shorten/shorten.c

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/param.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#define BUF_CAP 1024

int main(int argc, char **argv) {
  if (argc != 3) {
    printf("%s: Invalid number of arguments.\n", argv[0]);
    printf("Usage: %s linecount filename\n", argv[0]);
    printf("to remove linecount lines from the end of the file\n");
  }

  int number = atoi(argv[1]);
  char *file = argv[2]; // No copy
  int count = 0;

  if (number <= 0) {
    printf("Bad number of line\n");
    exit(1);
  }

  struct stat file_stat;
  if (stat(file, &file_stat) < 0) {
    printf("Stat failed: %d\n", errno);
    return 2;
  }
  int file_size = file_stat.st_size;
  if (file_size == 0) {
    printf("No change: file does not end with a newline\n");
    return 1;
  }

  int fd = open(file, O_RDWR | O_APPEND);

  if (lseek(fd, -1, SEEK_END) < 0) {
    printf("Lseek1 failed: %d\n", errno);
    close(fd);
    return 2;
  }
  char c;
  if (read(fd, &c, 1) < 0) {
    printf("Read1 failed: %d\n", errno);
    close(fd);
    return 2;
  }

  if (c != '\n') {
    printf("No change: file does not end with a newline\n");
    close(fd);
    return 1;
  }

  while (1) {
    // We can't seek back before the beginning of the file
    int buf_size = MIN(BUF_CAP, file_size);
    char buf[buf_size];

    // Go back in the file to read the length of the buffer
    if (lseek(fd, -buf_size, SEEK_CUR) < 0) {
      printf("Lseek2 failed: %d\n", errno);
      close(fd);
      return 2;
    }

    // ~lseek(fd, buf_size, os.SEEK_CUR)
    int buf_len = read(fd, buf, buf_size);
    if (buf_len < 0) {
      printf("Read2 failed: %d\n", errno);
      close(fd);
      return 2;
    }

    char *char_pos = memrchr(buf, '\n', buf_len);
    if (char_pos != NULL) {
      count++;
      // Go to position found ; a read would read a '\n'
      if (lseek(fd, (ssize_t)(char_pos - buf) - buf_len, SEEK_CUR) < 0) {
        printf("Lseek3 failed: %d\n", errno);
        close(fd);
        return 2;
      }
      // "resize" the file to look for next '\n'  from this point
      file_size -= buf_len - (ssize_t)(char_pos - buf);
    } else {
      // Go back to previous position
      if (lseek(fd, -buf_len, SEEK_CUR) < 0) {
        printf("Lseek4 failed: %d\n", errno);
        close(fd);
        return 2;
      }
      file_size -= buf_len;
    }

    if (file_size < 0)
      break;

    if (count == number + 1) {
      file_size++; // Keep the last '\n'
      if (ftruncate(fd, file_size) < 0) {
        printf("Ftruncate failed: %d\n", errno);
        close(fd);
        return 2;
      }
      close(fd);
      return 0;
    }

    // Beginning of file?
    if (lseek(fd, 0, SEEK_CUR) == 0) {
      printf("No change: file does not end with a newline\n");
      return 1;
    }
  }

  close(fd);

  if (count < number + 1) {
    printf("No change: requested removal would leave empty file\n");
    return 3;
  }
}