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# 9. Roots

Roots tell the garbage collector where to start tracing. The garbage collector determines which blocks are reachable from the roots, and (in automatically managed pools) reclaims the unreachable blocks. This is quite efficient and can be a very good approximation to liveness.

It is therefore important that all references that the client program can directly access are registered as roots, otherwise the garbage collector might recycle an object that would be used in the future. Some collectors, for example Boehm’s, assume that all references stored in static data are roots; the Memory Pool System is more flexible, but requires the client program to declare which references are roots.

## 9.1. Registering roots

You can register a root at any time by calling one of the `mps_root_create` functions. Roots may not be registered twice, and no two roots may overlap (that is, each reference is fixed by at most one root). Roots may be:

1. on the program’s control stack;
2. in the program’s static data;
3. in heap not managed by the MPS (provided that you destroy the root before freeing it; see the Scheme interpreter’s global symbol table for an example);
4. in manually managed pools (provided that you remove the root before freeing it).

Roots must not be in memory that is subject to garbage collection (and so roots must not be in automatically managed pools).

When you register a root you describe to the MPS how to scan it for references, providing your own scanning function in the cases of `mps_root_create()` and `mps_root_create_fmt()`. Such a root scanning function must follow the Scanning protocol.

All the references in a root are of the same rank (just as in a formatted object). So they are all exact, ambiguous or weak.

Note

If the rank of the root is exact, or weak, the references in the root must always be valid while the root is registered: that is, they must be references to actual objects or null pointers. This could be immediately after the root is registered, so the root must be valid before it is registered.

Note

As with scanning in general, it’s safe to fix references that point to memory not managed by the MPS. These will be ignored.

Roots can be deregistered at any time by calling `mps_root_destroy()`. All roots registered in an arena must be deregistered before the arena is destroyed.

There are five ways to register a root, depending on how you need to scan it for references:

1. `mps_root_create()` if you need a custom root scanning function (of type `mps_root_scan_t`);
2. `mps_root_create_fmt()` if the root consists of a block of objects belonging to an object format, which can be scanned by the format’s scan method (of type `mps_fmt_scan_t`);
3. `mps_root_create_table()` if the root consists of a table of references;
4. `mps_root_create_table_masked()` if the root consists of a table of tagged references;
5. `mps_root_create_reg()` if the root consists of the registers and control stack of a thread. See Thread roots below.

## 9.2. Cautions

Creating a root and then registering is similar to reserving a block and then committing it (in the Allocation point protocol), and similar cautions apply. Before registering a root:

1. The root must be valid (that is, the appropriate root scanning function can scan it).
2. All exact references in the root (references that are fixed by the root scanning function) must contain valid references or null pointers.
3. You must not store a reference in the root to a block in an automatically managed pool (such a reference is hidden from the MPS until you register the root, and may become invalid).

So the typical sequence of operations when creating a root is:

1. Initialize references in the root with null pointers or other safe values.
2. Register the root.
3. Fill in the references in the root.

## 9.3. Thread roots

Every thread’s registers and control stack potentially contain references to allocated objects, so should be registered as a root by calling `mps_root_create_reg()`. It’s not easy to write a scanner for the registers and the stack: it depends on the operating system, the processor architecture, and in some cases on the compiler. For this reason, the MPS provides `mps_stack_scan_ambig()` (and in fact, this is the only supported stack scanner).

A stack scanner needs to know how to find the bottom of the part of the stack to scan. The bottom of the relevant part of stack can be found by taking the address of a local variable in the function that calls the main work function of your thread. You should take care to ensure that the work function is not inlined so that the address is definitely in the stack frame below any potential roots.

