只實現了head/tail的基本功能，默認顯示十行及-n參數。 一、使用帶緩沖的系統調用。 write/read等系統調用是不帶緩沖的，可以包裝一層，使其帶緩沖。

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 == '\n'){if (++num == count){break;}}}else if (rc == 0){if (i == 1){return 0;}else{break;}}else{return -1;}}*buf = '\0';return i;
}

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? ? ?二、head命令實現

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include "rio.h"#define MAXSIZE 4096int main(int argc, char **argv)
{if (argc < 2){fprintf(stderr, "usage %s [-n n] filename\n", 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\n", 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;
}

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? ? ?三、tail命令實現

#include "rio.h"#define MAXSIZE 4096void show_info(char *buf, char **ptr, int count);int main(int argc, char **argv)
{if (argc < 2){fprintf(stderr, "usage %s [-n n] filename\n", 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\n", 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, '\n');flag = 1;**ptr = '\0';show_info(buf, ptr + 1, --times);}if (flag){printf("%s\n", *ptr + 1);}
}

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? ? ?通過遞歸show_info來實現按順序打印，其實也可以用鏈表來實現，不過遞歸寫起來簡單。

轉載于:https://www.cnblogs.com/shenlinken/p/5828253.html

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