pw_assert

Overview

Pigweed’s assert module enables applications to check preconditions, triggering a crash if the condition is not met. Consistent use of asserts is one aspect of defensive programming that can lead to more reliable and less buggy code.

The assert API facilitates flexible crash handling through Pigweed’s facade mechanism. The API is desigend to enable features like:

  • Optional ancillary printf-style messages along assertions

  • Capturing actual values of binary operator assertions like a < b

  • Compatibility with pw_tokenizer for reduced binary code size

The pw_assert API provides three classes of macros:

  • PW_CRASH(format, …) - Trigger a crash with a message.

  • PW_CHECK(condition[, format, …]) - Assert a condition, optionally with a message.

  • PW_CHECK_<type>_<cmp>(a, b[, fmt, …]) - Assert that the expression a <cmp> b is true, optionally with a message.

  • PW_ASSERT(condition) - Header- and constexpr-safe assert.

Tip

All of the CHECK macros optionally support a message with additional arguments, to assist in debugging when an assert triggers:

PW_CHECK_INT_LE(ItemCount(), 100);
PW_CHECK_INT_LE(ItemCount(), 100, "System state: %s", GetStateStr());

Example

#include "pw_assert/check.h"

int main() {
  bool sensor_running = StartSensor(&msg);
  PW_CHECK(sensor_running, "Sensor failed to start; code: %s", msg);

  int temperature_c = ReadSensorCelcius();
  PW_CHECK_INT_LE(temperature_c, 100,
                  "System is way out of heat spec; state=%s",
                  ReadSensorStateString());
}

Tip

All macros have both a CHECK and DCHECK variant. The CHECK variant is always enabled, even in production. Generally, we advise making most asserts CHECK rather than DCHECK, unless there is a critical performance or code size reason to use DCHECK.

// This assert is always enabled, even in production.
PW_CHECK_INT_LE(ItemCount(), 100);

// This assert disabled for release builds, where NDEBUG is defined.
// The functions ItemCount() and GetStateStr() are never called.
PW_DCHECK_INT_LE(ItemCount(), 100, "System state: %s", GetStateStr());

Tip

Use PW_ASSERT from pw_assert/assert.h for asserts in headers or asserting in constexpr contexts.

Structure of assert modules

The module is split into two components:

  1. The facade (this module) which is only a macro interface layer, and performs the actual checks for the conditions.

  2. The backend, provided elsewhere, that handles the consequences of an assert failing. Example backends include pw_assert_basic, which prints a useful message and either quits the application (on host) or hangs in a while loop (on device). In the future, there will be a tokenized assert backend. This is also where application or product specific crash handling would go.

blockdiag facade backend

See the Backend API section below for more details.

Facade API

The below functions describe the assert API functions that applications should invoke to assert. These macros are found in the pw_assert/check.h header.

PW_CRASH(format, ...)

Trigger a crash with a message. Replaces LOG_FATAL() in other systems. Can include a message with format arguments; for example:

PW_CRASH("Unexpected: frobnitz in state: %s", frobnitz_state);

Note: PW_CRASH is the equivalent of LOG_FATAL in other systems, where a device crash is triggered with a message. In Pigweed, logging and crashing/asserting are separated. There is a LOG_CRITICAL level in the logging module, but it does not have side effects; for LOG_FATAL, instead use this macro (PW_CRASH).

PW_CHECK(condition)
PW_CHECK(condition, format, ...)
PW_DCHECK(condition)
PW_DCHECK(condition, format, ...)

Assert that a condition is true, optionally including a message with arguments to report if the codition is false.

The DCHECK variants only run if NDEBUG is defined; otherwise, the entire statement is removed (and the expression not evaluated).

Example:

PW_CHECK(StartTurbines());
PW_CHECK(StartWarpDrive(), "Oddly warp drive couldn't start; ruh-roh!");
PW_CHECK(RunSelfTest(), "Failure in self test; try %d", TestAttempts());

Attention

Don’t use use PW_CHECK for binary comparisons or status checks!

Instead, use the PW_CHECK_<TYPE>_<OP> macros. These macros enable capturing the value of the operands, and also tokenizing them if using a tokenizing assert backend. For example, if x and b are integers, use instead PW_CHECK_INT_LT(x, b).

