---
title: "wasi-descriptors — Mova code model"
description: "Mova code model of wasi-host/src/lib.rs: components with their labels, obernants and the model text."
url: "https://groupants.elamur.ai/free/en/mova/wasi-host/src/lib.rs.groupant/wasi-descriptors.groupant/"
---
Elamur: [Mova](https://md.mova.elamur.ai/free/en/) · [Dictionary](https://md.dictionary.elamur.ai/free/en/) · [Research](https://md.research.elamur.ai/free/en/) · **Groupants**

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# wasi-descriptors

The groupant beside wasi-host/src/lib.rs in the Mova repository: what the code computes, what it never does, how to go back from its outputs, its invariants, cost and checks.

## Facts

- **kind:** groupant_fragment
- **status:** partial source bound model
- **source:** [`wasi-host/src/lib.rs`](https://github.com/ElamurAI/mova-free/blob/main/wasi-host/src/lib.rs)
- **source SHA-256:** `91213ce903810791…`
- **covers:** read decodes table, reads/copies Vfs or stdin and advances implicit offset before guest writes. write gathers all input bytes before descriptor lookup, writes/echoes then stores count. Explicit positional IO leaves file offset. seek computes i128 base+delta in0..i64::MAX then updates before output. readdir materializes dot/dotdot/list and copies a possibly truncated prefix from cookie. (bytes 12594–18805)
- **part of:** [wasi-runtime](https://md.groupants.elamur.ai/free/en/mova/wasi-host/src/lib.rs.groupant/)
- **repository folder:** [`wasi-host/src/lib.rs.groupant/wasi-descriptors.groupant`](https://github.com/ElamurAI/mova-free/tree/main/wasi-host/src/lib.rs.groupant/wasi-descriptors.groupant)
- **repository commit:** `855da7bd4a7b`

> **Implementation material.** A Mova code model describes this code at one source revision: partial models, finite checks and their stated scope. It is not a reviewed general exposition, and its labels are shown as written, never raised.

## Components

| component | label | where |
|---|---|---|
| **approximant** | partial | [approximant](#approximant) |
| **ast** | partial | `../syntax.json` |
| **cost** | symbolic source derived | [invariant](#invariant) |
| **descriptant** | present | [descriptant](#descriptant) |
| **invariant** | partial | [invariant](#invariant) |
| **tests** | executed bounded checks | [tests](#tests) |

## Obernants

The typed relations around the forward model: the behaviour complement (what the code never does), the reverse reconstruction (every input compatible with an output) and specialised views.

| id | type | label | where |
|---|---|---|---|
| `behavior_complement` | behavior_complement | partial source derived model | [behavior_complement](#behavior-complement) |
| `reverse_reconstruction` | reverse_reconstruction | partial source derived model | [reverse_reconstruction](#reverse-reconstruction) |
| `append_position` | append_position | partial source derived model | [append_position](#append-position) |
| `rights_and_kind` | rights_and_kind | partial source derived model | [rights_and_kind](#rights-and-kind) |
| `seek_range` | seek_range | partial source derived model | [seek_range](#seek-range) |
| `cookie_prefix` | cookie_prefix | partial source derived model | [cookie_prefix](#cookie-prefix) |

## Model text


### descriptant

#### wasi-descriptors

read decodes table, reads/copies Vfs or stdin and advances implicit offset before guest writes. write gathers all input bytes before descriptor lookup, writes/echoes then stores count. Explicit positional IO leaves file offset. seek computes i128 base+delta in0..i64::MAX then updates before output. readdir materializes dot/dotdot/list and copies a possibly truncated prefix from cookie.

##### Epoch 2026-10-31

Prospective review recorded before edits. Source review found late output FAULT effects and weaker-than-capability rights metadata; these are modeled as current behavior rather than silently repaired. Only bounded new test blocks were added; production code remains unchanged. Actual finite results and exact restoration receipts live under $MOVA_DATA/runs/octopus-wave31-wasi-2026-10-31. No full mlab guest, frozen suite, GPU or corpus run. This node retains its scoped observations for future refinement.

### approximant

#### Forward relation

read decodes table, reads/copies Vfs or stdin and advances implicit offset before guest writes. write gathers all input bytes before descriptor lookup, writes/echoes then stores count. Explicit positional IO leaves file offset. seek computes i128 base+delta in0..i64::MAX then updates before output. readdir materializes dot/dotdot/list and copies a possibly truncated prefix from cookie.

Scope: `wasi-descriptors`. State and transitions follow the authoritative source, including partial effects before errors. No executable approximate replacement is attached: categorical errno/effects and module execution lack a useful validated percentage metric; future numerical profiles may only use1/2/5/10% caps after domain/reference evidence.

### invariant

#### Invariant and cost

read decodes table, reads/copies Vfs or stdin and advances implicit offset before guest writes. write gathers all input bytes before descriptor lookup, writes/echoes then stores count. Explicit positional IO leaves file offset. seek computes i128 base+delta in0..i64::MAX then updates before output. readdir materializes dot/dotdot/list and copies a possibly truncated prefix from cookie.

The invariant is conditional on the admitted transition: Successful read-only file descriptor writes are excluded; explicit positional write cannot alter fd position. Output FAULT does not exclude earlier write/emission.

Cost: O(iovs+payload) allocation/copy plus Vfs read/write; directory listing O(entries+names), output prefix construction may exceed len by one full entry.

No wall-time gain, universal resource bound or atomic rollback is inferred. External Vfs/wasmi/native-mlab work remains separately charged.

### models

## Typed model catalog

### behavior_complement

Subject: `wasi-host/src/lib.rs` at SHA-256 `91213ce9038107912c2dd707c3a6639a2d723cdfceb02981a166a6853800248e`, scope `wasi-descriptors`. Domain: the source transition and bounded fixtures described in approximant.md; readable valid host resources where required. Observation: returned values, byte coordinates, state/effect order, not total-system semantics. Operator/relation: Successful read-only file descriptor writes are excluded; explicit positional write cannot alter fd position. Output FAULT does not exclude earlier write/emission.

Status: partial source-derived relation; complement is a scoped exclusion hypothesis, reverse preserves all compatible antecedents described rather than a chosen inverse. Assumptions: production branch order, declared coordinate/resource premises; no completeness claim for untested inputs. Evidence: tests.md distinguishes actual execution from source review; finite observations can falsify but not prove the relation.

### reverse_reconstruction

Subject: `wasi-host/src/lib.rs` at SHA-256 `91213ce9038107912c2dd707c3a6639a2d723cdfceb02981a166a6853800248e`, scope `wasi-descriptors`. Domain: the source transition and bounded fixtures described in approximant.md; readable valid host resources where required. Observation: returned values, byte coordinates, state/effect order, not total-system semantics. Operator/relation: Observed bytes and count constrain a prefix at old offset; EOF makes many offsets compatible. Appended file state admits many prior contents and writes unless original size and input bytes are fixed.

Status: partial source-derived relation; complement is a scoped exclusion hypothesis, reverse preserves all compatible antecedents described rather than a chosen inverse. Assumptions: production branch order, declared coordinate/resource premises; no completeness claim for untested inputs. Evidence: tests.md distinguishes actual execution from source review; finite observations can falsify but not prove the relation.

### append_position

Subject: `wasi-host/src/lib.rs` at SHA-256 `91213ce9038107912c2dd707c3a6639a2d723cdfceb02981a166a6853800248e`, scope `wasi-descriptors`. Domain: the source transition and bounded fixtures described in approximant.md; readable valid host resources where required. Observation: returned values, byte coordinates, state/effect order, not total-system semantics. Operator/relation: Explicit pos_at overrides append; implicit append uses current file size. Non-positional write updates fd position by payload length.

Status: partial source-derived relation; complement is a scoped exclusion hypothesis, reverse preserves all compatible antecedents described rather than a chosen inverse. Assumptions: production branch order, declared coordinate/resource premises; no completeness claim for untested inputs. Evidence: tests.md distinguishes actual execution from source review; finite observations can falsify but not prove the relation.

### rights_and_kind

Subject: `wasi-host/src/lib.rs` at SHA-256 `91213ce9038107912c2dd707c3a6639a2d723cdfceb02981a166a6853800248e`, scope `wasi-descriptors`. Domain: the source transition and bounded fixtures described in approximant.md; readable valid host resources where required. Observation: returned values, byte coordinates, state/effect order, not total-system semantics. Operator/relation: read/write booleans cached in Kind control access; stdout/stderr reject reads; directories map ISDIR and other nonfile seek maps SPIPE.

Status: partial source-derived relation; complement is a scoped exclusion hypothesis, reverse preserves all compatible antecedents described rather than a chosen inverse. Assumptions: production branch order, declared coordinate/resource premises; no completeness claim for untested inputs. Evidence: tests.md distinguishes actual execution from source review; finite observations can falsify but not prove the relation.

### seek_range

Subject: `wasi-host/src/lib.rs` at SHA-256 `91213ce9038107912c2dd707c3a6639a2d723cdfceb02981a166a6853800248e`, scope `wasi-descriptors`. Domain: the source transition and bounded fixtures described in approximant.md; readable valid host resources where required. Observation: returned values, byte coordinates, state/effect order, not total-system semantics. Operator/relation: Negative/new>i64::MAX returns INVAL before set_pos; invalid output pointer can fail after valid position change.

Status: partial source-derived relation; complement is a scoped exclusion hypothesis, reverse preserves all compatible antecedents described rather than a chosen inverse. Assumptions: production branch order, declared coordinate/resource premises; no completeness claim for untested inputs. Evidence: tests.md distinguishes actual execution from source review; finite observations can falsify but not prove the relation.

### cookie_prefix

Subject: `wasi-host/src/lib.rs` at SHA-256 `91213ce9038107912c2dd707c3a6639a2d723cdfceb02981a166a6853800248e`, scope `wasi-descriptors`. Domain: the source transition and bounded fixtures described in approximant.md; readable valid host resources where required. Observation: returned values, byte coordinates, state/effect order, not total-system semantics. Operator/relation: Cookie skips entries, . and .. count; truncated entry allowed, used==len suggests retry but does not itself prove remaining entries.

Status: partial source-derived relation; complement is a scoped exclusion hypothesis, reverse preserves all compatible antecedents described rather than a chosen inverse. Assumptions: production branch order, declared coordinate/resource premises; no completeness claim for untested inputs. Evidence: tests.md distinguishes actual execution from source review; finite observations can falsify but not prove the relation.


### tests

#### Checks and limits

Named actual-source tests run twice per debug/release; exact commands and numeric statuses retained externally. Test success is finite evidence only.

Exact names: tests::open_read_seek_write, tests::readdir_with_small_buffer_and_cookies, tests::limits_reach_the_guest_as_errno, tests::wave31_late_faults_keep_prior_effects.

Memory tests bypass linker ABI; tiny empty/unreachable guests exercise engine outcomes only. The CLI test uses isolated import/export/journal fixtures.

Five compiled production/helper-source controls: append offset, virtual clock advance, trap status, CLI export gate and newest-mtime max. Each compiles, fails its selected assertion, restores exact original bytes and passes again. These controls cover their target relations, not every node. No assertion-only mutants. See external source-test-map.json, native-commands.json, verify.rs and numeric/raw receipts in $MOVA_DATA/runs/octopus-wave31-wasi-2026-10-31. The example main and frozen mlab paths are never executed.