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# 2. Error handing

Operations in the Memory Pool System that might fail return a result code of type `mps_res_t`, rather than a “special value” of the return type.

Success is always indicated by the result code `MPS_RES_OK`, which is defined to be zero. Other result codes indicate failure, and are non-zero.

The modular nature of the MPS means that it is not usually possible for a function description to list the possible error codes that it might return. A function in the public interface typically calls methods of an arena class and one or more pool classes, any of which might fail. The MPS is extensible with new arena and pool classes, which might fail in new and interesting ways, so the only future-proof behaviour is for a client program to assume that any MPS function that returns a result code can return any result code.

`mps_res_t`

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

  A result code indicates the success or failure of an operation, along with the reason for failure. As with error numbers in Unix, the meaning of a result code depends on the call that returned it. Refer to the documentation of the function for the exact meaning of each result code.

  The result codes are:

  - `MPS_RES_OK`: operation succeeded.
  - `MPS_RES_FAIL`: operation failed.
  - `MPS_RES_IO`: an input/output error occurred.
  - `MPS_RES_LIMIT`: an internal limitation was exceeded.
  - `MPS_RES_MEMORY`: needed memory could not be obtained.
  - `MPS_RES_RESOURCE`: a needed resource could not be obtained.
  - `MPS_RES_UNIMPL`: operation is not implemented.
  - `MPS_RES_COMMIT_LIMIT`: the arena’s commit limit would be exceeded.
  - `MPS_RES_PARAM`: an invalid parameter was passed.

## 2.1. Result codes

`MPS_RES_COMMIT_LIMIT`

  A result code indicating that an operation could not be completed as requested without exceeding the commit limit.

  You need to deallocate something to make more space, or increase the commit limit by calling `mps_arena_commit_limit_set()`.

`MPS_RES_FAIL`

  A result code indicating that something went wrong that does not fall under the description of any other result code. The exact meaning depends on the function that returned this result code.

`MPS_RES_IO`

  A result code indicating that an input/output error occurred. The exact meaning depends on the function that returned this result code.

`MPS_RES_LIMIT`

  A result code indicating that an operation could not be completed as requested because of an internal limitation of the MPS. The exact meaning depends on the function that returned this result code.

`MPS_RES_MEMORY`

  A result code indicating that an operation could not be completed because there wasn’t enough memory available.

  You need to deallocate something or allow the garbage collector to reclaim something to free enough memory, or expand the arena (if you’re using an arena for which that does not happen automatically).

  Note

  Failing to acquire enough memory because the commit limit would have been exceeded is indicated by returning `MPS_RES_COMMIT_LIMIT`, not `MPS_RES_MEMORY`.

  Running out of address space (as might happen in virtual memory systems) is indicated by returning `MPS_RES_RESOURCE`, not `MPS_RES_MEMORY`.

`MPS_RES_OK`

  A result code indicating that an operation succeeded.

  If a function takes an out parameter or an in/out parameter, this parameter will only be updated if `MPS_RES_OK` is returned. If any other result code is returned, the parameter will be left untouched by the function.

  `MPS_RES_OK` is zero.

`MPS_RES_PARAM`

  A result code indicating that an operation could not be completed as requested because an invalid parameter was passed to the operation. The exact meaning depends on the function that returned this result code.

`MPS_RES_RESOURCE`

  A result code indicating that an operation could not be completed as requested because the MPS could not obtain a needed resource. The resource in question depends on the operation.

  Two special cases have their own result codes: when the MPS runs out of committed memory, it returns `MPS_RES_MEMORY`, and when it cannot proceed without exceeding the commit limit, it returns `MPS_RES_COMMIT_LIMIT`.

  This result code can be returned when the MPS runs out of virtual memory. If this happens, you need to reclaim memory within your process (as for the result code `MPS_RES_MEMORY`), or terminate other processes running on the same machine.

`MPS_RES_UNIMPL`

  A result code indicating that an operation, or some vital part of it, is not implemented.

  This might be returned by functions that are no longer supported, or by operations that are included for future expansion, but not yet supported.

## 2.2. Assertions

Bugs in the client program may violate the invariants that the MPS relies on. Most functions in the MPS (in most varieties; see below) assert the correctness of their data structures, so these bugs will often be discovered by an assertion failure in the MPS. The section Common assertions and their causes below lists commonly encountered assertions and explains the kinds of client program bugs that can provoke these assertions.

It is very rare for an assertion to indicate a bug in the MPS rather than the client program, but it is not unknown, so if you have made every effort to track down the cause (see Debugging with the Memory Pool System) without luck, get in touch.

If you are running your MPS-enabled program from Emacs via the `compile` command, you will probably want to recognize MPS assertions automatically, by adding the following to your `.emacs`:

    (add-to-list 'compilation-error-regexp-alist
                 '("MPS ASSERTION FAILURE: .*\n\\(.*\\)\n\\([0-9]+\\)" 1 2))

## 2.3. Common assertions and their causes

This section lists some commonly encountered assertions and suggests likely causes. If you encounter an assertion not listed here (or an assertion that is listed here but for which you discovered a different cause), please let us know so that we can improve this documentation.

`dbgpool.c: fencepost check on free`

> The client program wrote to a location after the end, or before the beginning of an allocated block. See Debugging pools.

`dbgpool.c: free space corrupted on release`

> The client program used an object after it was reclaimed. See Debugging pools.

`format.c: SigCheck Format: format`

> The client program called `mps_pool_create()` for a pool class like AMC (Automatic Mostly-Copying) that requires a object format, but passed something other than a `mps_fmt_t` for this argument.

`lockix.c: res == 0`

`lockw3.c: lock->claims == 0`

> The client program has made a re-entrant call into the MPS. Look at the backtrace to see what it was. Common culprits are format methods and stepper functions.

`mpsi.c: SizeIsAligned(size, BufferPool(buf)->alignment)`

> The client program reserved a block by calling `mps_reserve()` but neglected to round the size up to the alignment required by the pool’s object format.

`pool.c: (pool->class->attr & AttrALLOC) != 0`

> The client program called `mps_alloc()` on a pool that does not support this form of allocation. Use an allocation point instead.

`poolams.c: !AMS_IS_INVALID_COLOUR(seg, i)`

> The client program failed to fix a reference to an object in an AMS (Automatic Mark and Sweep) pool, violating the tri-colour invariant that the MPS depends on for the correctness of its incremental garbage collection.

`poolams.c: AMS_ALLOCED(seg, i)`

> The client program tried to fix a reference to a block in an AMS (Automatic Mark and Sweep) pool that died. This may mean that there was a previous collection in which a reference that should have kept the block alive failed to be scanned. Perhaps a formatted object was updated in some way that has a race condition?

## 2.4. Varieties

The MPS has three behaviours with respect to internal checking and telemetry, which need to be selected at compile time, by defining one of the following preprocessor constants. If none is specified then `CONFIG_VAR_HOT` is the default.

`CONFIG_VAR_COOL`

  The cool variety is intended for development and testing.

  All functions check the consistency of their data structures and may assert, including functions on the critical path.

  All events are sent to the telemetry stream, including events on the critical path.

`CONFIG_VAR_HOT`

  The hot variety is intended for production and deployment.

  Some functions check the consistency of their data structures and may assert, namely those not on the critical path.

  Some events are sent to the telemetry stream, namely those not on the critical path.

`CONFIG_VAR_RASH`

  The rash variety is intended for mature integrations, or for developers who like living dangerously.

  No functions check the consistency of their data structures and consequently there are no assertions.

  No events are sent to the telemetry stream.

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