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 == '\n') { if (++num == count) { break; } } } else if (rc == 0) { if (i == 1) { return 0; } else { break; } } else { return -1; } } *buf = '\0'; return i;} 二、head命令实现
#include#include #include #include #include #include "rio.h" #define MAXSIZE 4096 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, 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;}
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\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); }} 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。