head/tail实现

     只实现了head/tail的基本功能，默认显示十行及-n参数。

     

　　一、使用带缓冲的系统调用。

     

　　write/read等系统调用是不带缓冲的，可以包装一层，使其带缓冲。

typedef struct
{
    int rio_fd;
    int rio_cnt;
    char *rio_bufptr;
    char rio_buf[RIO_BUFFSIZE];
}rio_t;
 
void rio_readinitb(rio_t *rp, int fd)
{
    rp->rio_fd = fd;
    rp->rio_cnt = 0;
    rp->rio_bufptr = rp->rio_buf;
}
 
ssize_t rio_read(rio_t *rp, void *usrbuf, size_t n)
{
    int cnt = 0;
 
    while (rp->rio_cnt <= 0)
    {
        if ((rp->rio_cnt = read(rp->rio_fd, rp->rio_buf, sizeof(rp->rio_buf))) < 0)
        {
            if (errno != EINTR)
            {
                return -1;
            }
        }
        else if (rp->rio_cnt == 0)
        {
            return 0;
        }
        else
        {
            rp->rio_bufptr = rp->rio_buf;
        }
    }
 
    //cnt = n > rp->rio_cnt?rp->rio_cnt:n;
 
    cnt = n;
    if (n > rp->rio_cnt)
    {
        cnt = rp->rio_cnt;
    }
 
    memcpy(usrbuf, rp->rio_bufptr, cnt);
    rp->rio_cnt -= cnt;
    rp->rio_bufptr += cnt;
 
    return cnt;
}
 
ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen, int count)
{
    int i = 0, rc = 0, num = 0;
    char c = 0, *buf = usrbuf;
 
    lseek(rp->rio_fd, maxlen, SEEK_END);
 
    for (i = 1; i < maxlen; i++)
    {
        if ((rc = rio_read(rp, &c, 1)) == 1)
        {
            *buf++ = c;
            if (c == '
')
            {
                if (++num == count)
                {
                    break;
                }
            }
        }
        else if (rc == 0)
        {
            if (i == 1)
            {
                return 0;
            }
            else
            {
                break;
            }
        }
        else
        {
            return -1;
        }
    }
 
    *buf = '';
    return i;
}

     二、head命令实现

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include "rio.h"
 
#define MAXSIZE 4096
 
int main(int argc, char **argv)
{
    if (argc < 2)
    {
        fprintf(stderr, "usage %s [-n n] filename
", argv[0]);
        return -1;
    }
 
    int times = 10, i = 0, in_fd = -1, n_char = 0;
    char filename[16] = {0};
    char buf[MAXSIZE] = {0};
    rio_t rio_buf = {0};
 
    for (i = 1; i < argc; i++)
    {
        if (!strcmp(argv[i], "-n"))
        {
            times = atoi(argv[++i]);
        }
        else
        {
            snprintf(filename, sizeof(filename), "%s", argv[i]);
        }
    }
 
    if ((in_fd = open(filename, O_RDONLY)) == -1)
    {
        fprintf(stderr, "open %s failed
", filename);
        return -1;
    }
 
    rio_readinitb(&rio_buf, in_fd);
    if ((n_char = rio_readlineb(&rio_buf, buf, MAXSIZE, times)) > 0)
    {
        write(STDOUT_FILENO, buf, n_char);
    }
 
    close(in_fd);
 
    return 0;
}

     三、tail命令实现

#include "rio.h"
 
#define MAXSIZE 4096
 
void show_info(char *buf, char **ptr, int count);
 
int main(int argc, char **argv)
{
    if (argc < 2)
    {
        fprintf(stderr, "usage %s [-n n] filename
", argv[0]);
        return -1;
    }
 
    int times = 10, i = 0, in_fd = -1;
    char filename[16] = {0};
    char buf[MAXSIZE] = {0};
    rio_t rio_buf = {0};
    char *ptr[MAXSIZE];
 
    for (i = 1; i < argc; i++)
    {
        if (!strcmp(argv[i], "-n"))
        {
            times = atoi(argv[++i]);
        }
        else
        {
            snprintf(filename, sizeof(filename), "%s", argv[i]);
        }
    }
 
    if ((in_fd = open(filename, O_RDONLY)) == -1)
    {
        fprintf(stderr, "open %s failed
", filename);
        return -1;
    }
 
    rio_readinitb(&rio_buf, in_fd);
    rio_read(&rio_buf, buf, MAXSIZE);
 
    show_info(buf, ptr, times);
 
    return 0;
}
 
void show_info(char *buf, char **ptr, int times)
{
    int num = 0;
    int flag = 0;
 
    if (num < times)
    {
        *ptr = strrchr(buf, '
');
        flag = 1;
        **ptr = '';
        show_info(buf, ptr + 1, --times);
    }
 
    if (flag)
    {
        printf("%s
", *ptr + 1);
    }
}

     通过递归show_info来实现按顺序打印，其实也可以用链表来实现，不过递归写起来简单。