pw_log_backend_printf/varargs.rs
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// Copyright 2023 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
//! # Infrastructure for building calls to `printf`.
//!
//! The `varargs` modules solves a particularly tricky problem: Some arguments
//! passed to `pw_log` result in a single argument passed to printf (a `u32`
//! is passed directly) while some may result in multiple arguments (a `&str`
//! is passed as a length argument and a data argument). Since function like
//! proc macros don't know the types of the arguments we rely on chained generic
//! types.
//!
//! ## VarArgs trait
//! The [`VarArgs`] both encapsulates the structure of the arguments as well
//! as provides a method for calling `printf`. It accomplishes this through a
//! recursive, chained/wrapped type through it's `OneMore<T>` associated type.
//! We then provide generic implementations for [`VarArgs`] for tuples
//! ov values that implement [`Clone`] such that:
//!
//! ```
//! # use pw_log_backend_printf::varargs::VarArgs;
//! # use core::any::TypeId;
//! # use core::ffi::{c_int, c_uchar};
//! type VarArgsType =
//! <<<<() as VarArgs>
//! ::OneMore<u32> as VarArgs>
//! ::OneMore<i32> as VarArgs>
//! ::OneMore<c_int> as VarArgs>
//! ::OneMore<*const c_uchar>;
//! type TupleType = (u32, i32, c_int, *const c_uchar);
//! assert_eq!(TypeId::of::<VarArgsType>(), TypeId::of::<TupleType>());
//! ```
//!
//! [`VarArgs`] provides an `append()` method that allows building up these
//! typed tuple values:
//!
//! ```
//! # use pw_log_backend_printf::varargs::VarArgs;
//! # use core::ffi::{c_int, c_uchar};
//! let string = "test";
//! let args = ()
//! .append(0u32)
//! .append(-1i32)
//! .append(string.len() as c_int)
//! .append(string.as_ptr().cast::<*const c_uchar>());
//! assert_eq!(args, (
//! 0u32,
//! -1i32,
//! string.len() as c_int,
//! string.as_ptr().cast::<*const c_uchar>()));
//! ```
//!
//! Lastly [`VarArgs`] exposes an unsafe `call_printf()` method that calls
//! printf and passes the build up arguments to `printf()`:
//!
//! ```
//! # use pw_log_backend_printf::varargs::VarArgs;
//! # use core::ffi::{c_int, c_uchar};
//! let string = "test";
//! unsafe {
//! // This generates a call to:
//! // `printf("[%s] %u %d %.*s\0".as_ptr(), "INF".as_ptr(), 0u32, -1i32, 4, string.as_ptr())`.
//! ()
//! .append(0u32)
//! .append(-1i32)
//! .append(string.len() as c_int)
//! .append(string.as_ptr().cast::<*const c_uchar>())
//! // Note that we always pass the null terminated log level string here
//! // as an argument to `call_printf()`.
//! .call_printf("[%s] %u %d %.*s\0".as_ptr().cast(), "INF\0".as_ptr().cast());
//! }
//! ```
//!
//! ## Arguments Trait
//! The [`Arguments`] trait is the final piece of the puzzle. It is used to
//! generate one or more calls to [`VarArgs::append()`] for each argument to
//! `pw_log`. Most simple cases that push a single argument look like:
//!
//! ```ignore
//! # // Ignored as a test as the test is neither the crate that defines
//! # // `Arguments` nor `i32`.
//! impl Arguments<i32> for i32 {
//! type PushArg<Head: VarArgs> = Head::OneMore<i32>;
//! fn push_arg<Head: VarArgs>(head: Head, arg: &i32) -> Self::PushArg<Head> {
//! head.append(*arg)
//! }
//! }
//! ```
//!
//! A more complex case like `&str` can push multiple arguments:
//! ```ignore
//! # // Ignored as a test as the test is neither the crate that defines
//! # // `Arguments` nor `&str`.