For example, here’s the code from the toy Scheme interpreter that registers a thread root and then calls the program:

    mps_thr_t thread;
    mps_root_t reg_root;
    int exit_code;
    void *marker = &marker;

    res = mps_thread_reg(&thread, arena);
    if (res != MPS_RES_OK) error("Couldn't register thread");

    res = mps_root_create_reg(&reg_root,
                              arena,
                              mps_rank_ambig(),
                              0,
                              thread,
                              mps_stack_scan_ambig,
                              marker,
                              0);
    if (res != MPS_RES_OK) error("Couldn't create root");

    exit_code = start(argc, argv);

    mps_root_destroy(reg_root);
    mps_thread_dereg(thread);

## 9.4. Ranks

`mps_rank_t`

  The type of ranks. It is a transparent alias for `unsigned int`, provided for convenience and clarity.

mps_rank_t `mps_rank_ambig`(void)

  Return the rank of ambiguous roots.

mps_rank_t `mps_rank_exact`(void)

  Return the rank of exact roots.

mps_rank_t `mps_rank_weak`(void)

  Return the rank of weak roots.

## 9.5. Root modes

The root mode provides a way for the client to declare various facts about a root that allow the MPS to make optimizations. Roots that are declared to be constant need not be re-scanned, and roots that are declared to be protectable may have barriers placed on them, allowing the MPS to detect whether they have changed.

Note

The MPS does not currently perform either of these optimizations, so root modes have no effect. These features may be added in a future release.

`mps_rm_t`

  The type of root modes.

  It should be zero (meaning neither constant or protectable), or the sum of some subset of `MPS_RM_CONST` and `MPS_RM_PROT`.

`MPS_RM_CONST`

  Deprecated

  starting with version 1.111.

  This was introduced in the hope of being able to maintain a remembered set for the root without needing a write barrier, but it can’t work as described, since you can’t reliably create a valid registered constant root that contains any references. (If you add the references before registering the root, they may have become invalid; but you can’t add them afterwards because the root is supposed to be constant.)

  The root mode for constant roots. This tells the MPS that the client program will not change the root after it is registered: that is, scanning the root will produce the same set of references every time. Furthermore, for roots registered by `mps_root_create_fmt()` and `mps_root_create_table()`, the client program will not write to the root at all.

`MPS_RM_PROT`

  The root mode for protectable roots. This tells the MPS that it may place a barrier(1) on any page which any part of the root covers. No format method or scan method (except for the one for this root) may write data in this root. They may read it.

  Note

  You must not specify `MPS_RM_PROT` on a root allocated by the MPS.

  No page may contain parts of two or more protectable roots. You mustn’t specify `MPS_RM_PROT` if the client program or anything other than (this instance of) the MPS is going to protect or unprotect the relevant pages.

  This mode may not be suitable if the client program wants the operating system to be able to access the root. Many operating systems can’t cope with writing to protected pages.

## 9.6. Root interface

`mps_root_t`

  The type of root descriptions.

  The arena uses root descriptions to find references within the client program’s roots.

mps_res_t `mps_root_create`(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_root_scan_t root_scan, void *p, size_t s)

  Register a root that consists of the references fixed by a scanning function.

  `root_o` points to a location that will hold the address of the new root description.

  `arena` is the arena.

  `rank` is the rank of references in the root.

  `rm` is the root mode.

  `root_scan` is the root scanning function. See `mps_root_scan_t`.

  `p` and `s` are arguments that will be passed to `root_scan` each time it is called. This is intended to make it easy to pass, for example, an array and its size as parameters.

  Returns `MPS_RES_OK` if the root was registered successfully, `MPS_RES_MEMORY` if the new root description could not be allocated, or another result code if there was another error.

  The registered root description persists until it is destroyed by calling `mps_root_destroy()`.

mps_res_t `(*mps_root_scan_t)`(mps_ss_t ss, void *p, size_t s)

  The type of root scanning functions for `mps_root_create()`.

  `ss` is the scan state. It must be passed to `MPS_SCAN_BEGIN()` and `MPS_SCAN_END()` to delimit a sequence of fix operations, and to the functions `MPS_FIX1()` and `MPS_FIX2()` when fixing a reference.

  `p` and `s` are the corresponding values that were passed to `mps_root_create()`.

