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 = ;
rp->rio_bufptr = rp->rio_buf;
} ssize_t rio_read(rio_t *rp, void *usrbuf, size_t n)
{
int cnt = ; while (rp->rio_cnt <= )
{
if ((rp->rio_cnt = read(rp->rio_fd, rp->rio_buf, sizeof(rp->rio_buf))) < )
{
if (errno != EINTR)
{
return -;
}
}
else if (rp->rio_cnt == )
{
return ;
}
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 = , rc = , num = ;
char c = , *buf = usrbuf; lseek(rp->rio_fd, maxlen, SEEK_END); for (i = ; i < maxlen; i++)
{
if ((rc = rio_read(rp, &c, )) == )
{
*buf++ = c;
if (c == '\n')
{
if (++num == count)
{
break;
}
}
}
else if (rc == )
{
if (i == )
{
return ;
}
else
{
break;
}
}
else
{
return -;
}
} *buf = '\0';
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 < )
{
fprintf(stderr, "usage %s [-n n] filename\n", argv[]);
return -;
} int times = , i = , in_fd = -, n_char = ;
char filename[] = {};
char buf[MAXSIZE] = {};
rio_t rio_buf = {}; for (i = ; 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)) == -)
{
fprintf(stderr, "open %s failed\n", filename);
return -;
} rio_readinitb(&rio_buf, in_fd);
if ((n_char = rio_readlineb(&rio_buf, buf, MAXSIZE, times)) > )
{
write(STDOUT_FILENO, buf, n_char);
} close(in_fd); return ;
}
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 < )
{
fprintf(stderr, "usage %s [-n n] filename\n", argv[]);
return -;
} int times = , i = , in_fd = -;
char filename[] = {};
char buf[MAXSIZE] = {};
rio_t rio_buf = {};
char *ptr[MAXSIZE]; for (i = ; 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)) == -)
{
fprintf(stderr, "open %s failed\n", filename);
return -;
} rio_readinitb(&rio_buf, in_fd);
rio_read(&rio_buf, buf, MAXSIZE); show_info(buf, ptr, times); return ;
} void show_info(char *buf, char **ptr, int times)
{
int num = ;
int flag = ; if (num < times)
{
*ptr = strrchr(buf, '\n');
flag = ;
**ptr = '\0';
show_info(buf, ptr + , --times);
} if (flag)
{
printf("%s\n", *ptr + );
}
}
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。