blob: a2de75fa534ffe2ee4b15267e75481cfd01b49ed [file] [log] [blame]
.. _module-pw_status:
``pw_status`` provides features for communicating the result of an operation.
The classes in ``pw_status`` are used extensively throughout Pigweed.
The primary feature of ``pw_status`` is the ``pw::Status`` class.
``pw::Status`` (``pw_status/status.h``) is a simple, zero-overhead status
object that wraps a status code.
``pw::Status`` uses Google's standard status codes (see the `Google APIs
repository <>`_).
These codes are used extensively in Google projects including `Abseil
<>`_ (`status/status.h
) and `gRPC <>`_ (`doc/
A ``Status`` is created with a ``static constexpr`` member function
corresponding to the code.
.. code-block:: cpp
// Ok (gRPC code "OK") does not indicate an error; this value is returned on
// success. It is typical to check for this value before proceeding on any
// given call across an API or RPC boundary. To check this value, use the
// `Status::ok()` member function rather than inspecting the raw code.
// Cancelled (gRPC code "CANCELLED") indicates the operation was cancelled,
// typically by the caller.
// Unknown (gRPC code "UNKNOWN") indicates an unknown error occurred. In
// general, more specific errors should be raised, if possible. Errors raised
// by APIs that do not return enough error information may be converted to
// this error.
// InvalidArgument (gRPC code "INVALID_ARGUMENT") indicates the caller
// specified an invalid argument, such a malformed filename. Note that such
// errors should be narrowly limited to indicate to the invalid nature of the
// arguments themselves. Errors with validly formed arguments that may cause
// errors with the state of the receiving system should be denoted with
// `FailedPrecondition` instead.
// DeadlineExceeded (gRPC code "DEADLINE_EXCEEDED") indicates a deadline
// expired before the operation could complete. For operations that may change
// state within a system, this error may be returned even if the operation has
// completed successfully. For example, a successful response from a server
// could have been delayed long enough for the deadline to expire.
// NotFound (gRPC code "NOT_FOUND") indicates some requested entity (such as
// a file or directory) was not found.
// `NotFound` is useful if a request should be denied for an entire class of
// users, such as during a gradual feature rollout or undocumented allow list.
// If, instead, a request should be denied for specific sets of users, such as
// through user-based access control, use `PermissionDenied` instead.
// AlreadyExists (gRPC code "ALREADY_EXISTS") indicates the entity that a
// caller attempted to create (such as file or directory) is already present.
// PermissionDenied (gRPC code "PERMISSION_DENIED") indicates that the caller
// does not have permission to execute the specified operation. Note that this
// error is different than an error due to an *un*authenticated user. This
// error code does not imply the request is valid or the requested entity
// exists or satisfies any other pre-conditions.
// `PermissionDenied` must not be used for rejections caused by exhausting
// some resource. Instead, use `ResourceExhausted` for those errors.
// `PermissionDenied` must not be used if the caller cannot be identified.
// Instead, use `Unauthenticated` for those errors.
// ResourceExhausted (gRPC code "RESOURCE_EXHAUSTED") indicates some resource
// has been exhausted, perhaps a per-user quota, or perhaps the entire file
// system is out of space.
// FailedPrecondition (gRPC code "FAILED_PRECONDITION") indicates that the
// operation was rejected because the system is not in a state required for
// the operation's execution. For example, a directory to be deleted may be
// non-empty, an "rmdir" operation is applied to a non-directory, etc.
// Some guidelines that may help a service implementer in deciding between
// `FailedPrecondition`, `Aborted`, and `Unavailable`:
// (a) Use `Unavailable` if the client can retry just the failing call.
// (b) Use `Aborted` if the client should retry at a higher transaction
// level (such as when a client-specified test-and-set fails, indicating
// the client should restart a read-modify-write sequence).
// (c) Use `FailedPrecondition` if the client should not retry until
// the system state has been explicitly fixed. For example, if an "rmdir"
// fails because the directory is non-empty, `FailedPrecondition`
// should be returned since the client should not retry unless
// the files are deleted from the directory.
// Aborted (gRPC code "ABORTED") indicates the operation was aborted,
// typically due to a concurrency issue such as a sequencer check failure or a
// failed transaction.
// See the guidelines above for deciding between `FailedPrecondition`,
// `Aborted`, and `Unavailable`.
