Debugging features for client objects

author Pekka P. Pirinen
date 1998-09-10
index terms pair: debugging; design
revision //
status incomplete design
tag design.mps.object-debug


.intro: This is the design for all the various debugging features that MPS clients (and sometimes MPS developers) can use to discover what is happening to their objects and the memory space.

.readership: MPS developers.


.over.fenceposts: In its current state, this document mostly talks about fenceposts, straying a little into tagging where theses features have an effect on each other.


There exist other documents that list other required features, and propose interfaces and implementations. These will eventually be folded into this one. Pekka P. Pirinen, 1998-09-10.


.req.fencepost: Try to detect overwrites and underwrites of allocated blocks by adding fenceposts (source req.product.??? VC++, [TODO: Locate the relevant product requirement. RB 2023-02-23]

.req.fencepost.size: The fenceposts should be at least 4 bytes on either side or 8 bytes if on one side only, with an adjustable content (although VC++ only has 4 bytes with pattern 0xFDFDFDFD, having unwisely combined the implementation with other debug features).

.req.fencepost.check: There should be a function to check all the fenceposts (source Try to detect attempts to write and read free blocks.

.req.walk: There should be a way to map ("walk") a user function over all allocated objects (except PS VM objects), possibly only in a separate debugging variety/mode (source

.req.tag: There should be a way to store at least a word of user data (a "tag", borrowing the SW term) with every object in debugging mode, to be used in memory dumps (source req.product.??? VC++). [TODO: Locate the relevant product requirement. RB 2023-02-23]

.req.tag.walk: The walking function (as required by .req.walk) should have access to this data (source

.req.dump.aver: It must be possible to perform a memory dump after an AVER() has fired. Naturally, if the information required for the dump has been corrupted, it will fail, as softly as possible (source @@@@).

.req.portable: Client code that uses these features must be easily portable to all the supported platforms. (Source: job003749.)


There are more requirements, especially about memory dumps and allocation locations. Pekka P. Pirinen, 1998-09-10.

Solution ideas

.note.assumptions: I've tried not to assume anything about the coincidence of manual/automatic, formatted/unformatted, and ap/mps_alloc. I think those questions deserve to be decided on their own merits. instead of being constrained by a debug feature.

.fence.content.repeat: The content of a fencepost could be specified as a byte/word which used repeatedly to fill the fencepost.

.fence.content.template: The content could be given as a template which is of the right size and is simply copied onto the fencepost.

.fence.content.template.repeat: The content could be given as a template which is copied repeatedly until the fencepost is full. (This would avoid the need to specify different templates on different architectures, and so help meet .req.portable.)

.fence.walk: .req.fencepost.check requires the ability to find all the allocated objects. In formatted pools, this is not a problem. In unformatted pools, we could use the walker. It's a feasible strategy to bet that any pool that might have to support fenceposting will also have a walking requirement.

.fence.tag: Fenceposting also needs to keep track which objects have fenceposts. unless we manage to do them all. It would be easiest to put this in the tags.

.fence.check.object: A function to check the fenceposts on a given object would be nice.

.fence.ap: AP's could support fenceposting transparently by having a mode where mps_reserve() always goes out-of-line and fills in the fenceposts (the pool's BufferFill() method isn't involved). This would leave the MPS with more freedom of implementation, especially when combined with some of the other ideas. We think doing a function call for every allocation is not too bad for debugging.

.fence.outside-ap: We could also let the client insert their own fenceposts outside the MPS allocation mechanism. Even if fenceposting were done like this, we'd still want it to be an MPS feature, so we'd offer sample C macros for adding the size of the fencepost and filling in the fencepost pattern. Possibly something like this (while we could still store the parameters in the pool or allocation point, there seems little point in doing so in this case, and having them as explicit parameters to the macros allows the client to specify constants to gain effiency):

#define mps_add_fencepost(size, fp_size)
#define mps_fill_fenceposts(obj, size, fp_size, fp_pattern)

The client would need to supply their own fencepost checking function, obviously, but again we could offer one that matches the sample macros.

.fence.tail-only: In automatic pools, the presence of a fencepost at the head of the allocated block results in the object reference being an internal pointer. This means that the format or the pool would need to know about fenceposting and convert between references and pointers. This would slow down the critical path when fenceposting is used. This can be ameliorated by putting a fencepost at the tail of the block only: this obviates the internal pointer problem and could provide almost the same degree of checking (provided the size was twice as large), especially in copying pools, where there are normally no gaps between allocated blocks. In addition to the inescapable effects on allocation and freeing (including copying and reclaim thereunder), only scanning would have to know about fenceposts.

