C语言用read封装/实现一个readline函数，读取文件一行 or 最后一行

Posted 2022-12-06 狱典司

注意，readline函数可以在windows和linux下通用，但是linux和windows下的空行表示有所区别，下面封装的read_last_line只适用于Linux操作系统。

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#include <fcntl.h>

/* 读取fd的一行内容 */
ssize_t readline(int fd, char *buf, ssize_t maxlen)

	ssize_t count = 0;
	memset(buf, '\\0', sizeof(buf));
	
	char *ptr = buf;
	char tmp = 0;
	while(1)
		read(fd, &tmp, sizeof(char));
		if(tmp == '\\n' || tmp == '\\0')		break;
		
		(*ptr) = tmp;
		ptr ++;
		count ++;
		if(count >= maxlen)
			printf("Out of buffer!\\n");
			return -1;
		
	
	return count;	



/* 读取实际有内容的最后一行 */
ssize_t read_last_line(int fd, char *buf, ssize_t maxlen)
	if(fd < 0)	return -1;
	if( lseek(fd, 0, SEEK_END) == 0 ) 
		printf("空文件\\n");
		return -1;
	

	lseek(fd, -1, SEEK_CUR);	
	char tmp = '\\0';
	while(lseek(fd, -1, SEEK_CUR) > 0)
		read(fd, &tmp, sizeof(char));
		if(tmp == '\\n')		break;
		else	lseek(fd, -1, SEEK_CUR);
	
	
	ssize_t offset;
	while((offset = readline(fd, buf, maxlen)) != -1)
		if(offset > 0)	return offset;
		else if( offset == 0 )
			lseek(fd, -2, SEEK_CUR);	
			tmp = '\\0';
			while(lseek(fd, -1, SEEK_CUR) > 0)
				read(fd, &tmp, sizeof(char));
				if(tmp == '\\n')		break;
				else	lseek(fd, -1, SEEK_CUR);
			
			continue;
		
	
	return -1;

int main()
	int fd = -1;
	fd = open("t.txt", O_RDONLY);
	if( fd == -1 )
		printf("failed~!\\n");
	
	
	char readbuf[1024] = 0;
	read_last_line(fd, readbuf, sizeof(readbuf));
	printf("最后一行：%s\\n",readbuf);	

	close(fd);
	return 0;