Additionally, use PW_CHECK_OK(status) when checking for an OK status, since it enables showing a human-readable status string rather than an integer (e.g. status == RESOURCE_EXHAUSTED instead of status == 5.

Do NOT do this

Do this instead

PW_CHECK(a_int < b_int)

PW_CHECK_INT_LT(a_int, b_int)

PW_CHECK(a_ptr <= b_ptr)

PW_CHECK_PTR_LE(a_ptr, b_ptr)

PW_CHECK(Temp() <= 10.0)

PW_CHECK_FLOAT_EXACT_LE( `` Temp(), 10.0)``

PW_CHECK(Foo() == OkStatus())

PW_CHECK_OK(Foo())

PW_CHECK_NOTNULL(ptr)
PW_CHECK_NOTNULL(ptr, format, ...)
PW_DCHECK_NOTNULL(ptr)
PW_DCHECK_NOTNULL(ptr, format, ...)

Assert that the given pointer is not NULL, optionally including a message with arguments to report if the pointer is NULL.

The DCHECK variants only run if NDEBUG is defined; otherwise, the entire statement is removed (and the expression not evaluated).

Foo* foo = GetTheFoo()
PW_CHECK_NOTNULL(foo);

Bar* bar = GetSomeBar();
PW_CHECK_NOTNULL(bar, "Weirdly got NULL bar; state: %d", MyState());
PW_CHECK_TYPE_OP(a, b)
PW_CHECK_TYPE_OP(a, b, format, ...)
PW_DCHECK_TYPE_OP(a, b)
PW_DCHECK_TYPE_OP(a, b, format, ...)

Asserts that a OP b is true, where a and b are converted to TYPE; with OP and TYPE described below.

If present, the optional format message is reported on failure. Depending on the backend, values of a and b will also be reported.

The DCHECK variants only run if NDEBUG is defined; otherwise, the entire statement is removed (and the expression not evaluated).

Example, with no message:

PW_CHECK_INT_LE(CurrentTemperature(), 100);
PW_CHECK_INT_LE(ItemCount(), 100);

Example, with an included message and arguments:

PW_CHECK_FLOAT_EXACT_GE(BatteryVoltage(), 3.2,
                        "System state=%s", SysState());

Below is the full list of binary comparison assert macros, along with the type specifier. The specifier is irrelevant to application authors but is needed for backend implementers.

Macro

a, b type

condition

a, b format specifier

PW_CHECK_INT_LE

int

a <= b

%d

PW_CHECK_INT_LT

int

a < b

%d

PW_CHECK_INT_GE

int

a >= b

%d

PW_CHECK_INT_GT

int

a > b

%d

PW_CHECK_INT_EQ

int

a == b

%d

PW_CHECK_INT_NE

int

a != b

%d

PW_CHECK_UINT_LE

unsigned int

a <= b

%u

PW_CHECK_UINT_LT

unsigned int

a < b

%u

PW_CHECK_UINT_GE

unsigned int

a >= b

%u

PW_CHECK_UINT_GT

unsigned int

a > b

%u

PW_CHECK_UINT_EQ

unsigned int

a == b

%u

PW_CHECK_UINT_NE

unsigned int

a != b

%u

PW_CHECK_PTR_LE

void*

a <= b

%p

PW_CHECK_PTR_LT

void*

a < b

%p

PW_CHECK_PTR_GE

void*

a >= b

%p

PW_CHECK_PTR_GT

void*

a > b

%p

PW_CHECK_PTR_EQ

void*

a == b

%p

PW_CHECK_PTR_NE

void*

a != b

%p

PW_CHECK_FLOAT_EXACT_LE

float

a <= b

%f

PW_CHECK_FLOAT_EXACT_LT

float

a < b

%f

PW_CHECK_FLOAT_EXACT_GE

float

a >= b

%f

PW_CHECK_FLOAT_EXACT_GT

float

a > b

%f

PW_CHECK_FLOAT_EXACT_EQ

float

a == b

%f

PW_CHECK_FLOAT_EXACT_NE

float

a != b

%f

The above CHECK_*_*() are also available in DCHECK variants, which will only evaluate their arguments and trigger if the NDEBUG macro is defined.