.fence.tail-only.under: Walking over all the objects in the pool would be necessary to detect underwrites, as one couldn't be sure that there is a fencepost before any given object (or where it's located exactly). If the pool were doing the checking, it could be sure: it would know about alignments and it could put fenceposts in padding objects (free blocks will have them because they were once allocated) so there'd be one on either side of any object (except at the head of a segment, which is not a major problem, and could be fixed by adding a padding object at the beginning of every segment). This requires some cleverness to avoid splinters smaller than the fencepost size, but it can be done.

.fence.wrapper: On formatted pools, fenceposting could be implemented by "wrapping" the client-supplied format at creation time. The wrapper can handle the conversion from the fenceposted object and back. This will be invisible to the client and gives the added benefit that the wrapper can validate fenceposts on every format operation, should it desire. That is, the pool would see the fenceposts as part of the client object, but the client would only see its object; the format wrapper would translate between the two. Note that hiding the fenceposts from scan methods, which are required to take a contiguous range of objects, is a bit complicated.

.fence.client-format: The MPS would supply such a wrapper, but clients could also be allowed to write their own fenceposted formats (provided they coordinate with allocation, see below). This would make scanning fenceposted segments more efficient.

.fence.wrapper.variable: Furthermore, you could create different classes of fencepost within a pool, because the fencepost itself could have a variable format. For instance, you might choose to have the fencepost be minimal (one to two words) for small objects, and more detailed/complex for large objects (imagining that large objects are likely vector-ish and subject to overruns). You could get really fancy and have the fencepost class keyed to the object class (for example, different allocation points create different classes of fenceposting).

.fence.wrapper.alloc: Even with a wrapped format, allocation and freeing would still have know about the fenceposts. If allocation points are used, either MPS-side (.fence.ap) or client-side (.fence.outside-ap) fenceposting could be used, with the obvious modifications.

.fence.wrapper.alloc.format: We could add three format methods, to adjust the pointer and the size for alloc and free, to put down the fenceposts during alloc, and to check them; to avoid slowing down all allocation, this would require some MOPping to make the format class affect the choice of the alloc and free methods (see mail.pekka.1998-06-11.18-18).

.fence.wrapper.alloc.size: We could just communicate the size of the fenceposts between the format and the allocation routines, but then you couldn't use variable fenceposts (.fence.wrapper.variable).


All this applies to copying and reclaim in a straight-forward manner, I think.

.fence.pool.wrapper: Pools can be wrapped as well. This could be a natural way to represent/implement the fenceposting changes to the Alloc and Free methods. [@@@@alignment] We could simply offer a debugging version of each pool class (e.g., mps_pool_class_mv_debug()). As we have seen, debugging features have synergies which make it advantageous to have a coordinated implementation, so splitting them up would not just complicate the client interface, it would also be an implementation problem; we can turn features on or off with pool init parameters.

.fence.pool.abstract: We could simply use pool init parameters only to control all debugging features (optargs would be useful here). While there migh be subclasses and wrappers internally, the client would only see a single pool class; in the internal view, this would be an abstract class, and the parameters would determine which concrete class actually gets instantiated.

.tag.out-of-line: It would be nice if tags were stored out-of-line, so they can be used to study allocation patterns and fragmentation behaviours. Such an implementation of tagging could also easily be shared among several pools.


.pool: The implementation is at the pool level, because pools manage allocated objects. A lot of the code will be generic, naturally, but the data structures and the control interfaces attach to pools. In particular, clients will be able to use tagging and fenceposting separately on each pool.

.fence.size: Having fenceposts of adjustable size and pattern is useful. Restricting the size to an integral multiple of the [pool or format?] alignment would simplify the implementation but breaks .req.portable.

.fence.template: We use templates (.fence.content.template) to fill in the fenceposts, but we do not give any guarantees about the location of the fenceposts. This leaves us the opportunity to do tail-only fenceposting, if we choose.

.fence.slop: [see impl.c.dbgpool.FenceAlloc @@@@] We check the fenceposts when freeing an object.

.unified-walk: Combine the walking and tagging requirements (.req.tag.walk and @@@@) into a generic facility for walking and tagging objects with just one interface and one name: tagging. Also combine the existing formatted object walker into this metaphor, but allowing the format and tag parameters of the step function be optional.


This part has not been implemented yet Pekka P. Pirinen, 1998-09-10.

.init: It simplifies the implementation of both tagging and fenceposting if they are always on, so that we don't have to keep track of which objects have been fenceposted and which have not, and don't have to have three kinds of tags: for user data, for fenceposting, and for both. So we determine this at pool init time (and let fenceposting turn on tagging, if necessary).

