模仿unix tail程序的源代码，只支持 -n 指定行数

//
// Created by xia on 2022/3/16.
//

#include "common.h"

int main(int argc,char *argv[])
{
    if(argc <2 || strcmp(argv[1],"--help")==0)
        errExit("%s [-n lines] file",argv[0]);
    int opt,n=10;
    while ((opt = getopt(argc,argv,"n:")) != -1)
    {
        switch (opt) {
            case 'n':
                n = getInt(optarg,GN_ANY_BASE,"n");
                break;
            case '?':
            case ':':
                errExit("unknown [%c]",optopt);
            default:
                fatal("Unexpected case in switch()");
        }
    }
    if(optind>=argc)
        errExit("%s [-n lines] file",argv[0]);
    FILE *fp = fopen(argv[optind],"r");
    int fd = fileno(fp);
    off_t offset = lseek(fd,0,SEEK_END);
    if((off_t)-1 == offset)
        errExit("lseek");
    offset--;
    int line_count = -1;
    int c;
    while(offset >= 0 && line_count < n)
    {
        fseek(fp,offset--,SEEK_SET);
        c = fgetc(fp);
        if(c == -1)
            errExit("fgetc");
        if(c == '\n')
            line_count++;
    }
    if(offset == -1)
        fseek(fp,0,SEEK_SET);
#define BUF_SIZE  1024
    char buf[BUF_SIZE];
    while (!feof(fp))
    {
        if(fgets(buf,BUF_SIZE,fp)==NULL)
            break;
        printf("%s",buf);
    }
    exit(EXIT_SUCCESS);
}

其中common头文件里只包含了一些常用的头文件和处理错误的函数，对理解程序没啥影响，需要这部分的代码可以评论留邮箱。