Switch to stdio for output. It's simpler

[overonion] / reverse_lib.c

#define _FILE_OFFSET_BITS 64

#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sysexits.h>
#include <unistd.h>

void reverse_file(const char* input_filename, FILE* output_stream) {
  int fd = open(input_filename, O_RDONLY);
  if (fd == -1) err(EX_NOINPUT, "Could not open specified file");

  struct stat stats;
  if (fstat(fd, &stats) == -1) err(EX_NOINPUT, "Could not stat input");

  long page_size = sysconf(_SC_PAGE_SIZE);
  off_t pages = (stats.st_size - 1) / page_size + 1;
  long map_size = pages * page_size;
  char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, 0);
  if (m == MAP_FAILED) err(EX_NOINPUT, "Could not mmap input");

  for (off_t p = stats.st_size - 1; p >= 0; p--) {
    if (fputc(m[p], output_stream) == EOF) errx(EX_IOERR, "Could not write");
  }

  if (munmap(m, map_size) == -1) err(EX_IOERR, "Could not unmap input");
  if (close(fd) == -1) err(EX_IOERR, "Could not close input");
}