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application-abi.md

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WASI Application ABI

In addition to the APIs defined by the various WASI modules there are also certain expectations that the WASI runtime places on an application that wishes to be portable across WASI implementations.

This document describes how a conforming WASI application is expected to behave in terms of lifecycle (startup, shutdown, etc) and any exports it is expected to include.

Current Unstable ABI

There are two kinds of modules:

  • A command exports a function named _start, with no arguments and no return values.

    _start is the default export which is called when the user doesn't select a specific function to call. Commands may also export additional functions, (similar to "multi-call" executables), which may be explicitly selected by the user to run instead.

    Except as noted below, commands shall not export any mutable globals, tables, or linear memories.

    Command instances may assume that they will be called from the environment at most once. Command instances may assume that none of their exports are accessed outside the duration of that call.

  • All other modules are reactors. A reactor may export a function named _initialize, with no arguments and no return values.

    If an _initialize export is present, reactor instances may assume that it will be called by the environment at most once, and that none of their other exports are accessed before that call.

    After being instantiated, and after any _initialize function is called, a reactor instance remains live, and its exports may be accessed.

These kinds are mutually exclusive; implementations should report an error if asked to instantiate a module containing exports which declare it to be of multiple kinds.

Regardless of the kind, all modules accessing WASI APIs also export a linear memory with the name memory. Data pointers in WASI API calls are relative to this memory's index space.

Regardless of the kind, all modules accessing WASI APIs also export a table with the name __indirect_function_table. Function pointers in WASI API calls are relative to this table's index space.

When _start or _initialize is called, environments shall provide file descriptors with indices 0, 1, and 2 representing stream resources for standard input, standard output, and standard error. Environments may provide additional "preopen" file descriptors that can be inspected with fd_prestat_get and fd_prestat_dir_name. These resources may be closed at any time.

Environments shall not access exports named __heap_base or __data_end. Toolchains are encouraged to avoid providing these exports.

In the future, as the underlying WebAssembly platform offers more features, we hope to eliminate the requirement to export all of linear memory or all of the indirect function table.

Planned Stable ABI

There is ongoing discussion about what the stable ABI might look like: