git.scottworley.com Git - overonion/blob - reverse.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 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
  26 int main(int argc, char** argv) {
  27   if (argc != 2) {
  28     errx(EX_USAGE, "Usage: reverse filename");
  29   }
  30
  31   int fd = open(argv[1], O_RDONLY);
  32   if (fd == -1) {
  33     err(EX_NOINPUT, "Could not open specified file");
  34   }
  35
  36   struct stat stats;
  37   if (fstat(fd, &stats) == -1) {
  38     err(EX_NOINPUT, "Could not stat input");
  39   }
  40
  41   long page_size = sysconf(_SC_PAGE_SIZE);
  42   off_t pages = (stats.st_size - 1) / page_size + 1;
  43   long map_size = pages * page_size;
  44   char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, 0);
  45   if (m == MAP_FAILED) {
  46     err(EX_NOINPUT, "Could not mmap input");
  47   }
  48
  49   char buf[BUFFER_SIZE];
  50   off_t buf_offset = 0;
  51   for (off_t p = stats.st_size - 1; p >= 0; p--) {
  52     buf[buf_offset++] = m[p];
  53     if (buf_offset >= BUFFER_SIZE) {
  54       write_all(1, buf, buf_offset);
  55       buf_offset = 0;
  56     }
  57   }
  58   if (buf_offset) {
  59     write_all(1, buf, buf_offset);
  60   }
  61
  62   if (munmap(m, map_size) == -1) {
  63     err(EX_IOERR, "Could not unmap input");
  64   }
  65   if (close(fd) == -1) {
  66     err(EX_IOERR, "Could not close input");
  67   }
  68
  69   return 0;
  70 }