errors.h

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/*
 * Copyright (C) 2016 The Android Open Source Project
 *
 * 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
 *
 *      http://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.
 */
 
// Portable error handling functions. This is only necessary for host-side
// code that needs to be cross-platform; code that is only run on Unix should
// just use errno and strerror() for simplicity.
//
// There is some complexity since Windows has (at least) three different error
// numbers, not all of which share the same type:
//   * errno: for C runtime errors.
//   * GetLastError(): Windows non-socket errors.
//   * WSAGetLastError(): Windows socket errors.
// errno can be passed to strerror() on all platforms, but the other two require
// special handling to get the error string. Refer to Microsoft documentation
// to determine which error code to check for each function.
 
#pragma once
 
#include <assert.h>
 
#include <string>
 
namespace android {
namespace base {
 
// Returns a string describing the given system error code. |error_code| must
// be errno on Unix or GetLastError()/WSAGetLastError() on Windows. Passing
// errno on Windows has undefined behavior.
std::string SystemErrorCodeToString(int error_code);
 
}  // namespace base
}  // namespace android
 
// Convenient macros for evaluating a statement, checking if the result is error, and returning it
// to the caller.
//
// Usage with Result<T>:
//
// Result<Foo> getFoo() {...}
//
// Result<Bar> getBar() {
//   Foo foo = OR_RETURN(getFoo());
//   return Bar{foo};
// }
//
// Usage with status_t:
//
// status_t getFoo(Foo*) {...}
//
// status_t getBar(Bar* bar) {
//   Foo foo;
//   OR_RETURN(getFoo(&foo));
//   *bar = Bar{foo};
//   return OK;
// }
//
// Actually this can be used for any type as long as the OkOrFail<T> contract is satisfied. See
// below.
// If implicit conversion compilation errors occur involving a value type with a templated
// forwarding ref ctor, compilation with cpp20 or explicitly converting to the desired
// return type is required.
#define OR_RETURN(expr)                                                                  \
  ({                                                                                     \
    decltype(expr)&& __or_return_expr = (expr);                                          \
    typedef android::base::OkOrFail<std::remove_reference_t<decltype(__or_return_expr)>> \
        ok_or_fail;                                                                      \
    if (!ok_or_fail::IsOk(__or_return_expr)) {                                           \
      return ok_or_fail::Fail(std::move(__or_return_expr));                              \
    }                                                                                    \
    ok_or_fail::Unwrap(std::move(__or_return_expr));                                     \
  })
 
// Same as OR_RETURN, but aborts if expr is a failure.
#if defined(__BIONIC__)
#define OR_FATAL(expr)                                                                  \
  ({                                                                                    \
    decltype(expr)&& __or_fatal_expr = (expr);                                          \
    typedef android::base::OkOrFail<std::remove_reference_t<decltype(__or_fatal_expr)>> \
        ok_or_fail;                                                                     \
    if (!ok_or_fail::IsOk(__or_fatal_expr)) {                                           \
      __assert(__FILE__, __LINE__, ok_or_fail::ErrorMessage(__or_fatal_expr).c_str());  \
    }                                                                                   \
    ok_or_fail::Unwrap(std::move(__or_fatal_expr));                                     \
  })
#else
#define OR_FATAL(expr)                                                                  \
  ({                                                                                    \
    decltype(expr)&& __or_fatal_expr = (expr);                                          \
    typedef android::base::OkOrFail<std::remove_reference_t<decltype(__or_fatal_expr)>> \
        ok_or_fail;                                                                     \
    if (!ok_or_fail::IsOk(__or_fatal_expr)) {                                           \
      fprintf(stderr, "%s:%d: assertion \"%s\" failed", __FILE__, __LINE__,             \
              ok_or_fail::ErrorMessage(__or_fatal_expr).c_str());                       \
      abort();                                                                          \
    }                                                                                   \
    ok_or_fail::Unwrap(std::move(__or_fatal_expr));                                     \
  })
#endif
 
namespace android {
namespace base {
 
// The OkOrFail contract for a type T. This must be implemented for a type T if you want to use
// OR_RETURN(stmt) where stmt evalues to a value of type T.
template <typename T, typename = void>
struct OkOrFail {
  // Checks if T is ok or fail.
  static bool IsOk(const T&);
 
  // Turns T into the success value.
  template <typename U>
  static U Unwrap(T&&);
 
  // Moves T into OkOrFail<T>, so that we can convert it to other types
  OkOrFail(T&& v);
  OkOrFail() = delete;
  OkOrFail(const T&) = delete;
 
  // And there need to be one or more conversion operators that turns the error value of T into a
  // target type. For example, for T = Result<V, E>, there can be ...
  //
  // // for the case where OR_RETURN is called in a function expecting E
  // operator E()&& { return val_.error().code(); }
  //
  // // for the case where OR_RETURN is called in a function expecting Result<U, E>
  // template <typename U>
  // operator Result<U, E>()&& { return val_.error(); }
 
  // Returns the string representation of the fail value.
  static std::string ErrorMessage(const T& v);
};
 
}  // namespace base
}  // namespace android