  Returns a result code. If a fix function returns a value other than `MPS_RES_OK`, the scan method must return that value, and may return without fixing any further references. Generally, it is better if it returns as soon as possible. If the scanning is completed successfully, the function should return `MPS_RES_OK`.

mps_res_t `mps_root_create_fmt`(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_fmt_scan_t fmt_scan, mps_addr_t base, mps_addr_t limit)

  Register a root that consists of the references fixed by a scanning function in a block of formatted objects.

  `root_o` points to a location that will hold the address of the new root description.

  `arena` is the arena.

  `rank` is the rank of references in the root.

  `rm` is the root mode.

  `fmt_scan` is a scanning function. See `mps_fmt_scan_t`.

  `base` is the address of the base of the block of formatted objects.

  `limit` is the address just beyond the end of the block of formatted objects.

  Returns `MPS_RES_OK` if the root was registered successfully, `MPS_RES_MEMORY` if the new root description could not be allocated, or another result code if there was another error.

  The registered root description persists until it is destroyed by calling `mps_root_destroy()`.

mps_res_t `mps_root_create_reg`(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_thr_t thr, mps_reg_scan_t reg_scan, void *p, size_t s)

  Register a root that consists of the references fixed in a thread’s stack by a scanning function.

  `root_o` points to a location that will hold the address of the new root description.

  `arena` is the arena.

  `rank` is the rank of references in the root.

  `rm` is the root mode.

  `thr` is the thread.

  `reg_scan` is a scanning function. See `mps_reg_scan_t`.

  `p` and `s` are arguments that will be passed to `reg_scan` each time it is called. This is intended to make it easy to pass, for example, an array and its size as parameters.

  Returns `MPS_RES_OK` if the root was registered successfully, `MPS_RES_MEMORY` if the new root description could not be allocated, or another result code if there was another error.

  The registered root description persists until it is destroyed by calling `mps_root_destroy()`.

  Note

  It is not supported for client programs to pass their own scanning functions to this function. The built-in MPS function `mps_stack_scan_ambig()` must be used.

  This function is intended as a hook should we ever need to allow client-specific extension or customization of stack and register scanning. If you’re in a position where you need this, for example, if you’re writing a compiler and have control over what goes in the registers, contact us.

mps_res_t `(*mps_reg_scan_t)`(mps_ss_t ss, mps_thr_t thr, void *p, size_t s)

  The type of a root scanning function for roots created with `mps_root_create_reg()`.

  `ss` is the scan state. It must be passed to `MPS_SCAN_BEGIN()` and `MPS_SCAN_END()` to delimit a sequence of fix operations, and to the functions `MPS_FIX1()` and `MPS_FIX2()` when fixing a reference.

  `thr` is the thread.

  `p` and `s` are the corresponding values that were passed to `mps_root_create_reg()`.

  Returns a result code. If a fix function returns a value other than `MPS_RES_OK`, the scan method must return that value, and may return without fixing any further references. Generally, it is better if it returns as soon as possible. If the scanning is completed successfully, the function should return `MPS_RES_OK`.

  A root scan method is called whenever the MPS needs to scan the root. It must then indicate references within the root by calling `MPS_FIX1()` and `MPS_FIX2()`.

  See also

  Scanning.

  Note

  Client programs are not expected to write scanning functions of this type. The built-in MPS function `mps_stack_scan_ambig()` must be used.

mps_reg_scan_t `mps_stack_scan_ambig`()

  A root scanning function for ambiguous scanning of threads, suitable for passing to `mps_root_create_reg()`.

  It scans all integer registers and everything on the stack of the thread given, and can therefore only be used with ambiguous roots. It only scans locations that are at, or higher on the stack (that is, more recently added), the stack bottom that was passed to `mps_thread_reg()`. References are assumed to be represented as machine words, and are required to be 4-byte-aligned; unaligned values are ignored.