// OutOfRange (gRPC code "OUT_OF_RANGE") indicates the operation was
// attempted past the valid range, such as seeking or reading past an
// end-of-file.
// Unlike `InvalidArgument`, this error indicates a problem that may
// be fixed if the system state changes. For example, a 32-bit file
// system will generate `InvalidArgument` if asked to read at an
// offset that is not in the range [0,2^32-1], but it will generate
// `OutOfRange` if asked to read from an offset past the current
// file size.
// There is a fair bit of overlap between `FailedPrecondition` and
// `OutOfRange`. We recommend using `OutOfRange` (the more specific
// error) when it applies so that callers who are iterating through
// a space can easily look for an `OutOfRange` error to detect when
// they are done.
// Unimplemented (gRPC code "UNIMPLEMENTED") indicates the operation is not
// implemented or supported in this service. In this case, the operation
// should not be re-attempted.
// Internal (gRPC code "INTERNAL") indicates an internal error has occurred
// and some invariants expected by the underlying system have not been
// satisfied. This error code is reserved for serious errors.
// Unavailable (gRPC code "UNAVAILABLE") indicates the service is currently
// unavailable and that this is most likely a transient condition. An error
// such as this can be corrected by retrying with a backoff scheme. Note that
// it is not always safe to retry non-idempotent operations.
// See the guidelines above for deciding between `FailedPrecondition`,
// `Aborted`, and `Unavailable`.
// DataLoss (gRPC code "DATA_LOSS") indicates that unrecoverable data loss or
// corruption has occurred. As this error is serious, proper alerting should
// be attached to errors such as this.
// Unauthenticated (gRPC code "UNAUTHENTICATED") indicates that the request
// does not have valid authentication credentials for the operation. Correct
// the authentication and try again.
.. attention::
Some code may use all-caps status values such as ``Status::UNKNOWN`` instead
of ``Status::Unknown()``. These all-caps status codes are deprecated and will
be removed in the future. Do not use them; use the functions above instead.
The all-caps status aliases were deprecated because they do not comply with
the style guide and potentially conflict with macro definitions. For example,
projects might define an ``INTERNAL`` macro, which would prevent ``status.h``
or code that uses ``Status::INTERNAL`` from compiling.
The Python tool ``pw_status/`` may be used to migrate code in a
Git repo to the new status style.
C compatibility
``pw_status`` provides the C-compatible ``pw_Status`` enum for the status codes.
For ease of use, ``pw::Status`` implicitly converts to and from ``pw_Status``.
However, the ``pw_Status`` enum should never be used in C++; instead use the
``Status`` class.
The values of the ``pw_Status`` enum are all-caps and prefixed with
``PW_STATUS_``. For example, ``PW_STATUS_DATA_LOSS`` corresponds with the C++
``pw::StatusWithSize`` (``pw_status/status_with_size.h``) is a convenient,
efficient class for reporting a status along with an unsigned integer value.
It is similar to the ``pw::Result<T>`` class, but it stores both a size and a
status, regardless of the status value, and only supports a limited range (27
``pw::StatusWithSize`` values may be created with functions similar to
``pw::Status``. For example,
.. code-block:: cpp
// An OK StatusWithSize with a size of 123.
// A NOT_FOUND StatusWithSize with a size of 0.
// A RESOURCE_EXHAUSTED StatusWithSize with a size of 10.
``PW_TRY`` (``pw_status/try.h``) is a convenient set of macros for working
with Status and StatusWithSize objects in functions that return Status or
StatusWithSize. The PW_TRY and PW_TRY_WITH_SIZE macros call a function and
do an early return if the function's return status is not ok.
.. code-block:: cpp
Status PwTryExample() {
// Do something, only executed if both functions above return OK.
StatusWithSize PwTryWithSizeExample() {
// Do something, only executed if both functions above return OK.
PW_TRY_ASSIGN is for working with StatusWithSize objects in in functions
that return Status. It is similar to PW_TRY with the addition of assigning
the size from the StatusWithSize on ok.
.. code-block:: cpp
Status PwTryAssignExample() {
size_t size_value
PW_TRY_ASSIGN(size_value, FunctionThatReturnsStatusWithSize());
// Do something that uses size_value. size_value is only assigned and this
// following code executed if the PW_TRY_ASSIGN function above returns OK.