src/runtime/var-io.c

   1 /*
   2  * This software is part of the SBCL system. See the README file for
   3  * more information.
   4  *
   5  * This software is derived from the CMU CL system, which was
   6  * written at Carnegie Mellon University and released into the
   7  * public domain. The software is in the public domain and is
   8  * provided with absolutely no warranty. See the COPYING and CREDITS
   9  * files for more information.
  10  */
  11
  12 // Read a variable-length encoded 32-bit integer from SOURCE and
  13 // return its value.
  14 //
  15 // If OFFSET is not NULL, start decoding at OFFSET bytes from SOURCE
  16 // and increment the value pointed to by OFFSET by the length of the
  17 // encoded representation.
  18 //
  19 // Keep in sync with {READ,WRITE}-VAR-INTEGER in
  20 // src/code/debug-var-io.lisp
  21 int read_var_integer(unsigned char *source, int *offset) {
  22     unsigned char *ptr = source + (offset ? *offset : 0);
  23     int result = 0;
  24     unsigned char octet;
  25     int k = 0;
  26     for (;; k += 7, ++ptr){
  27         octet = *ptr;
  28         result |= (octet & 0x7f) << k;
  29         if (!(octet & 0x80)) {
  30             break;
  31         }
  32     }
  33     if (offset) {
  34         *offset += (ptr - source);
  35     }
  36     return result;
  37 }