
			*pdst++ = *src++;
			n--;
		
		while (n--) 
			*pdst++ = '\\0';
		
    

    return dest;


char *strcat(char *dest, char *src)

	assert(dest != NULL && src != NULL);

    char *head = dest;

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

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

    *dest = '\\0';

    return head;


int strcmp(const char* s1,const char* s2)

    assert(s1 != NULL && s2 != NULL);

    while (s1 && s2 && *s1 == *s2) 
        s1++;
        s2++;
    
    return *s1 - *s2;


size_t strlen(const char *str)

    assert(str != NULL);

    int num = 0;
    while(*str != '\\0')
    
        str++;
        num++;
    
    return num;


const char* strstr(const char *str1, const char *str2)

    if (str1== NULL || str2 == NULL) 
        return NULL;
	

    const char *temp = str1;
    const char *res = str2;

	while (*str1 != '\\0') 
        temp = str1;
        res = str2;
        while (*temp== *res) 
            temp++;
            res++;
        
        if (*res == '\\0') 
			return str1;
		
        str1++;

    

    return NULL;


char* strchr(const char* s, int c)

    if (NULL == s) 
        return NULL;
    

	do 
		if (*s == ((char)c)) 
			return (char *)s;
		
	 while (*s++);

	return NULL;


char* strdup(char *src)

    if (NULL == src) 
        return NULL;
    
    char *p_src = src;
    int count = 0 ;

    while (*p_src != '\\0') 
         count++;
         p_src++;
    

    char *new_addr  = (char *)malloc(sizeof(char) * (count + 1));
	if (new_addr == NULL) 
		return NULL;
	

    p_src = src;
    while (*p_src != '\\0') 
        *new_addr++ = *p_src++;
    
    *new_addr = '\\0';
    return (new_addr -(count));


int isspace(int x)

    if (x == ' ' || x == '\\t' || x == '\\n' || x =='\\f' || x =='\\b' || x == '\\r')
        return 1;
    else
        return 0;


int atoi(const char* nptr)

    int ret;
    int sign;

    while (isspace((int)(unsigned char)*nptr))
	
		++nptr;
	

	if ((*nptr == '-') || (*nptr == '+'))
	
		sign = *nptr;
		nptr++;
	
	else
	
		sign = '+';
	

    ret = 0;
    while ((*nptr <= '9') && (*nptr >= '0'))
    
        ret = 10 * ret + (*nptr - '0');
        nptr++;
    

    if (sign == '-')
        return -ret;
    else
        return ret;