Plugin calling convention

Purpose

Determine how to pass input and output values between plugins and consumers using the types available in WASM.

Solution

Internally we use a struct to store information about the location of the arguments and return value in memory:

input_offset: is a pointer to the input data
input_length: is the length of the input data
output_offset: is a pointer to the output data
output_length: is the length of the output data

These values are not exposed outside of the core Rust implementation, exception through the following functions:

extism_input_offset() -> i64
- Returns the start offset of the input data
extism_input_length() -> i64
- Returns the length of the input data
extism_output_set(offset: i64, length: i64)
- Sets the offset/length for output data
extism_alloc(length: i64) -> i64
- Allocates length bytes and returns the offset, this can be used to allocate the output data

On the SDK side we provide the following API:

extism_output_length(plugin_id: i64) -> i64
- Returns the length of the output buffer, this is used so the caller can allocate a big enough buffer to copy the data
extism_output_get(plugin_id: i64, buffer: *mut void, length: i64)
- Copies the output buffer into buffer