Smart Memory Allocation
Keywords: developers

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Smart Memory Allocation

Few things are as embarrassing as a program that leaks, yet few errors are so easy to commit or as difficult to track down in a large, complicated program as failure to release allocated memory. SMARTALLOC replaces the standard C library memory allocation functions with versions which keep track of buffer allocations and releases and report all orphaned buffers at the end of program execution. By including this package in your program during development and testing, you can identify code that loses buffers right when it's added and most easily fixed, rather than as part of a crisis debugging push when the problem is identified much later in the testing cycle (or even worse, when the code is in the hands of a customer). When program testing is complete, simply recompiling with different flags removes SMARTALLOC from your program, permitting it to run without speed or storage penalties.

In addition to detecting orphaned buffers, SMARTALLOC also helps to find other common problems in management of dynamic storage including storing before the start or beyond the end of an allocated buffer, referencing data through a pointer to a previously released buffer, attempting to release a buffer twice or releasing storage not obtained from the allocator, and assuming the initial contents of storage allocated by functions that do not guarantee a known value. SMARTALLOC's checking does not usually add a large amount of overhead to a program (except for programs which use `realloc()` extensively; see below). SMARTALLOC focuses on proper storage management rather than internal consistency of the heap as checked by the malloc_debug facility available on some systems. SMARTALLOC does not conflict with malloc_debug and both may be used together, if you wish. SMARTALLOC makes no assumptions regarding the internal structure of the heap and thus should be compatible with any C language implementation of the standard memory allocation functions.

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Installing SMARTALLOC

SMARTALLOC is provided as a Zipped archive, smartall.ziphttp://www.fourmilab.ch/smartall/smartall.zip; see the download instructions below.

To install SMARTALLOC in your program, simply add the statement:

to every C program file which calls any of the memory allocation functions (`malloc`, `calloc`, `free`, etc.). SMARTALLOC must be used for all memory allocation with a program, so include file for your entire program, if you have such a thing. Next, define the symbol SMARTALLOC in the compilation before the inclusion of smartall.h. I usually do this by having my Makefile add the ```-DSMARTALLOC`'' option to the C compiler for non-production builds. You can define the symbol manually, if you prefer, by adding the statement:

`#define SMARTALLOC`

At the point where your program is all done and ready to relinquish control to the operating system, add the call:

`sm_dump(`datadump`);`

where datadump specifies whether the contents of orphaned buffers are to be dumped in addition printing to their size and place of allocation. The data are dumped only if datadump is nonzero, so most programs will normally use ```sm_dump(0);`''. If a mysterious orphaned buffer appears that can't be identified from the information this prints about it, replace the statement with ```sm_dump(1)`;''. Usually the dump of the buffer's data will furnish the additional clues you need to excavate and extirpate the elusive error that left the buffer allocated.

Finally, add the files ``smartall.h'' and ``smartall.c'' from this release to your source directory, make dependencies, and linker input. You needn't make inclusion of smartall.c in your link optional; if compiled with SMARTALLOC not defined it generates no code, so you may always include it knowing it will waste no storage in production builds. Now when you run your program, if it leaves any buffers around when it's done, each will be reported by `sm_dump()` on stderr as follows:

    Orphaned buffer:     120 bytes allocated at line 50 of gutshot.c

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Squelching a SMARTALLOC

Usually, when you first install SMARTALLOC in an existing program you'll find it nattering about lots of orphaned buffers. Some of these turn out to be legitimate errors, but some are storage allocated during program initialisation that, while dynamically allocated, is logically static storage not intended to be released. Of course, you can get rid of the complaints about these buffers by adding code to release them, but by doing so you're adding unnecessary complexity and code size to your program just to silence the nattering of a SMARTALLOC, so an escape hatch is provided to eliminate the need to release these buffers.

