【C】 Introduction and Simulation Implementation of ATOI and offsetof

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Preface

Here are atoi、offsetof And their simulation implementation ,atoi This library function is used to convert a string into a number ;offsetof Used to calculate the offset , It looks like a function , In fact, it is a macro ！

One . atoi Library function

1. Introduce

function ：

• Convert a numeric string to its corresponding integer

int atoi (const char * str);

The header file ：

• <stdlib.h>

Parameters ：

• str—— Pointer to the string to be converted

Return value ：

• success , The function takes the converted integer as int Value returns ;
• If conversion is not possible , return 0;
• If the converted value exceeds int Range of representable values of , Then act Undefined .

matters needing attention ：

• atoi When use , If the string to be converted is preceded by a blank character , Will skip these white space characters , Until you find the first non white space character ;
• The string to be converted can start with + - Characters such as number , Will be recognized as one of the arithmetic + - Number ;
• If there are non numeric characters between the numeric characters to be converted , Then the numeric characters after non numeric characters cannot be converted .

Using examples ：

2. Simulation Implementation

atoi The simulation of should be as comprehensive as possible , Consider the following points ：

1. The parameter is a null pointer
2. The parameter is an empty string
3. The space at the beginning of the string
4. Before the first numeric character + - Number
5. The converted value is out of bounds , beyond int The scope of the
6. Non numeric characters in string

#include<stdio.h>
#include<assert.h>
#include<limits.h>
#include<ctype.h>

enum Status
{
    
	VALID,
	INVALID
}sta = INVALID;// Illegal default 

int My_atoi(const char* str)
{
    
	int flag = 1;

	assert(str);
	if (*str == '\0')
	{
    
		return 0;// Illegal return 0
	}
	// Skip white space 
	while (isspace(*str))
	{
    
		str++;
	}
	// solve +- Number 
	if (*str == '+')
	{
    
		flag = 1;
		str++;
	}
	else if (*str == '-')
	{
    
		flag = -1;
		str++;
	}
	long long ret = 0;
	while (*str)
	{
    
		if (isdigit(*str))
		{
    
			ret = ret * 10 + flag * (*str - '0');
			// Judge out of bounds 
			if (ret > INT_MAX || ret < INT_MIN)
			{
    
				return 0;
			}
		}
		else
		{
    
			return (int)ret; 
		}
		str++;
	}
	if (*str == '\0')
	{
    
		sta = VALID;
	}
	return (int)ret;
}
int main()
{
    
	char arr[50] = "-1234";
	int ret = My_atoi(arr);
	if (sta == INVALID)
	{
    
		printf(" Illegal return ：%d\n", ret);
	}
	else if(sta == VALID)
	{
    
		printf(" Legal conversion ：%d\n", ret);
	}

	return 0;
}

Two . macro offsetof

1. Introduce

function ：

• Calculate the offset of the structure member relative to the first address of the structure

offsetof (type,member)

Parameters ：

• type—— Structure or union type
• member—— Members in types

Return value ：

• Returns the offset of the member relative to the first address of the type , One size_t Type value .

Using examples ：

2. Simulation Implementation

If you look at the address of the structure and the address of the structure member, you will find , The address of the structure member minus the address of the structure is the offset of the structure member from the first address of the structure ;

hypothesis 0 The address is the address of the structure , Then the address of the structure member is its offset from the first address of the structure

#include<stdio.h>
#define OFFSETOF(type, name) (int)&(((struct S*)0)->name)

struct S
{
    
	int a;
	char b;
	double c;
};

int main()
{
    
	printf("%d\n", OFFSETOF(struct S, a));
	printf("%d\n", OFFSETOF(struct S, b));
	printf("%d\n", OFFSETOF(struct S, c));

	return 0;
}

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