# 算法：实现自己的tail（读取大文件的最后n行）

2021年3月15日09:11:03 发表评论 972 次浏览

1.程序不应读取整个文件。

2.程序应处理传入的动态数据, 并在任何时候返回最后n行。

3.在读取最后n行之前, 程序不应关闭输入流。

``````// C++ program to implement your own tail
#include <bits/stdc++.h>
using namespace std;

#define SIZE 100

// Utility function to sleep for n seconds
void sleep(unsigned int n)
{
clock_t goal = n * 1000 + clock ();
while (goal > clock ());
}

// function to read last n lines from the file
// at any point without reading the entire file
void tail( FILE * in, int n)
{
int count = 0;  // To count '\n' characters

// unsigned long long pos (stores upto 2^64 – 1
// chars) assuming that long long int takes 8
// bytes
unsigned long long pos;
char str[2*SIZE];

// Go to End of file
if ( fseek (in, 0, SEEK_END))
perror ( "fseek() failed" );
else
{
// pos will contain no. of chars in
// input file.
pos = ftell (in);

// search for '\n' characters
while (pos)
{
// Move 'pos' away from end of file.
if (! fseek (in, --pos, SEEK_SET))
{
if ( fgetc (in) == '\n' )

// stop reading when n newlines
// is found
if (count++ == n)
break ;
}
else
perror ( "fseek() failed" );
}

// print last n lines
printf ( "Printing last %d lines -\n" , n);
while ( fgets (str, sizeof (str), in))
printf ( "%s" , str);
}
printf ( "\n\n" );
}

// Creates a file and prints and calls tail() for
// 10 different values of n (from 1 to 10)
int main()
{
FILE * fp;
char buffer[SIZE];
// Open file in binary mode
// wb+ mode for reading and writing simultaneously
fp = fopen ( "input.txt" , "wb+" );
if (fp == NULL)
{
printf ( "Error while opening file" );
exit (EXIT_FAILURE);
}
srand ( time (NULL));

// Dynamically add lines to input file
// and call tail() each time
for ( int index = 1; index <= 10; index++)
{
/* generate random logs to print in input file*/
for ( int i = 0; i < SIZE - 1; i++)
buffer[i] = rand () % 26 + 65; // A-Z
buffer[SIZE] = '\0' ;

/* code to print timestamp in logs */
// get current calendar time
time_t ltime = time (NULL);

// asctime() returns a pointer to a string
// which represents the day and time
char * date = asctime ( localtime (&ltime));

// replace the '\n' character in the date string
// with '\0' to print on the same line.
date[ strlen (date)-1] = '\0' ;

/* Note in text mode '\n' appends two characters, so we have opened file in binary mode */
fprintf (fp, "\nLine #%d [%s] - %s" , index, date, buffer);

// flush the input stream before calling tail
fflush (fp);

// read last index lines from the file
tail(fp, index);

// sleep for 3 seconds
// note difference in timestamps in logs
sleep(3);
}

/* close the file before ending program */
fclose (fp);

return 0;
}``````

1.此代码无法在在线编译器上使用, 因为它需要文件创建权限。在本地计算机上运行时, 它将生成示例输入文件" input.txt", 并向其动态写入数据10次, 并每次都调用tail()函数。

2.我们应该避免对大型文件(以GB为单位)使用fseek()和ftell(), 因为它们在long int类型的文件位置上运行。请改用_fseeki64()和_ftelli64()。

3. unsigned long的最大允许值为232– 1(假设无符号long占用4个字节)。它可用于文件大小小于4 GB的文件。

4. unsigned long long的最大允许值为264– 1(假设unsigned long long占用8个字节)。它可用于文件大小超过4 GB的文件。