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

   1 #define _FILE_OFFSET_BITS 64
   2
   3 #define BUFFER_SIZE 4096
   4
   5 #include <sys/types.h>
   6 #include <sys/stat.h>
   7 #include <sys/mman.h>
   8 #include <fcntl.h>
   9 #include <unistd.h>
  10 #include <err.h>
  11 #include <sysexits.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) {
  19       err(EX_IOERR, "Could not write");
  20     }
  21     written += ret;
  22   }
  23 }
  24
  25 void reverse_file(const char* input_filename, int output_fd) {
  26   int fd = open(input_filename, O_RDONLY);
  27   if (fd == -1) {
  28     err(EX_NOINPUT, "Could not open specified file");
  29   }
  30
  31   struct stat stats;
  32   if (fstat(fd, &stats) == -1) {
  33     err(EX_NOINPUT, "Could not stat input");
  34   }
  35
  36   long page_size = sysconf(_SC_PAGE_SIZE);
  37   off_t pages = (stats.st_size - 1) / page_size + 1;
  38   long map_size = pages * page_size;
  39   char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, 0);
  40   if (m == MAP_FAILED) {
  41     err(EX_NOINPUT, "Could not mmap input");
  42   }
  43
  44   char buf[BUFFER_SIZE];
  45   off_t buf_offset = 0;
  46   for (off_t p = stats.st_size - 1; p >= 0; p--) {
  47     buf[buf_offset++] = m[p];
  48     if (buf_offset >= BUFFER_SIZE) {
  49       write_all(output_fd, buf, buf_offset);
  50       buf_offset = 0;
  51     }
  52   }
  53   if (buf_offset) {
  54     write_all(output_fd, buf, buf_offset);
  55   }
  56
  57   if (munmap(m, map_size) == -1) {
  58     err(EX_IOERR, "Could not unmap input");
  59   }
  60   if (close(fd) == -1) {
  61     err(EX_IOERR, "Could not close input");
  62   }
  63 }