tools/reorder/mcount.c

   1 /* This Source Code Form is subject to the terms of the Mozilla Public
   2  * License, v. 2.0. If a copy of the MPL was not distributed with this
   3  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   4
   5 /*
   6
   7   A library that can be LD_PRELOAD-ed to dump a function's address on
   8   function entry.
   9
  10  */
  11
  12 #include <unistd.h>
  13
  14 /**
  15  * When compiled with `-pg' (or even just `-p'), gcc inserts a call to
  16  * |mcount| in each function prologue. This implementation of |mcount|
  17  * will grab the return address from the stack, and write it to stdout
  18  * as a binary stream, creating a gross execution trace the
  19  * instrumented program.
  20  *
  21  * For more info on gcc inline assembly, see:
  22  *
  23  *   <http://www.ibm.com/developerworks/linux/library/l-ia.html?dwzone=linux>
  24  *
  25  */
  26 void
  27 mcount()
  28 {
  29     register unsigned int caller;
  30     unsigned int buf[1];
  31
  32     // Grab the return address.
  33     asm("movl 4(%%esp),%0"
  34         : "=r"(caller));
  35
  36     buf[0] = caller;
  37     write(STDOUT_FILENO, buf, sizeof buf[0]);
  38 }