misc/sbrk.c

   1 /* Copyright (C) 1991-2021 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <libc-internal.h>
  20 #include <stdbool.h>
  21 #include <stdint.h>
  22 #include <unistd.h>
  23
  24 /* Defined in brk.c.  */
  25 extern void *__curbrk;
  26 extern int __brk (void *addr);
  27
  28 /* Extend the process's data space by INCREMENT.
  29    If INCREMENT is negative, shrink data space by - INCREMENT.
  30    Return start of new space allocated, or -1 for errors.  */
  31 void *
  32 __sbrk (intptr_t increment)
  33 {
  34   /* Controls whether __brk (0) is called to read the brk value from
  35      the kernel.  */
  36   bool update_brk = __curbrk == NULL;
  37
  38 #if defined (SHARED) && ! IS_IN (rtld)
  39   if (!__libc_initial)
  40     {
  41       if (increment != 0)
  42         {
  43           /* Do not allow changing the brk from an inner libc because
  44              it cannot be synchronized with the outer libc's brk.  */
  45           __set_errno (ENOMEM);
  46           return (void *) -1;
  47         }
  48       /* Querying the kernel's brk value from an inner namespace is
  49          fine.  */
  50       update_brk = true;
  51     }
  52 #endif
  53
  54   if (update_brk)
  55     if (__brk (0) < 0)          /* Initialize the break.  */
  56       return (void *) -1;
  57
  58   if (increment == 0)
  59     return __curbrk;
  60
  61   void *oldbrk = __curbrk;
  62   if (increment > 0
  63       ? ((uintptr_t) oldbrk + (uintptr_t) increment < (uintptr_t) oldbrk)
  64       : ((uintptr_t) oldbrk < (uintptr_t) -increment))
  65     {
  66       __set_errno (ENOMEM);
  67       return (void *) -1;
  68     }
  69
  70   if (__brk (oldbrk + increment) < 0)
  71     return (void *) -1;
  72
  73   return oldbrk;
  74 }
  75 libc_hidden_def (__sbrk)
  76 weak_alias (__sbrk, sbrk)