Normally all storage allocated with the functions `malloc()`, `calloc()`, and `realloc()` is monitored by SMARTALLOC. If you make the function call:

            sm_static(1);

you declare that subsequent storage allocated by `malloc()`, `calloc()`, and `realloc()` should not be considered orphaned if found to be allocated when `sm_dump()` is called. I use a call on ```sm_static(1);`'' before I allocate things like program configuration tables so I don't have to add code to release them at end of program time. After allocating unmonitored data this way, be sure to add a call to:

            sm_static(0);

to resume normal monitoring of buffer allocations. Buffers allocated while `sm_static(1`) is in effect are not checked for having been orphaned but all the other safeguards provided by SMARTALLOC remain in effect. You may release such buffers, if you like; but you don't have to.

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Living with Libraries

Some library functions for which source code is unavailable may gratuitously allocate and return buffers that contain their results, or require you to pass them buffers which they subsequently release. If you have source code for the library, by far the best approach is to simply install SMARTALLOC in it, particularly since this kind of ill-structured dynamic storage management is the source of so many storage leaks. Without source code, however, there's no option but to provide a way to bypass SMARTALLOC for the buffers the library allocates and/or releases with the standard system functions.

For each function xxx redefined by SMARTALLOC, a corresponding routine named ```actually`xxx'' is furnished which provides direct access to the underlying system function, as follows:

> Standard function | Direct access function
> `malloc(`size`)` | `actuallymalloc(`size`)`
> `calloc(`nelem`,` elsize`)` | `actuallycalloc(`nelem, elsize`)`
> `realloc(`ptr`,` size`)` | `actuallyrealloc(`ptr, size`)`
> `free(`ptr`)` | `actuallyfree(`ptr`)`

For example, suppose there exists a system library function named ```getimage()`'' which reads a raster image file and returns the address of a buffer containing it. Since the library routine allocates the image directly with `malloc()`, you can't use SMARTALLOC's `free()`, as that call expects information placed in the buffer by SMARTALLOC's special version of `malloc()`, and hence would report an error. To release the buffer you should call `actuallyfree()`, as in this code fragment:

            struct image *ibuf = getimage("ratpack.img");
            display_on_screen(ibuf);
            actuallyfree(ibuf);

Conversely, suppose we are to call a library function, ```putimage()`'', which writes an image buffer into a file and then releases the buffer with `free()`. Since the system `free()` is being called, we can't pass a buffer allocated by SMARTALLOC's allocation routines, as it contains special information that the system `free()` doesn't expect to be there. The following code uses `actuallymalloc()` to obtain the buffer passed to such a routine.

            struct image *obuf =
               (struct image *) actuallymalloc(sizeof(struct image));
            dump_screen_to_image(obuf);
            putimage("scrdump.img", obuf);  /* putimage() releases obuf */

It's unlikely you'll need any of the ``actually'' calls except under very odd circumstances (in four products and three years, I've only needed them once), but they're there for the rare occasions that demand them. Don't use them to subvert the error checking of SMARTALLOC; if you want to disable orphaned buffer detection, use the `sm_static(1)` mechanism described above. That way you don't forfeit all the other advantages of SMARTALLOC as you do when using `actuallymalloc()` and `actuallyfree()`.

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SMARTALLOC Details

When you include ``smartall.h'' and define SMARTALLOC, the following standard system library functions are redefined with the #define mechanism to call corresponding functions within smartall.c instead. (For details of the redefinitions, please refer to smartall.h.)

            void *malloc(size_t size)
            void *calloc(size_t nelem, size_t elsize)
            void *realloc(void *ptr, size_t size)
            void free(void *ptr)
            void cfree(void *ptr)

`cfree()` is a historical artifact identical to `free()`.

In addition to allocating storage in the same way as the standard library functions, the SMARTALLOC versions expand the buffers they allocate to include information that identifies where each buffer was allocated and to chain all allocated buffers together. When a buffer is released, it is removed from the allocated buffer chain. A call on `sm_dump()` is able, by scanning the chain of allocated buffers, to find all orphaned buffers. Buffers allocated while `sm_static(1)` is in effect are specially flagged so that, despite appearing on the allocated buffer chain, `sm_dump()` will not deem them orphans.

