lib/xsize.h

/* xsize.h -- Checked size_t computations.

   Copyright (C) 2003, 2008-2026 Free Software Foundation, Inc.

   This file is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation; either version 2.1 of the
   License, or (at your option) any later version.

   This file is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

#ifndef _XSIZE_H
#define _XSIZE_H

/* This file uses _GL_INLINE_HEADER_BEGIN, _GL_INLINE, HAVE_STDINT_H.  */
#if !_GL_CONFIG_H_INCLUDED
 #error "Please include config.h first."
#endif

/* Get size_t.  */
#include <stddef.h>

/* Get INT_MAX, SIZE_MAX.  */
#include <limits.h>
#if HAVE_STDINT_H
# include <stdint.h>
#endif

/* Get ATTRIBUTE_PURE.  */
#include "attribute.h"

_GL_INLINE_HEADER_BEGIN
#ifndef XSIZE_INLINE
# define XSIZE_INLINE _GL_INLINE
#endif

#ifdef __cplusplus
extern "C" {
#endif


/* The size of memory objects is often computed through expressions of
   type size_t. Example:
      void* p = malloc (header_size + n * element_size).
   These computations can lead to overflow.  When this happens, malloc()
   returns a piece of memory that is way too small, and the program then
   crashes while attempting to fill the memory.
   To avoid this, the functions and macros in this file check for overflow.
   The convention is that SIZE_MAX represents overflow.
   malloc (SIZE_MAX) is not guaranteed to fail -- think of a malloc
   implementation that uses mmap --, it's recommended to use size_overflow_p()
   or size_in_bounds_p() before invoking malloc().
   The example thus becomes:
      size_t size = xsum (header_size, xtimes (n, element_size));
      void *p = (size_in_bounds_p (size) ? malloc (size) : NULL);
*/

/* Convert an arbitrary N >= 0 to type size_t.
   N should not have side effects.  */
#define xcast_size_t(N) \
  ((N) <= SIZE_MAX ? (size_t) (N) : SIZE_MAX)

/* Sum of two sizes, with overflow check.  */
XSIZE_INLINE size_t ATTRIBUTE_PURE
xsum (size_t size1, size_t size2)
{
  if (INT_MAX < SIZE_MAX)
    {
      /* Optimize for the common case where size_t arithmetic wraps
         around without undefined behavior.  */
      size_t sum = size1 + size2;
      return size1 <= sum ? sum : SIZE_MAX;
    }

  return size1 <= SIZE_MAX - size2 ? size1 + size2 : SIZE_MAX;
}

/* Sum of three sizes, with overflow check.  */
XSIZE_INLINE size_t ATTRIBUTE_PURE
xsum3 (size_t size1, size_t size2, size_t size3)
{
  return xsum (xsum (size1, size2), size3);
}

/* Sum of four sizes, with overflow check.  */
XSIZE_INLINE size_t ATTRIBUTE_PURE
xsum4 (size_t size1, size_t size2, size_t size3, size_t size4)
{
  return xsum (xsum (xsum (size1, size2), size3), size4);
}

/* Maximum of two sizes, with overflow check.  */
XSIZE_INLINE size_t ATTRIBUTE_PURE
xmax (size_t size1, size_t size2)
{
  /* No explicit check is needed here, because for any n:
     max (SIZE_MAX, n) == SIZE_MAX and max (n, SIZE_MAX) == SIZE_MAX.  */
  return (size1 >= size2 ? size1 : size2);
}

/* Multiplication of a count with an element size, with overflow check.
   The count must be >= 0 and the element size must be > 0.
   Arguments should not have side effects.
   The element size's type should be no wider than size_t.
   This is a macro, not a function, so that it works correctly even
   when N is of a wider type and N > SIZE_MAX.  */
#define xtimes(N, ELSIZE) \
  ((N) <= SIZE_MAX / (ELSIZE) ? (size_t) (N) * (ELSIZE) : SIZE_MAX)

/* Check for overflow.  */
#define size_overflow_p(SIZE) \
  ((SIZE) == SIZE_MAX)
/* Check against overflow.  */
#define size_in_bounds_p(SIZE) \
  ((SIZE) != SIZE_MAX)


#ifdef __cplusplus
}
#endif

_GL_INLINE_HEADER_END

#endif /* _XSIZE_H */