.pool-parameters: Fencepost templates and tag formats are passed in as pool parameters.

.modularity: While a combined generic implementation of tags and fenceposts is provided, it is structured so that each part of it could be implemented by a pool-specific mechanism with a minimum of new protocol.


This will be improved, when we figure out formatted pools -- they don't need tags for fenceposting.

.out-of-space: If there's no room for tags, we just fail to allocate the tag. We free the block allocated for the object and fail the allocation, so that the client gets a chance to do whatever low-memory actions they might want to do.

This breaks the one-to-one relationship between tags and objects, so some checks cannot be made, but we do count the "lost" tags.


Need to hash out how to do fenceposting in formatted pools.

Client interface

.interface.fenceposting.check: mps_pool_check_fenceposts() is a function to check all fenceposts in a pool (AVER() if a problem is found)


From here on, these are tentative and incomplete.

mps_res_t mps_fmt_fencepost_wrap(mps_fmt_t *format_return, mps_arena_t arena, mps_fmt_t format, ...)

.interface.fenceposting.format: A function to wrap a format (class) to provide fenceposting.

typedef void (*mps_fmt_adjust_fencepost_t)(size_t *size_io)

.interface.fenceposting.adjust: A format method to adjust size of a block about to be allocted to allow for fenceposts.

typedef void (*mps_fmt_put_fencepost_t)(mps_addr_t * addr_io, size_t size)

.interface.fenceposting.add: A format method to add a fencepost around a block about to be allocated. The NULL method adds a tail fencepost.

typedef mps_bool_t (*mps_fmt_check_fenceposts_t)(mps_addr_t)

.interface.fenceposting.checker: A format method to check the fenceposts around an object. The NULL method checks tails.

mps_res_t mps_alloc_dbg(mps_addr_t *, mps_pool_t, size_t, ...) mps_res_t mps_alloc_dbg_v(mps_addr_t *, mps_pool_t, size_t, va_list)

.interface.tags.alloc: Two functions to extend the existing mps_alloc() (request.???.??? proposes to remove the varargs) [TODO: Locate the relevant Harlequin request. RB 2023-02-23]

typedef void (*mps_objects_step_t)(mps_addr_t addr, size_t size, mps_fmt_t format, mps_pool_t pool, void *tag_data, void *p)

.interface.tags.walker.type: Type of walker function for mps_pool_walk() and mps_arena_walk().

.interface.tags.walker: Functions to walk all the allocated objects in an arena (only client pools in this case), format and tag_data can be NULL (tag_data really wants to be void *, not mps_addr_t, because it's stored together with the internal tag data in an MPS internal pool)



#define MPS_ALLOC_DBG(res_io, addr_io, pool, size)
    static mps_tag_A_s _ts = { __FILE__, __LINE__ };

    *res_io = mps_alloc(addr_io, pool, size, _ts_)


.new-pool: The client interface to control fenceposting consists of the new classes mps_pool_class_mv_debug(), mps_pool_class_epdl_debug(), and mps_pool_class_epdr_debug(), and their new init parameter of type mps_pool_debug_option_s.


This is a temporary solution, to get it out without writing lots of new interface. Pekka P. Pirinen, 1998-09-10.

.new-pool.impl: The debug pools are implemented using the "class wrapper" EnsureDebugClass(), which produces a subclass with modified init, finish, alloc, and free methods. These methods are implemented in the generic debug class code (impl.c.dbgpool), and are basically wrappers around the superclass methods (invoked through the pool->class->super field). To find the data stored in the class for the debugging features, they use the debugMixin method provided by the subclass. So to make a debug subclass, three things should be provided: a structure definition of the instance containing a PoolDebugMixinStruct, a pool class function that uses EnsureDebugClass(), and a debugMixin method that locates the PoolDebugMixinStruct within an instance.

.tags.splay: The tags are stored in a splay tree of tags allocated from a subsidiary MFS pool. The client needs to specify the (maximum) size of the client data in a tag, so that the pool can be created.


Lots more should be said, eventually. Pekka P. Pirinen, 1998-09-10.

Document History

  • 1998-09-10 Pekka Pirinen The first draft merely records all the various ideas about fenceposting that came up in discussions in June, July and September 1998. This includes the format wrapping idea from mail.ptw.1998-06-19.21-13.
  • 2002-06-07 RB Converted from MMInfo database design document.
  • 2013-04-14 GDR Converted to reStructuredText.
  • 2014-04-09 GDR Added newly discovered requirement .req.portable.