Macro

a, b type

condition

a, b format specifier

PW_DCHECK_INT_LE

int

a <= b

%d

PW_DCHECK_INT_LT

int

a < b

%d

PW_DCHECK_INT_GE

int

a >= b

%d

PW_DCHECK_INT_GT

int

a > b

%d

PW_DCHECK_INT_EQ

int

a == b

%d

PW_DCHECK_INT_NE

int

a != b

%d

PW_DCHECK_UINT_LE

unsigned int

a <= b

%u

PW_DCHECK_UINT_LT

unsigned int

a < b

%u

PW_DCHECK_UINT_GE

unsigned int

a >= b

%u

PW_DCHECK_UINT_GT

unsigned int

a > b

%u

PW_DCHECK_UINT_EQ

unsigned int

a == b

%u

PW_DCHECK_UINT_NE

unsigned int

a != b

%u

PW_DCHECK_PTR_LE

void*

a <= b

%p

PW_DCHECK_PTR_LT

void*

a < b

%p

PW_DCHECK_PTR_GE

void*

a >= b

%p

PW_DCHECK_PTR_GT

void*

a > b

%p

PW_DCHECK_PTR_EQ

void*

a == b

%p

PW_DCHECK_PTR_NE

void*

a != b

%p

PW_DCHECK_FLOAT_EXACT_LE

float

a <= b

%f

PW_DCHECK_FLOAT_EXACT_LT

float

a < b

%f

PW_DCHECK_FLOAT_EXACT_GE

float

a >= b

%f

PW_DCHECK_FLOAT_EXACT_GT

float

a > b

%f

PW_DCHECK_FLOAT_EXACT_EQ

float

a == b

%f

PW_DCHECK_FLOAT_EXACT_NE

float

a != b

%f

Attention

For float, proper comparator checks which take floating point precision and ergo error accumulation into account are not provided on purpose as this comes with some complexity and requires application specific tolerances in terms of Units of Least Precision (ULP). Instead, we recommend developers carefully consider how floating point precision and error impact the data they are bounding and whether checks are appropriate.

PW_CHECK_FLOAT_NEAR(a, b, abs_tolerance)
PW_CHECK_FLOAT_NEAR(a, b, abs_tolerance, format, ...)
PW_DCHECK_FLOAT_NEAR(a, b, abs_tolerance)
PW_DCHECK_FLOAT_NEAR(a, b, abs_tolerance, format, ...)

Asserts that (a >= b - abs_tolerance) && (a <= b + abs_tolerance) is true, where a, b, and abs_tolerance are converted to float.

Note

This also asserts that abs_tolerance >= 0.

The DCHECK variants only run if NDEBUG is defined; otherwise, the entire statement is removed (and the expression not evaluated).

Example, with no message:

PW_CHECK_FLOAT_NEAR(cos(0.0f), 1, 0.001);

Example, with an included message and arguments:

PW_CHECK_FLOAT_NEAR(FirstOperation(), RedundantOperation(), 0.1,
                    "System state=%s", SysState());
PW_CHECK_OK(status)
PW_CHECK_OK(status, format, ...)
PW_DCHECK_OK(status)
PW_DCHECK_OK(status, format, ...)

Assert that status evaluates to pw::OkStatus() (in C++) or PW_STATUS_OK (in C). Optionally include a message with arguments to report.

The DCHECK variants only run if NDEBUG is defined; otherwise, the entire statement is removed (and the expression not evaluated).

pw::Status operation_status = DoSomeOperation();
PW_CHECK_OK(operation_status);

// Any expression that evaluates to a pw::Status or pw_Status works.
PW_CHECK_OK(DoTheThing(), "System state: %s", SystemState());

// C works too.
pw_Status c_status = DoMoreThings();
PW_CHECK_OK(c_status, "System state: %s", SystemState());

Note

Using PW_CHECK_OK(status) instead of PW_CHECK(status == OkStatus()) enables displaying an error message with a string version of the error code; for example status == RESOURCE_EXHAUSTED instead of status == 5.

Assert API

The normal PW_CHECK_* and PW_DCHECK_* family of macros are intended to provide rich debug information, like the file, line number, value of operands in boolean comparisons, and more. However, this comes at a cost: these macros depend directly on the backend headers, and may perform complicated call-site transformations like tokenization.