When a buffer is allocated by `malloc()` or expanded with `realloc()`, all bytes of newly allocated storage are set to the hexadecimal value 0x55 (alternating one and zero bits). Note that for `realloc()` this applies only to the bytes added at the end of buffer; the original contents of the buffer are not modified. Initializing allocated storage to a distinctive nonzero pattern is intended to catch code that erroneously assumes newly allocated buffers are cleared to zero; in fact their contents are random. The `calloc()` function, defined as returning a buffer cleared to zero, continues to zero its buffers under SMARTALLOC.

Buffers obtained with the SMARTALLOC functions contain a special sentinel byte at the end of the user data area. This byte is set to a special key value based upon the buffer's memory address. When the buffer is released, the key is tested and if it has been overwritten an assertion in the `free` function will fail. This catches incorrect program code that stores beyond the storage allocated for the buffer. At `free()` time the queue links are also validated and an assertion failure will occur if the program has destroyed them by storing before the start of the allocated storage.

In addition, when a buffer is released with `free()`, its contents are immediately destroyed by overwriting them with the hexadecimal pattern 0xAA (alternating bits, the one's complement of the initial value pattern). This will usually trip up code that keeps a pointer to a buffer that's been freed and later attempts to reference data within the released buffer. Incredibly, this is legal in the standard Unix memory allocation package, which permits programs to free() buffers, then raise them from the grave with `realloc()`. Such program ``logic'' should be fixed, not accommodated, and SMARTALLOC brooks no such Lazarus buffer`` nonsense.

Some C libraries allow a zero size argument in calls to `malloc()`. Since this is far more likely to indicate a program error than a defensible programming stratagem, SMARTALLOC disallows it with an assertion.

When the standard library `realloc()` function is called to expand a buffer, it attempts to expand the buffer in place if possible, moving it only if necessary. Because SMARTALLOC must place its own private storage in the buffer and also to aid in error detection, its version of `realloc()` always moves and copies the buffer except in the trivial case where the size of the buffer is not being changed. By forcing the buffer to move on every call and destroying the contents of the old buffer when it is released, SMARTALLOC traps programs which keep pointers into a buffer across a call on `realloc()` which may move it. This strategy may prove very costly to programs which make extensive use of `realloc()`. If this proves to be a problem, such programs may wish to use `actuallymalloc()`, `actuallyrealloc()`, and `actuallyfree()` for such frequently-adjusted buffers, trading error detection for performance. Although not specified in the System V Interface Definition, many C library implementations of `realloc()` permit an old buffer argument of NULL, causing `realloc()` to allocate a new buffer. The SMARTALLOC version permits this.

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When SMARTALLOC is Disabled

When SMARTALLOC is disabled by compiling a program with the symbol SMARTALLOC not defined, calls on the functions otherwise redefined by SMARTALLOC go directly to the system functions. In addition, compile-time definitions translate calls on the ''`actually`...`()``` functions into the corresponding library calls; ''`actuallymalloc(100)```, for example, compiles into ''`malloc(100)```. The two special SMARTALLOC functions, `sm_dump()` and `sm_static()`, are defined to generate no code (hence the null statement). Finally, if SMARTALLOC is not defined, compilation of the file smartall.c generates no code or data at all, effectively removing it from the program even if named in the link instructions.

Thus, except for unusual circumstances, a program that works with SMARTALLOC defined for testing should require no changes when built without it for production release.

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The `alloc()` Function

Many programs I've worked on use very few direct calls to `malloc()`, using the identically declared `alloc()` function instead. Alloc detects out-of-memory conditions and aborts, removing the need for error checking on every call of `malloc()` (and the temptation to skip checking for out-of-memory).

