C - 了解 malloc.h 差异:__attribute_malloc__

Question

有人可以帮我找到__attribute_malloc__的意思吗？ 还有__wur？

我正在使用一些关于 120 个 fortran f77 文件的旧代码，但使用了一些 C 文件，它只使用malloc()并且它们# include <malloc.h>.

我有 kdiff'dstdlib.h和malloc.h; 不太确定我能找到两个实现之间的任何区别？

所使用的 Linux 发行版可以对/usr/include/malloc.h的内容负责吗？ 我正在使用具有GCC 11.4的SLES 11.4 x86-64 4.3.4，这是/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

如果你的代码只使用标准的内存分配接口(malloc、realloc、calloc和free)，那么你应该包括stdlib.h，而不应该包括malloc.h。malloc.h是一个非标准标头，它存在有两个原因：向后兼容某些 1980 年代、标准化前的 C 库，以及作为声明一堆malloc接口扩展的地方，这样它们就不会污染不使用它们的人的命名空间。stdlib.h是从中获取内存分配函数的标准位置。 它还声明了一堆与内存分配无关的其他内容。 拿malloc.h和stdlib.h之间的差异不会告诉你任何有用的东西。

你不需要关心__attribute_malloc__或__wur(或__THROW，或者通常 C 库标头中的任何内容，未记录，其名称以两个下划线开头)的含义。 它们是 C 库用来与 C 编译器就不是您的问题进行通信的注释。malloc.h中的malloc声明应该与stdlib.h中的malloc声明完全相同，包括任何注释;如果不是，则它是 C 库中的错误。 由于这是您正在查看其标头的 GNU C 库，我通常会告诉您在 https://sourceware.org/bugzilla/提交错误报告，但是您引用的malloc.h比当前的开发系列早十年，我刚刚检查了一下，它们在当前的开发系列中是相同的，所以如果有区别是因为您的 Linux 发行版已经过时了， 你应该和他们一起讨论这个问题，而不是上游的GNU libc。

但是，为了满足您的好奇心，我将告诉您注释的含义。__attribute_malloc__和__wur是用sys/cdefs.h定义的宏(GNU libc中的每个公共标头都间接包含)。__attribute_malloc__扩展到__attribute__ ((__malloc__))，__wur扩展到__attribute__ ((__warn_unused_result__))或无，具体取决于_FORTIFY_SOURCE的值。

这两种__attribute__ ((...))结构都记录在 GCC 手册中。malloc属性告诉编译器这是一个"类似 malloc 的函数，即函数返回的指针 P 不能在函数返回时为任何其他有效指针提供别名，而且在 P 寻址的任何存储中都没有指向有效对象的指针"，这有助于优化器一点。 (当前版本的GCC已经知道malloc，即使它没有声明该注释，但旧版本没有。warn_unused_result属性"如果具有此属性的函数的调用者不使用其返回值，则会导致发出警告"，这对于malloc的安全功能是有意义的，因为如果您调用malloc并且不使用其返回值，则会出现内存泄漏。