There are several issues with the normal PW_CHECK_* suite of macros:

  1. PW_CHECK_* in headers can cause ODR violations in the case of tokenized asserts, due to differing module choices.

  2. PW_CHECK_* is not constexpr-safe.

  3. PW_CHECK_* can cause code bloat with some backends; this is the tradeoff to get rich assert information.

  4. PW_CHECK_* can trigger circular dependencies when asserts are used from low-level contexts, like in <span>.

PW_ASSERT solves all of the above problems: No risk of ODR violations, are constexpr safe, and have a tiny call site footprint; and there is no header dependency on the backend preventing circular include issues. However, there are no format messages, no captured line number, no captured file, no captured expression, or anything other than a binary indication of failure.

Example

// This example demonstrates asserting in a header.

#include "pw_assert/assert.h"

class InlinedSubsystem {
 public:
  void DoSomething() {
    // GOOD: No problem; PW_ASSERT is fine to inline and place in a header.
    PW_ASSERT(IsEnabled());
  }
  void DoSomethingElse() {
    // BAD: Generally avoid using PW_DCHECK() or PW_CHECK in headers. If you
    // want rich asserts or logs, move the function into the .cc file, and
    // then use PW_CHECK there.
    PW_DCHECK(IsEnabled());  // DON'T DO THIS
  }
};

PW_ASSERT API reference

PW_ASSERT(condition)

A header- and constexpr-safe version of PW_CHECK().

If the given condition is false, crash the system. Otherwise, do nothing. The condition is guaranteed to be evaluated. This assert implementation is guaranteed to be constexpr-safe.

PW_DASSERT(condition)

A header- and constexpr-safe version of PW_DCHECK().

Same as PW_ASSERT(), except that if PW_ASSERT_ENABLE_DEBUG == 1, the assert is disabled and condition is not evaluated.

Attention

Unlike the PW_CHECK_*() suite of macros, PW_ASSERT() and PW_DASSERT() capture no rich information like line numbers, the file, expression arguments, or the stringified expression. Use these macros only when absolutely necessary—in headers, constexpr contexts, or in rare cases where the call site overhead of a full PW_CHECK must be avoided.

Use PW_CHECK_*() whenever possible.

PW_ASSERT API backend

The PW_ASSERT API ultimately calls the C function pw_assert_HandleFailure(), which must be provided by the pw_assert backend. The pw_assert_HandleFailure() function must not return.

Avoiding circular dependencies with PW_ASSERT

Because asserts are so widely used, including in low-level libraries, it is common for the pw_assert backend to cause circular dependencies. Because of this, assert backends may avoid declaring explicit dependencies, instead relying on include paths to access header files.

In GN, the pw_assert backend’s full implementation with true dependencies is made available through the $dir_pw_assert:impl group. When pw_assert_BACKEND is set, $dir_pw_assert:impl must be listed in the pw_build_LINK_DEPS variable. See Link-only deps.

In the pw_assert, the backend’s full implementation is placed in the $pw_assert_BACKEND.impl target. $dir_pw_assert:impl depends on this backend target. The $pw_assert_BACKEND.impl target may be an empty group if the backend target can use its dependencies directly without causing circular dependencies.

In order to break dependency cycles, the pw_assert_BACKEND target may need to directly provide dependencies through include paths only, rather than GN public_deps. In this case, GN header checking can be disabled with check_includes = false.

Backend API

The backend controls what to do in the case of an assertion failure. In the most basic cases, the backend could display the assertion failure on something like sys_io and halt in a while loop waiting for a debugger. In other cases, the backend could store crash details like the current thread’s stack to flash.

This facade module (pw_assert) does not provide a backend. See pw_assert_basic for a basic implementation.

Attention

The facade macros (PW_CRASH and related) are expected to behave like they have the [[noreturn]] attribute set. This implies that the backend handler functions, PW_HANDLE_* defined by the backend, must not return.

In other words, the device must reboot.

The backend must provide the header

pw_assert_backend/backend.h

and that header must define the following macros:

PW_HANDLE_CRASH(message, ...)

Trigger a system crash or halt, and if possible, deliver the specified message and arguments to the user or developer.

PW_HANDLE_ASSERT_FAILURE(condition_str, message, ...)

