git.scottworley.com Git - overonion/blob - reverse_lib.c

   1 #define _FILE_OFFSET_BITS 64
   2
   3 #define BUFFER_SIZE 4096
   4
   5 #include <err.h>
   6 #include <fcntl.h>
   7 #include <sys/mman.h>
   8 #include <sys/stat.h>
   9 #include <sys/types.h>
  10 #include <sysexits.h>
  11 #include <unistd.h>
  12
  13 static void write_all(int fd, const void *buf, size_t count) {
  14   const char* cbuf = buf;
  15   size_t written = 0;
  16   while (written < count) {
  17     int ret = write(fd, &cbuf[written], count - written);
  18     if (ret == -1) err(EX_IOERR, "Could not write");
  19     written += ret;
  20   }
  21 }
  22
  23 void reverse_file(const char* input_filename, int output_fd) {
  24   int fd = open(input_filename, O_RDONLY);
  25   if (fd == -1) err(EX_NOINPUT, "Could not open specified file");
  26
  27   struct stat stats;
  28   if (fstat(fd, &stats) == -1) err(EX_NOINPUT, "Could not stat input");
  29
  30   long page_size = sysconf(_SC_PAGE_SIZE);
  31   off_t pages = (stats.st_size - 1) / page_size + 1;
  32   long map_size = pages * page_size;
  33   char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, 0);
  34   if (m == MAP_FAILED) err(EX_NOINPUT, "Could not mmap input");
  35
  36   char buf[BUFFER_SIZE];
  37   off_t buf_offset = 0;
  38   for (off_t p = stats.st_size - 1; p >= 0; p--) {
  39     buf[buf_offset++] = m[p];
  40     if (buf_offset >= BUFFER_SIZE) {
  41       write_all(output_fd, buf, buf_offset);
  42       buf_offset = 0;
  43     }
  44   }
  45   if (buf_offset) {
  46     write_all(output_fd, buf, buf_offset);
  47   }
  48
  49   if (munmap(m, map_size) == -1) err(EX_IOERR, "Could not unmap input");
  50   if (close(fd) == -1) err(EX_IOERR, "Could not close input");
  51 }