mirror of https://github.com/t1meshift/os_labs.git
92 lines
2.3 KiB
C
92 lines
2.3 KiB
C
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#include <assert.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/stat.h>
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#include <sys/time.h>
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#include <sys/wait.h>
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#include <unistd.h>
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#include <stdbool.h>
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/*
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3. Разработать собственную версию (mytail) команды tail.
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Формат: mytail -n file
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Она читает блок из конца файла, просматривает его с конца до заданного количества строк n и печатает эти строки
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в соответствующем порядке.
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Использовать: stat(), lseek(), open(), read(), close(), ...
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*/
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#define MAX(a,b) ((a)>(b)?(a):(b))
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#define FATAL(fmt, ...) do {\
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fprintf(stderr,fmt,##__VA_ARGS__);\
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exit(1);\
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} while (0)
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int main(int argc, char *argv[]) {
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const int BUFF_SIZE = 4096;
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long n = 10;
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char *filename = NULL;
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// Parse arguments (skip 0th arg)
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for (int i = 1; i < argc; ++i) {
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if ((argv[i][0] == '-') && (i < argc - 1)) {
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// -n flag (e.g. -20)
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n = strtol(argv[i], NULL, 10) * -1;
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if (n < 1 || errno == ERANGE) {
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FATAL("Usage: %s [-<number_of_lines>] <file>\n", argv[0]);
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}
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} else {
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filename = argv[i];
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break;
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}
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}
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if (filename == NULL) FATAL("No file name specified.\n");
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struct stat sb;
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int fd;
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if (stat(filename, &sb) == -1) {
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perror("stat()");
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FATAL("Couldn't stat %s\n", filename);
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}
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if ((fd = open(filename, O_RDONLY)) == -1) {
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perror("open()");
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FATAL("Couldn't open %s\n", filename);
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}
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char *buf = calloc(BUFF_SIZE, 1);
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int lf_cnt = 0;
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size_t read_sz = 0;
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off_t file_offset = MAX(sb.st_size - BUFF_SIZE, 0);
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bool seek_stop = false;
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lseek(fd, -BUFF_SIZE, SEEK_END);
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do {
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int data_sz = read(fd, buf, BUFF_SIZE);
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for (int i = data_sz - 1; i >= 0; --i) {
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if (buf[i] == '\n') {
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if (++lf_cnt >= n) {
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file_offset += i + 1;
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goto print_lines;
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}
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}
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}
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file_offset -= BUFF_SIZE;
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lseek(fd, -BUFF_SIZE, SEEK_CUR);
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} while ((lf_cnt <= n) && (file_offset > 0));
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print_lines:
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lseek(fd, file_offset, SEEK_SET);
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do {
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read_sz = read(fd, buf, BUFF_SIZE);
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write(STDOUT_FILENO, buf, read_sz);
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} while (read_sz == BUFF_SIZE);
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free(buf);
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close(fd);
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return 0;
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}
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