pw_log_backend_printf/
lib.rs

1// Copyright 2023 The Pigweed Authors
2//
3// Licensed under the Apache License, Version 2.0 (the "License"); you may not
4// use this file except in compliance with the License. You may obtain a copy of
5// the License at
6//
7//     https://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
11// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
12// License for the specific language governing permissions and limitations under
13// the License.
14
15//! `pw_log` backend that calls `libc`'s `printf` to emit log messages.  This
16//! module is useful when you have a mixed C/C++ and Rust code base and want a
17//! simple logging system that leverages an existing `printf` implementation.
18//!
19//! *Note*: This uses FFI to call `printf`.  This has two implications:
20//! 1. C/C++ macro processing is not done.  If a system's `printf` relies on
21//!    macros, this backend will likely need to be forked to make work.
22//! 2. FFI calls use `unsafe`.  Attempts are made to use `printf` in sound ways
23//!    such as bounding the length of strings explicitly but this is still an
24//!    off-ramp from Rust's safety guarantees.
25//!
26//! Varargs marshaling for call to printf is handled through a type expansion
27//! of a series of traits.  It is documented in the [`varargs`] module.
28//!
29//! TODO: <pwbug.dev/311232605> - Document how to configure facade backends.
30#![deny(missing_docs)]
31
32pub mod varargs;
33
34// Re-export dependences of backend proc macro to be accessed via `$crate::__private`.
35#[doc(hidden)]
36pub mod __private {
37    use core::ffi::{c_int, c_uchar};
38
39    pub use pw_bytes::concat_static_strs;
40    pub use pw_format_core::{PrintfHexFormatter, PrintfUpperHexFormatter};
41    use pw_log_backend_api::LogLevel;
42    pub use pw_log_backend_printf_macro::{_pw_log_backend, _pw_logf_backend};
43
44    pub use crate::varargs::{Arguments, VarArgs};
45
46    pub const fn log_level_tag(level: LogLevel) -> &'static str {
47        match level {
48            LogLevel::Debug => "DBG\0",
49            LogLevel::Info => "INF\0",
50            LogLevel::Warn => "WRN\0",
51            LogLevel::Error => "ERR\0",
52            LogLevel::Critical => "CRT\0",
53            LogLevel::Fatal => "FTL\0",
54        }
55    }
56
57    macro_rules! extend_args {
58      ($head:ty; $next:ty $(,$rest:ty)* $(,)?) => {
59          extend_args!(<$head as VarArgs>::OneMore<$next>; $($rest,)*)
60      };
61      ($head:ty;) => {
62          $head
63      };
64    }
65
66    /// The printf uses its own formatter trait because it needs strings to
67    /// resolve to `%.*s` instead of `%s`.
68    ///
69    /// The default [`PrintfHexFormatter`] and [`PrintfUpperHexFormatter`] are
70    /// used since they are not supported by strings.
71    pub trait PrintfFormatter {
72        /// The format specifier for this type.
73        const FORMAT_ARG: &'static str;
74    }
75
76    /// A helper to declare a [`PrintfFormatter`] trait for a given type.
77    macro_rules! declare_formatter {
78        ($ty:ty, $specifier:literal) => {
79            impl PrintfFormatter for $ty {
80                const FORMAT_ARG: &'static str = $specifier;
81            }
82        };
83    }
84
85    declare_formatter!(i32, "d");
86    declare_formatter!(u32, "u");
87    declare_formatter!(&str, ".*s");
88
89    /// A helper to declare an [`Argument<T>`] trait for a given type.
90    ///
91    /// Useful for cases where `Argument::push_args()` appends a single
92    /// argument of type `T`.
93    macro_rules! declare_simple_argument {
94        ($ty:ty) => {
95            impl Arguments<$ty> for $ty {
96                type PushArg<Head: VarArgs> = Head::OneMore<$ty>;
97                fn push_arg<Head: VarArgs>(head: Head, arg: &$ty) -> Self::PushArg<Head> {
98                    // Try expanding `CHECK` which should fail if we've exceeded
99                    // 12 arguments in our args tuple.
100                    let _ = Self::PushArg::<Head>::CHECK;
101                    head.append(*arg)
102                }
103            }
104        };
105    }
106
107    declare_simple_argument!(i32);
108    declare_simple_argument!(u32);
109    declare_simple_argument!(char);
110
111    // &str needs a more complex implementation of [`Argument<T>`] since it needs
112    // to append two arguments.
113    impl Arguments<&str> for &str {
114        type PushArg<Head: VarArgs> = extend_args!(Head; c_int, *const c_uchar);
115        fn push_arg<Head: VarArgs>(head: Head, arg: &&str) -> Self::PushArg<Head> {
116            // Try expanding `CHECK` which should fail if we've exceeded 12
117            // arguments in our args tuple.
118            #[allow(clippy::let_unit_value)]
119            let _ = Self::PushArg::<Head>::CHECK;
120            let arg = *arg;
121            head.append(arg.len() as c_int).append(arg.as_ptr().cast())
122        }
123    }
124}
125
126/// Implements the `pw_log` backend api.
127#[macro_export]
128macro_rules! pw_log_backend {
129    ($log_level:expr, $format_string:literal $(, $args:expr)* $(,)?) => {{
130        use $crate::__private as __pw_log_backend_crate;
131        $crate::__private::_pw_log_backend!($log_level, $format_string,  $($args),*)
132    }};
133}
134
135/// Implements the `pw_log` backend api.
136#[macro_export]
137macro_rules! pw_logf_backend {
138    ($log_level:expr, $format_string:literal $(, $args:expr)* $(,)?) => {{
139        use $crate::__private as __pw_log_backend_crate;
140        $crate::__private::_pw_logf_backend!($log_level, $format_string,  $($args),*)
141    }};
142}
143
144#[cfg(test)]
145mod tests {
146    use core::ffi::c_int;
147
148    use super::__private::*;
149
150    #[test]
151    fn pushed_args_produce_correct_tuple() {
152        let string = "test";
153        let args = ();
154        let args = <&str as Arguments<&str>>::push_arg(args, &(string as &str));
155        let args = <u32 as Arguments<u32>>::push_arg(args, &2u32);
156        assert_eq!(args, (string.len() as c_int, string.as_ptr().cast(), 2u32));
157    }
158}
pw_log_backend_printf/lib.rs

pw_log_backend_printf/
lib.rs