//! impl Arguments<&str> for &str {
//! // Arguments are a chain of two `OneMore` types. One for the string length
//! // and one for the pointer to the string data.
//! type PushArg<Head: VarArgs> =
//! <Head::OneMore<c_int> as VarArgs>::OneMore<*const c_uchar>;
//!
//! // `push_arg` pushes both the length and pointer to the string into the args tuple.
//! fn push_arg<Head: VarArgs>(head: Head, arg: &&str) -> Self::PushArg<Head> {
//! let arg = *arg;
//! head.append(arg.len() as c_int).append(arg.as_ptr().cast::<*const c_uchar>())
//! }
//! }
//! ```
//!
//! ## Putting it all together
//! With all of these building blocks, the backend proc macro emits the following
//! code:
//!
//! ```
//! # use pw_log_backend_printf::varargs::{Arguments, VarArgs};
//! // Code emitted for a call to `info!("Hello {}. It is {}:00", "Pigweed" as &str, 2 as u32)
//! let args = ();
//! let args = <&str as Arguments<&str>>::push_arg(args, &("Pigweed" as &str));
//! let args = <u32 as Arguments<u32>>::push_arg(args, &(2 as u32));
//! unsafe {
//! args.call_printf("[%s] Hello %.*s. It is %d:00".as_ptr().cast(), "INF\0".as_ptr().cast());
//! }
//! ```
use core::convert::Infallible;
use core::ffi::{c_int, c_uchar};
/// Implements a list of arguments to a vararg call.
///
/// See [module level docs](crate::varargs) for a detailed description on how
/// [`Arguments`] works and is used.
pub trait Arguments<T: ?Sized> {
/// Type produced by calling [`Self::push_arg()`].
type PushArg<Head: VarArgs>: VarArgs;
/// Push an argument onto the list of varargs.
///
/// This may actually push zero, one, or more arguments onto the list
/// depending on implementation.
fn push_arg<Head: VarArgs>(head: Head, arg: &T) -> Self::PushArg<Head>;
}
/// Represents a variable length list of arguments to printf.
///
/// See [module level docs](crate::varargs) for a detailed description on how
/// how [`VarArgs`] works and is used.
pub trait VarArgs: Clone {
/// The type that is produced by a call to `append()`
type OneMore<T: Clone>: VarArgs;
/// Used to check if there is space left in the argument list.
///
/// If the there is no space left in the argument list an [`Arguments<T>`]'s
/// PushArg type will expand to a type where `CHECK` us unable to be
/// compiled.
const CHECK: () = ();
/// Append an additional argument to this argument list.
fn append<T: Clone>(self, val: T) -> Self::OneMore<T>;
/// Calls `printf` with the arguments in `self` and the given format and log level string.
///
/// # Safety
///
/// Calls into `libc` printf without any input validation. The code further
/// up the stack is responsible for initializing valid [`VarArgs`] that
/// will cause printf to execute in a sound manner.
unsafe fn call_printf(self, format_str: *const c_uchar, log_level_str: *const c_uchar)
-> c_int;
}
#[derive(Clone)]
/// A sentinel type for trying to append too many (>12) arguments to a
/// [`VarArgs`] tuple.
pub struct TooMany(Infallible);
impl TooMany {
const fn panic() -> ! {
panic!("Too many arguments to logging call")
}
}
#[doc(hidden)]
/// Implementation VarArgs for TooMany. Usages of TooMany::CHECK will cause a
/// compile-time error.
impl VarArgs for TooMany {
type OneMore<T: Clone> = TooMany;
const CHECK: () = Self::panic();
fn append<T: Clone>(self, _: T) -> Self::OneMore<T> {
Self::panic()
}
unsafe fn call_printf(self, _: *const c_uchar, _: *const c_uchar) -> c_int {
Self::panic()
}
}
/// Used to implement [`VarArgs`] on tuples.
