[C++知识库]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

#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 bufffer!\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( 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;
	
	
}