Trigger a system crash or halt, and if possible, deliver the condition string (indicating what expression was false) and the message with format arguments, to the user or developer.

This macro is invoked from the PW_CHECK facade macro if condition is false.

PW_HANDLE_ASSERT_BINARY_COMPARE_FAILURE(a_str, a_val, op_str, b_str, b_val, type_fmt, message, ...)

Trigger a system crash or halt for a failed binary comparison assert (e.g. any of the PW_CHECK_<type>_<op> macros). The handler should combine the assert components into a useful message for the user; though in some cases this may not be possible.

Consider the following example:

int temp = 16;
int max_temp = 15;
PW_CHECK_INT_LE(temp, MAX_TEMP, "Got too hot; state: %s", GetSystemState());

In this block, the assert will trigger, which will cause the facade to invoke the handler macro. Below is the meaning of the arguments, referencing to the example:

  • a_str - Stringified first operand. In the example: "temp".

  • a_val - The value of the first operand. In the example: 16.

  • op_str - The string version of the operator. In the example: “<=”.

  • b_str - Stringified second operand. In the example: "max_temp".

  • b_val - The value of the second operand. In the example: 15.

  • type_fmt - The format code for the type. In the example: "%d".

  • message, ... - A formatted message to go with the assert. In the example: "Got too hot; state: %s", "ON_FIRE".

Tip

See pw_assert_basic for one way to combine these arguments into a meaningful error message.

Additionally, the backend must provide a link-time function for the PW_ASSERT assert handler. This does not need to appear in the backend header, but instead is in a .cc file.

pw_assert_HandleFailure()

Handle a low-level crash. This crash entry happens through pw_assert/assert.h. In this crash handler, there is no access to line, file, expression, or other rich assert information. Backends should do something reasonable in this case; typically, capturing the stack is useful.

Backend build targets

In GN, the backend must provide a pw_assert.impl build target in the same directory as the backend target. If the main backend target’s dependencies would cause dependency cycles, the actual backend implementation with its full dependencies is placed in the pw_assert.impl target. If this is not necessary, pw_assert.impl can be an empty group. Circular dependencies are a common problem with pw_assert because it is so widely used. See Avoiding circular dependencies with PW_ASSERT.

Frequently asked questions

When should DCHECK_* be used instead of CHECK_* and vice versa?

There is no hard and fast rule for when to use one or the other.

In theory, DCHECK_* macros should never be used and all the asserts should remain active in production. In practice, assert statements come at a binary size and runtime cost, even when using extensions like a tokenized assert backend that strips the stringified assert expression from the binary. Each assert is at least a branch with a function call; depending on the assert backend, that function call may take several arguments (like the message, the file line number, the module, etc). These function calls can take 10-20 bytes or more of ROM each. Thus, there is a balance to be struct between DCHECK_* and CHECK_*.

Pigweed uses these conventions to decide between CHECK_* and DCHECK_*:

  • Prefer to use CHECK_* at public API boundaries of modules, where an invalid value is a clear programmer bug. In certain cases use DCHECK_* to keep binary size small when in production; for example, in modules with a large public API surface, or modules with many inlined functions in headers.

  • Avoid using CHECK_* macros in headers. It is still OK to use CHECK_* macros in headers, but carefully consider the cost, since inlined use of the CHECK_* macros in headers will expand to the full assert cost for every translation unit that includes the header and calls the function with the CHECK_* instance. DCHECK_* macros are are better, but even they come at a cost, since it is preferable to be able to compile a binary in debug mode for as long as possible on the road to production.

  • Prefer to use DCHECK_* variants for internal asserts that attempt to catch module-author level programming errors. For example, use DCHECKs to verify internal function preconditions, or other invariants that should always be true but will likely never fire in production. In some cases using CHECK_* macros for internal consistency checking can make sense, if the runtime cost is low and there are only a couple of instances.

Tip

Do not return error status codes for obvious API misuse

Returning an error code may mask the earliest sign of a bug because notifying the developer of the problem depends on correct propagation of the error to upper levels of the system. Instead, prefer to use the CHECK_* or DCHECK_* macros to ensure a prompt termination and warning to the developer.

Error status codes should be reserved for system misbehaviour or expected exceptional cases, like a sensor is not yet ready, or a storage subsystem is full when writing. Doing CHECK_* assertions in those cases would be a mistake; so use error codes in those cases instead.

