tail2.c

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <getopt.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <errno.h>

#define READSIZE 4096

int  locate(int fd,int nline,char* rbuf); //计算要打印的行距文件尾的偏移量（fd:文件描述符，nline:打印行数，rbuf:内容缓冲）
int  printd(int fd,off_t offs2end,char* buf);//打印内容（fd:文件描述符，offset:距文件尾的偏移量，buf:内容缓冲）
void printerr(int);   //打印错误信息

int main(int argc ,char* argv[])
{
    int rfd =-1,   //文件描述付
        ret=-1,    //主程序返回值
        nline =10; //读取文件行数（默认10）
    char rbuf[READSIZE];//读取缓冲区
    char c;

    memset(rbuf,0,READSIZE);
    //程序参数解析
    while((c=getopt(argc,argv,"n:")) != -1){
        switch(c){
        case 'n'://指定显示行数
            nline = atoi(optarg);
            break;
        default:
            fprintf(stderr,"Usage: %s [-n lines]",argv[0]);
            return -1;
        }
    }
//打开文件
    if((rfd = open(argv[optind],O_RDONLY)) == -1){
        fprintf(stderr,"%s:",argv[0]);
        return -1;
    }

    //  if(pstat.st_size < READSIZE)
    printd(rfd,(off_t)(0-locate(rfd,nline,rbuf)),rbuf);
    //关闭文件，并返回
    ret = close(rfd);
    if(ret == -1)
        fprintf(stderr,"%s:\n",argv[0]);
    return ret;
}
int  locate(int fd,int nline,char* rbuf){
    off_t offs_seek;
    int len=0,
        l=1,
        i=0,
        nent=0,  //换行计数器
        offs2end=0,  //距文件结尾的偏移量
        off=0,
        whence=SEEK_END;

    char *pb;//指向行换行指针
     struct stat pstat;   //文件状态
     if(fstat(fd,&pstat) < 0){
        perror("ttail");
        return -1;
    }
     //处理缓冲区大小大于文件大小的
     if(pstat.st_size <= READSIZE){
         off = 0;
         whence = SEEK_CUR;
     }
     else{
         off = 0-READSIZE;
         whence = SEEK_END;
     }

    while((offs_seek=lseek(fd,l*off,whence)) != -1)
    {
        l++;
        if((len = read(fd,rbuf,READSIZE)) > 0)
        {
            for(i=len-1; i >=0; i--){
                if(rbuf[i] == '\n'){
                    pb = &rbuf[i];
                    if(++nent == nline+1)
                        break;
                }
            }
            //累计距文件尾的偏移量
            if(nent != nline+1){
                offs2end += len;
            }
            else {
                offs2end += (&rbuf[len-1]-pb);
                break;
            }
            memset(rbuf,0,READSIZE);
        }
    }
    if(offs_seek == -1)
        printerr(errno);
    return offs2end;
}

int  printd(int fd,off_t offs2end,char* buf){
    int len =0;
    if(lseek(fd,offs2end,SEEK_END) == -1)
        return -1;
    memset(buf,0,READSIZE);
    while((len=read(fd,buf,READSIZE)) > 0){
        printf("%s",buf);
        memset(buf,0,READSIZE);
    }
}

void printerr(int err){
    if(err == EBADF)
        printf("fd is not an open file descriptor.\n");
    else if(err == EINVAL)
        printf("whence  is  not  one  of  SEEK_SET,  SEEK_CUR,  SEEK_END; or the\
   resulting file offset would be negative, or beyond the end of a  seekable device.\n");
    else if(err == EOVERFLOW )
        printf(" The resulting file offset cannot be represented in an off_t.\n");
    else if(err == ESPIPE)
        printf("fd is associated with a pipe, socket, or FIFO.\n");
}