As a convenience, SMARTALLOC supplies a compatible version of `alloc()` in the file alloc.c, with its definition in the file alloc.h. This version of `alloc()` is sensitive to the definition of SMARTALLOC and cooperates with SMARTALLOC's orphaned buffer detection. In addition, when SMARTALLOC is defined and `alloc()` detects an out of memory condition, it takes advantage of the SMARTALLOC diagnostic information to identify the file and line number of the call on `alloc()` that failed.

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Overlays and Underhandedness

String constants in the C language are considered to be static arrays of characters accessed through a pointer constant. The arrays are potentially writable even though their pointer is a constant. SMARTALLOC uses the compile-time definition `./smartall.wml` to obtain the name of the file in which a call on buffer allocation was performed. Rather than reserve space in a buffer to save this information, SMARTALLOC simply stores the pointer to the compiled-in text of the file name. This works fine as long as the program does not overlay its data among modules. If data are overlayed, the area of memory which contained the file name at the time it was saved in the buffer may contain something else entirely when `sm_dump()` gets around to using the pointer to edit the file name which allocated the buffer.

If you want to use SMARTALLOC in a program with overlayed data, you'll have to modify smartall.c to either copy the file name to a fixed-length field added to the `abufhead` structure, or else allocate storage with `malloc()`, copy the file name there, and set the `abfname` pointer to that buffer, then remember to release the buffer in `sm_free`. Either of these approaches are wasteful of storage and time, and should be considered only if there is no alternative. Since most initial debugging is done in non-overlayed environments, the restrictions on SMARTALLOC with data overlaying may never prove a problem. Note that conventional overlaying of code, by far the most common form of overlaying, poses no problems for SMARTALLOC; you need only be concerned if you're using exotic tools for data overlaying on MS-DOS or other address-space-challenged systems.

Since a C language ''constant`` string can actually be written into, most C compilers generate a unique copy of each string used in a module, even if the same constant string appears many times. In modules that contain many calls on allocation functions, this results in substantial wasted storage for the strings that identify the file name. If your compiler permits optimization of multiple occurrences of constant strings, enabling this mode will eliminate the overhead for these strings. Of course, it's up to you to make sure choosing this compiler mode won't wreak havoc on some other part of your program.

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Test and Demonstration Program

A test and demonstration program, smtest.c, is supplied with SMARTALLOC. You can build this program with the Makefile included. Please refer to the comments in smtest.c and the Makefile for information on this program. If you're attempting to use SMARTALLOC on a new machine or with a new compiler or operating system, it's a wise first step to check it out with smtest first.

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Invitation to the Hack

SMARTALLOC is not intended to be a panacea for storage management problems, nor is it universally applicable or effective; it's another weapon in the arsenal of the defensive professional programmer attempting to create reliable products. It represents the current state of evolution of expedient debug code which has been used in several commercial software products which have, collectively, sold more than third of a million copies in the retail market, and can be expected to continue to develop through time as it is applied to ever more demanding projects.

The version of SMARTALLOC here has been tested on a Sun SPARCStation, Silicon Graphics Indigo2, and on MS-DOS using both Borland and Microsoft C. Moving from compiler to compiler requires the usual small changes to resolve disputes about prototyping of functions, whether the type returned by buffer allocation is `char *` or `void *`, and so forth, but following those changes it works in a variety of environments. I hope you'll find SMARTALLOC as useful for your projects as I've found it in mine.

Subsections

- - Installing SMARTALLOC
  - Squelching a SMARTALLOC
  - Living with Libraries
  - SMARTALLOC Details
  - When SMARTALLOC is Disabled
  - The `alloc()` Function
  - Overlays and Underhandedness
  - Test and Demonstration Program
  - Invitation to the Hack
- http://www.fourmilab.ch/smartall/smartall.zip Download smartall.ziphttp://www.fourmilab.ch/smartall/smartall.zip (Zipped archive)
  - Copying