How should objects be asserted against or compared?

Unfortunatly, there is no native mechanism for this, and instead the way to assert object states or comparisons is with the normal PW_CHECK_* macros that operate on booleans, ints, and floats.

This is due to the requirement of supporting C and also tokenization. It may be possible support rich object comparions by defining a convention for stringifying objects; however, this hasn’t been added yet. Additionally, such a mechanism would not work well with tokenization. In particular, it would require runtime stringifying arguments and rendering them with %s, which leads to binary bloat even with tokenization. So it is likely that a rich object assert API won’t be added.

Why was the assert facade designed this way?

The Pigweed assert API was designed taking into account the needs of several past projects the team members were involved with. Based on those experiences, the following were key requirements for the API:

  1. C compatibility - Since asserts are typically invoked from arbitrary contexts, including from vendor or third party code, the assert system must have a C-compatible API. Some API functions working only in C++ is acceptable, as long as the key functions work in C.

  2. Capturing both expressions and values - Since asserts can trigger in ways that are not repeatable, it is important to capture rich diagnostic information to help identifying the root cause of the fault. For asserts, this means including the failing expression text, and optionally also capturing failing expression values. For example, instead of capturing an error with the expression (x < y), capturing an error with the expression and values(x < y, with x = 10, y = 0).

  3. Tokenization compatible - It’s important that the assert expressions support tokenization; both the expression itself (e.g. a < b) and the message attached to the expression. For example: PW_CHECK(ItWorks(), "Ruh roh: %d", some_int).

  4. Customizable assert handling - Most products need to support custom handling of asserts. In some cases, an assert might trigger printing out details to a UART; in other cases, it might trigger saving a log entry to flash. The assert system must support this customization.

The combination of #1, #2, and #3 led to the structure of the API. In particular, the need to support tokenized asserts and the need to support capturing values led to the choice of having PW_CHECK_INT_LE(a, b) instead of PW_CHECK(a <= b). Needing to support tokenization is what drove the facade & backend arrangement, since the backend must provide the raw macros for asserting in that case, rather than terminating at a C-style API.

Why isn’t there a PW_CHECK_LE? Why is the type (e.g. INT) needed?

The problem with asserts like PW_CHECK_LE(a, b) instead of PW_CHECK_INT_LE(a, b) or PW_CHECK_FLOAT_EXACT_LE(a, b) is that to capture the arguments with the tokenizer, we need to know the types. Using the preprocessor, it is impossible to dispatch based on the types of a and b, so unfortunately having a separate macro for each of the types commonly asserted on is necessary.

Compatibility

The facade is compatible with both C and C++.

Roadmap & Status

The Pigweed assert subsystem consiststs of several modules that work in coordination. This module is the facade (API), then a number of backends are available to handle assert failures. Products can also define their own backends. In some cases, the backends will have backends (like pw_log_tokenized).

Below is a brief summary of what modules are ready for use:

Available assert backends

  • pw_assert - Stable - The assert facade (this module). This module is stable, and in production use. The documentation is comprehensive and covers the functionality. There are (a) tests for the facade macro processing logic, using a fake assert backend; and (b) compile tests to verify that the selected backend compiles with all supported assert constructions and types.

  • pw_assert_basic - Stable - The assert basic module is a simple assert handler that displays the failed assert line and the values of captured arguments. Output is directed to pw_sys_io. This module is a great ready-to-roll module when bringing up a system, but is likely not the best choice for production.

  • pw_assert_log - Stable - This assert backend redirects to logging, but with a logging flag set that indicates an assert failure. This is our advised approach to get tokenized asserts–by using tokenized logging, then using the pw_assert_log backend.

Note: If one desires a null assert module (where asserts are removed), use pw_assert_log in combination with pw_log_null. This will direct asserts to logs, then the logs are removed due to the null backend.

Missing functionality

  • Stack traces - Pigweed doesn’t have a reliable stack walker, which makes displaying a stack trace on crash harder. We plan to add this eventually.

  • Snapshot integration - Pigweed doesn’t yet have a rich system state capture system that can capture state like number of tasks, available memory, and so on. Snapshot facilities are the obvious ones to run inside an assert handler. It’ll happen someday.