///
/// This recursive macro divides it's arguments into a set of arguments for use
/// in the recursive case in `[]`s and arguments used for the current
/// implementation of [`VarArgs`] following the `[]`'s.
macro_rules! impl_args_list {
// Entry point into the macro that directly invokes `@impl` with its
// arguments.
//
// Take a list of arguments of the form `ident => position`. `ident`
// is used to name the generic type argument for the implementation
// of `[VarArgs]` (i.e. `impl<ident: Clone, ...> VarArgs for (ident, ...)`).
// `position` is used to index into the argument tuple when calling
// printf (i.e. `printf(format_str, ..., args.position, ...)`).
($($arg:ident => $arg_ident:tt),* $(,)?) => {
impl_args_list!(@impl [$($arg => $arg_ident),*]);
};
// Recursive case for [`VarArgs`] implementation.
//
// Implements [`VarArgs`] for a tuple with length equal to the number
// of arguments listed after the `[]`. It then recurses with taking
// the first argument between the `[]`s and appending it to the list
// after the `[]`s.
(@impl [$next:ident => $next_num:tt
$(, $remaining:ident => $next_remaining:tt)*]
$($current:ident => $current_num:tt),*) => {
impl<$($current: Clone),*> VarArgs for ($($current,)*) {
type OneMore<$next: Clone> = ($($current,)* $next,);
fn append<$next>(self, val: $next) -> ($($current,)* $next,) {
($(self. $current_num,)* val,)
}
unsafe fn call_printf(
self,
format_str: *const c_uchar,
log_level_str: *const c_uchar,
) -> c_int {
extern "C" {
fn printf(fmt: *const c_uchar, ...) -> c_int;
}
printf(format_str, log_level_str, $(self. $current_num),*)
}
}
impl_args_list!(@impl
[$($remaining => $next_remaining),*]
$($current => $current_num,)* $next => $next_num);
};
// Base for [`VarArgs`] implementation.
//
// Implements [`VarArgs`] for the full list of arguments and sets
// its `OneMore` type to `TooMany` to cause a compilation error
// if code tries to instantiate an argument list longer that this.
(@impl [] $($current:ident => $current_num:tt),*) => {
impl<$($current: Clone),*> VarArgs for ($($current),*) {
type OneMore<T: Clone> = TooMany;
fn append<T: Clone>(self, _: T) -> TooMany {
panic!("Too many arguments to logging call")
}
unsafe fn call_printf(
self,
format_str: *const c_uchar,
log_level_str: *const c_uchar,
) -> c_int {
extern "C" {
fn printf(fmt: *const c_uchar, ...) -> c_int;
}
printf(format_str, log_level_str, $(self. $current_num),*)
}
}
};
}
// Expands to implementations of [`VarArgs`] for tuples of length 0-12.
impl_args_list!(
ARGS0 => 0,
ARGS1 => 1,
ARGS2 => 2,
ARGS3 => 3,
ARGS4 => 4,
ARGS5 => 5,
ARGS6 => 6,
ARGS7 => 7,
ARGS8 => 8,
ARGS9 => 9,
ARGS10 => 10,
ARGS11 => 11,
ARGS12 => 12,
);
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn appended_args_yields_correct_tuple() {
let string = "test";
let args =
().append(0u32)
.append(-1i32)
.append(string.len() as c_int)
.append(string.as_ptr().cast::<*const c_uchar>());
assert_eq!(
args,
(
0u32,
-1i32,
string.len() as c_int,
string.as_ptr().cast::<*const c_uchar>()
)
);
}
#[test]
fn twelve_argument_long_tuples_are_supported() {
let args =
().append(0u32)
.append(1u32)
.append(2u32)
.append(3u32)
.append(4u32)
.append(5u32)
.append(6u32)
.append(7u32)
.append(8u32)
.append(9u32)
.append(10u32)
.append(11u32);
assert_eq!(args, (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11));
}
}