tavianator

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

int
main(int argc, char *argv[])
{
  if (argc != 3) {
    fprintf(stderr, "usage: %s <filename> <line number>\n", argv[0]);
    return EXIT_FAILURE;
  }

  int fd = open(argv[1], O_RDWR);
  if (fd < 0) {
    perror("open");
    return EXIT_FAILURE;
  }

  struct stat stat_buf;
  if (fstat(fd, &stat_buf) < 0) {
    perror("fstat");
    close(fd);
    return EXIT_FAILURE;
  }

  void *map = mmap(NULL, stat_buf.st_size, PROT_READ | PROT_WRITE, MAP_SHARED,
                   fd, 0);
  if (map == MAP_FAILED) {
    perror("mmap");
    return EXIT_FAILURE;
  }

  int line = atoi(argv[2]);
  if (line < 1) {
    fprintf(stderr, "Line %d makes no sense.\n", line);
    return EXIT_FAILURE;
  }

  char *linen = map;
  int i;
  for (i = 1; i < line; ++i) {
    linen = memchr(linen, '\n', stat_buf.st_size - (linen - (char *)map));
    if (!linen) {
      fprintf(stderr, "Line %d is past EOF.\n", line);
      return EXIT_FAILURE;
    }
    ++linen;
  }

  char *linen1 = memchr(linen, '\n', stat_buf.st_size - (linen - (char *)map));
  if (linen1) {
    ++linen1;
    memmove(linen, linen1, stat_buf.st_size - (linen1 - (char *)map));
    if (munmap(map, stat_buf.st_size) < 0) {
      perror("munmap");
      return EXIT_FAILURE;
    }
    if (ftruncate(fd, stat_buf.st_size - (linen1 - linen)) < 0) {
      perror("ftruncate");
      return EXIT_FAILURE;
    }
  } else {
    munmap(map, stat_buf.st_size);
    if (ftruncate(fd, linen - (char *)map) < 0) {
      perror("ftruncate");
      return EXIT_FAILURE;
    }
  }

  close(fd);
  return EXIT_SUCCESS;
}