How deeply does Refactoring to Rust cover function pointer type?

Defines the Rust and C callback signatures and demonstrates invocation, but defers general function-pointer behavior beyond this FFI case.

Function pointer type

As explained in Refactoring to Rust

A function pointer is a value that refers to a callable function and can be passed as an argument or stored for later invocation.

A function pointer type describes a callable function passed by its memory location instead of by name. In Rust FFI, an NGINX body-reading API accepts a callback that runs after the request body loads. section 4.4

Passing an NGINX callback

Rust declares the C-compatible callback type as `Option<unsafe extern "C" fn(r: *mut ngx_http_request_t)>`. The handler passes `Some(read_body_handler)` to `ngx_http_read_client_request_body`; the callback does not need to be exported by name because C receives its location. The equivalent C declaration is `typedef void (*ngx_http_client_body_handler_pt)(ngx_http_request_t *r);`.

How Refactoring to Rust covers it

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Also explained in

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Modern CA function pointer is a pointer value that designates a function and can be passed to or invoked by other code.
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Modern C, Third EditionA function pointer is a C pointer value that designates a function and can be used to invoke or pass that function indirectly.
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Rust in Action: Systems programming concepts and techniquesA function pointer is a value that identifies a function and can be passed or stored so that the function can be invoked indirectly.

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Related concepts

function decayproduced by · Function decay is the C conversion of a function designator to a pointer to that function except in contexts such as sizeof and unary &.
callused by · A function call is an operation or expression that invokes a function or other callable, optionally supplies arguments, and initiates its execution.
referencekind of · A reference is an indirect value or relationship that identifies or provides access to another value, object, resource, or source.
closurecontrasts with · A closure is a function or callable value together with access to bindings from the lexical environment in which it was created.
callbackused by · A callback is a function or other callable supplied to software for invocation later in response to a specified event, condition, or point in execution.

This explainer was generated by Manning's knowledge graph from the text of Refactoring to Rust. The section links point to the parts of the book it draws on.