understanding malloc.h difference: __attribute_malloc__

Question

can someone help me find where or explain what __attribute_malloc__ means? And also __wur ?

I am working with some old code about 120 fortran f77 files but uses a few C files where it only uses malloc() and they do # include <malloc.h> .

I have kdiff'd stdlib.h and malloc.h ; not really sure I can find any difference between the two implementations ?

Can the linux distribution being used be responsible for the contents of /usr/include/malloc.h ? I am using SLES 11.4 x86-64 having GCC 4.3.4 and here is /usr/include/malloc.h :

/* Prototypes and definition for malloc implementation.
   Copyright (C) 1996, 1997, 1999, 2000, 2002-2004, 2005, 2007, 2009
   Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library 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.

   The GNU C Library 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 the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#ifndef _MALLOC_H
#define _MALLOC_H 1

#include <features.h>
#include <stddef.h>
#include <stdio.h>

# define __malloc_ptr_t  void *

/* Used by GNU libc internals. */
#define __malloc_size_t size_t
#define __malloc_ptrdiff_t ptrdiff_t

#ifdef __GNUC__

# define __MALLOC_P(args)   args __THROW
/* This macro will be used for functions which might take C++ callback functions.  */
# define __MALLOC_PMT(args) args

#else   /* Not GCC.  */

# define __MALLOC_P(args)   args
# define __MALLOC_PMT(args) args

#endif  /* GCC.  */


__BEGIN_DECLS

/* Allocate SIZE bytes of memory.  */
extern void *malloc __MALLOC_P ((size_t __size)) __attribute_malloc__ __wur;

/* Allocate NMEMB elements of SIZE bytes each, all initialized to 0.  */
extern void *calloc __MALLOC_P ((size_t __nmemb, size_t __size)) __attribute_malloc__ __wur;

/* Re-allocate the previously allocated block in __ptr, making the new block SIZE bytes long.  */
/* __attribute_malloc__ is not used, because if realloc returns
   the same pointer that was passed to it, aliasing needs to be allowed
   between objects pointed by the old and new pointers.  */
extern void *realloc __MALLOC_P ((void *__ptr, size_t __size)) __attribute_warn_unused_result__;

/* Free a block allocated by `malloc', `realloc' or `calloc'.  */
extern void free __MALLOC_P ((void *__ptr));

/* Free a block allocated by `calloc'. */
extern void cfree __MALLOC_P ((void *__ptr));

/* Allocate SIZE bytes allocated to ALIGNMENT bytes.  */
extern void *memalign __MALLOC_P ((size_t __alignment, size_t __size)) __attribute_malloc__ __wur;

/* Allocate SIZE bytes on a page boundary.  */
extern void *valloc __MALLOC_P ((size_t __size)) __attribute_malloc__ __wur;

/* Equivalent to valloc(minimum-page-that-holds(n)), that is, round up __size to nearest pagesize. */
extern void * pvalloc __MALLOC_P ((size_t __size)) __attribute_malloc__ __wur;

/* Underlying allocation function; successive calls should return contiguous pieces of memory.  */
extern void *(*__morecore) __MALLOC_PMT ((ptrdiff_t __size));

/* Default value of `__morecore'.  */
extern void *__default_morecore __MALLOC_P ((ptrdiff_t __size)) __attribute_malloc__;

/* SVID2/XPG mallinfo structure */

struct mallinfo {
  int arena;    /* non-mmapped space allocated from system */
  int ordblks;  /* number of free chunks */
  int smblks;   /* number of fastbin blocks */
  int hblks;    /* number of mmapped regions */
  int hblkhd;   /* space in mmapped regions */
  int usmblks;  /* maximum total allocated space */
  int fsmblks;  /* space available in freed fastbin blocks */
  int uordblks; /* total allocated space */
  int fordblks; /* total free space */
  int keepcost; /* top-most, releasable (via malloc_trim) space */
};

/* Returns a copy of the updated current mallinfo. */
extern struct mallinfo mallinfo __MALLOC_P ((void));

/* SVID2/XPG mallopt options */
#ifndef M_MXFAST
# define M_MXFAST  1    /* maximum request size for "fastbins" */
#endif
#ifndef M_NLBLKS
# define M_NLBLKS  2    /* UNUSED in this malloc */
#endif
#ifndef M_GRAIN
# define M_GRAIN   3    /* UNUSED in this malloc */
#endif
#ifndef M_KEEP
# define M_KEEP    4    /* UNUSED in this malloc */
#endif