  Note

  The MPS provides this function because it’s hard to write: it depends on the operating system, the processor architecture, and in some cases on the compiler.

mps_res_t `mps_root_create_table`(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_addr_t *base, size_t count)

  Register a root that consists of a vector of references.

  `root_o` points to a location that will hold the address of the new root description.

  `arena` is the arena.

  `rank` is the rank of references in the root.

  `rm` is the root mode.

  `base` points to a vector of references.

  `count` is the number of references in the vector.

  Returns `MPS_RES_OK` if the root was registered successfully, `MPS_RES_MEMORY` if the new root description could not be allocated, or another result code if there was another error.

  The registered root description persists until it is destroyed by calling `mps_root_destroy()`.

mps_res_t `mps_root_create_table_masked`(mps_root_t *root_o, mps_arena_t arena, mps_rank_t rank, mps_rm_t rm, mps_addr_t *base, size_t count, mps_word_t mask)

  Register a root that consists of a vector of tagged references.

  `root_o` points to a location that will hold the address of the new root description.

  `arena` is the arena.

  `rank` is the rank of references in the root.

  `rm` is the root mode.

  `base` points to a vector of tagged references.

  `count` is the number of tagged references in the vector.

  `mask` is a bitmask whose set bits specify the location of the tag. References are assumed to have a tag of zero: any value in the vector with a non-zero tag is ignored.

  Returns `MPS_RES_OK` if the root was registered successfully, `MPS_RES_MEMORY` if the new root description could not be allocated, or another result code if there was another error.

  The registered root description persists until it is destroyed by calling `mps_root_destroy()`.

  For example:

      #define TAG_MASK 0x3            /* bottom two bits */

      /* Global symbol table. */
      size_t symtab_size;
      struct {
          obj_t symbol;
          obj_t value;
      } *symtab;

      mps_res_t res;
      mps_root_t root;
      res = mps_root_create_table_masked(&root, arena,
                                         mps_rank_exact(),
                                         (mps_rm_t)0,
                                         symtab, symtab_size * 2,
                                         (mps_word_t)TAG_MASK);
      if (res != MPS_RES_OK) errror("can't create symtab root");

void `mps_root_destroy`(mps_root_t root)

  Deregister a root and destroy its description.

  `root` is the root.

## 9.7. Root introspection

void `mps_arena_roots_walk`(mps_arena_t arena, mps_roots_stepper_t f, void *p, size_t s)

  Deprecated

  starting with version 1.111.

  If you think you need this, there’s probably a better way to achieve what you’re trying to do. Contact us.

  Visit references in registered roots in an arena.

  `arena` is the arena whose roots you want to visit.

  `f` is a function that will be called for each reference to an object in an automatically managed pool class that was found in a registered root belonging to the arena. It takes four arguments: `ref` is the address of a reference to an object in the arena, `root` is the root in which `ref` was found, and `p` and `s` are the corresponding arguments that were passed to `mps_arena_roots_walk()`.

  `p` and `s` are arguments that will be passed to `f` each time it is called. This is intended to make it easy to pass, for example, an array and its size as parameters.

  This function may only be called when the arena is in the parked state.

  See also

  Arenas.

  Note

  If a root is ambiguous then the reference might not be to the start of an object; the client program should handle this case. There is no guarantee that the reference corresponds to the actual location that holds the pointer to the object (since this might be a register, for example), but the actual location will be passed if possible. This may aid analysis of roots via a debugger.

void `(*mps_roots_stepper_t)`(mps_addr_t *ref, mps_root_t root, void *p, size_t s)

  The type of a root stepper function.

  A function of this type can be passed to `mps_arena_roots_walk()`, in which case it will be called for each reference into the arena from a root registered with the arena. It receives four arguments:

  `ref` points to a reference in a root. The reference points to something in the arena. If the root is exact then the reference points to the start of an allocated block, but if the root is ambiguous it might point to somewhere in the middle of an allocated block.

  `root` is the description of the root which contains `ref`.

  `p` and `s` are the corresponding values that were passed to `mps_arena_roots_walk()`.

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