/* mallopt options that actually do something */
#define M_TRIM_THRESHOLD    -1
#define M_TOP_PAD           -2
#define M_MMAP_THRESHOLD    -3
#define M_MMAP_MAX          -4
#define M_CHECK_ACTION      -5
#define M_PERTURB       -6
#define M_ARENA_TEST        -7
#define M_ARENA_MAX     -8

/* General SVID/XPG interface to tunable parameters. */
extern int mallopt __MALLOC_P ((int __param, int __val));

/* Release all but __pad bytes of freed top-most memory back to the
   system. Return 1 if successful, else 0. */
extern int malloc_trim __MALLOC_P ((size_t __pad));

/* Report the number of usable allocated bytes associated with allocated chunk __ptr. */
extern size_t malloc_usable_size __MALLOC_P ((void *__ptr));

/* Prints brief summary statistics on stderr. */
extern void malloc_stats __MALLOC_P ((void));

/* Output information about state of allocator to stream FP.  */
extern int malloc_info (int __options, FILE *__fp);

/* Record the state of all malloc variables in an opaque data structure. */
extern void *malloc_get_state __MALLOC_P ((void));

/* Restore the state of all malloc variables from data obtained with malloc_get_state(). */
extern int malloc_set_state __MALLOC_P ((void *__ptr));

/* Called once when malloc is initialized; redefining this variable in
   the application provides the preferred way to set up the hook pointers. */
extern void (*__malloc_initialize_hook) __MALLOC_PMT ((void));

/* Hooks for debugging and user-defined versions. */
extern void (*__free_hook) __MALLOC_PMT ((void *__ptr, __const __malloc_ptr_t));
extern void *(*__malloc_hook) __MALLOC_PMT ((size_t __size, __const __malloc_ptr_t));
extern void *(*__realloc_hook) __MALLOC_PMT ((void *__ptr, size_t __size, __const __malloc_ptr_t));
extern void *(*__memalign_hook) __MALLOC_PMT ((size_t __alignment, size_t __size, __const __malloc_ptr_t));
extern void (*__after_morecore_hook) __MALLOC_PMT ((void));

/* Activate a standard set of debugging hooks. */
extern void __malloc_check_init __MALLOC_P ((void));


__END_DECLS

#endif /* malloc.h */

Answer 1

If your code uses only the standard memory allocation interfaces ( malloc , realloc , calloc , and free ), then you should include stdlib.h , and you should not include malloc.h . malloc.h is a nonstandard header which exists for two reasons: backward compatibility with certain 1980s-era, pre-standardization C libraries, and as a place to declare a bunch of extensions to the malloc interface, so that they don't pollute the namespace for people who aren't using them. stdlib.h is the standard place to get the memory allocation functions from. It also declares a bunch of other stuff that isn't related to memory allocation. Taking the diff between malloc.h and stdlib.h will not tell you anything useful.

You don't need to care what either __attribute_malloc__ or __wur (or __THROW , or in general anything that's in a C library header, isn't documented, and whose name begins with two underscores) means. They are annotations used by the C library to communicate with the C compiler about things which are Not Your Problem. The declaration of malloc in malloc.h is supposed to be exactly the same as the declaration of malloc in stdlib.h , including any annotations; if they are not, it is a bug in the C library. Since this is the GNU C Library whose headers you're looking at, I would normally tell you to file a bug report at https://sourceware.org/bugzilla/ , but the malloc.h you quoted is ten years older than the current development series, and I just checked and they are the same in the current development series, so if there's a difference it is because your Linux distribution is out of date, and you should take that up with them, not with GNU libc upstream.

To satisfy your curiosity, however, I will tell you what the annotations mean. __attribute_malloc__ and __wur are macros defined in sys/cdefs.h (which is included, indirectly, by every public header in GNU libc). __attribute_malloc__ expands to __attribute__ ((__malloc__)) , and __wur expands to __attribute__ ((__warn_unused_result__)) or to nothing, depending on the value of _FORTIFY_SOURCE .

Both of these __attribute__ ((...)) constructs are documented in the GCC manual . The malloc attribute tells the compiler that this is a function that "is malloc-like, ie, that the pointer P returned by the function cannot alias any other pointer valid when the function returns, and moreover no pointers to valid objects occur in any storage addressed by P", which helps the optimizer out a bit. (Current versions of GCC already know this about malloc , even if it's declared without that annotation, but older versions didn't.) The warn_unused_result attribute "causes a warning to be emitted if a caller of the function with this attribute does not use its return value", which makes sense as a safety feature for malloc , because if you call malloc and don't use its return value you have a memory leak.