# Miscellaneous¶

## Error handling¶

Like in any other asynchronous invocation scheme, it is important to be able to handle error conditions occurring while the asynchronous (and possibly remote) operation is executed. In HPX all error handling is based on standard C++ exception handling. Any exception thrown during the execution of an asynchronous operation will be transferred back to the original invocation locality, where it will be rethrown during synchronization with the calling thread.

The source code for this example can be found here: error_handling.cpp.

### Working with exceptions¶

For the following description assume that the function raise_exception() is executed by invoking the plain action raise_exception_type.

//[error_handling_raise_exception
void raise_exception()
{
HPX_THROW_EXCEPTION(hpx::no_success, "raise_exception", "simulated error");


The exception is thrown using the macro HPX_THROW_EXCEPTION. The type of the thrown exception is hpx::exception. This associates additional diagnostic information with the exception, such as file name and line number, locality id and thread id, and stack backtrace from the point where the exception was thrown.

Any exception thrown during the execution of an action is transferred back to the (asynchronous) invocation site. It will be rethrown in this context when the calling thread tries to wait for the result of the action by invoking either future<>::get() or the synchronous action invocation wrapper as shown here:

        // Error reporting using exceptions
//[exception_diagnostic_information
hpx::cout << "Error reporting using exceptions\n";
try {
// invoke raise_exception() which throws an exception
raise_exception_action do_it;
do_it(hpx::find_here());
}
catch (hpx::exception const& e) {
// Print just the essential error information.
hpx::cout << "caught exception: " << e.what() << "\n\n";

// Print all of the available diagnostic information as stored with
// the exception.
hpx::cout << "diagnostic information:"
<< hpx::diagnostic_information(e) << "\n";


Note

The exception is transferred back to the invocation site even if it is executed on a different locality.

Additionally, this example demonstrates how an exception thrown by an (possibly remote) action can be handled. It shows the use of hpx::diagnostic_information, which retrieves all available diagnostic information from the exception as a formatted string. This includes, for instance, the name of the source file and line number, the sequence number of the OS thread and the HPX thread id, the locality id and the stack backtrace of the point where the original exception was thrown.

Under certain circumstances it is desirable to output only some of the diagnostics, or to output those using different formatting. For this case, HPX exposes a set of lower-level functions as demonstrated in the following code snippet:

        // Detailed error reporting using exceptions
//[exception_diagnostic_elements
hpx::cout << "Detailed error reporting using exceptions\n";
try {
// Invoke raise_exception() which throws an exception.
raise_exception_action do_it;
do_it(hpx::find_here());
}
catch (hpx::exception const& e) {
// Print the elements of the diagnostic information separately.
hpx::cout << "{what}: "        << hpx::get_error_what(e) << "\n";
hpx::cout << "{locality-id}: " << hpx::get_error_locality_id(e) << "\n";
hpx::cout << "{hostname}: "    << hpx::get_error_host_name(e) << "\n";
hpx::cout << "{pid}: "         << hpx::get_error_process_id(e) << "\n";
hpx::cout << "{function}: "    << hpx::get_error_function_name(e) << "\n";
hpx::cout << "{file}: "        << hpx::get_error_file_name(e) << "\n";
hpx::cout << "{line}: "        << hpx::get_error_line_number(e) << "\n";
<< "\n";
hpx::cout << "{state}: "       << std::hex << hpx::get_error_state(e)
<< "\n";
hpx::cout << "{stack-trace}: " << hpx::get_error_backtrace(e) << "\n";
hpx::cout << "{env}: "         << hpx::get_error_env(e) << "\n";


### Working with error codes¶

Most of the API functions exposed by HPX can be invoked in two different modes. By default those will throw an exception on error as described above. However, sometimes it is desirable not to throw an exception in case of an error condition. In this case an object instance of the hpx::error_code type can be passed as the last argument to the API function. In case of an error, the error condition will be returned in that hpx::error_code instance. The following example demonstrates extracting the full diagnostic information without exception handling:

        {
//[error_handling_diagnostic_information
hpx::cout << "Error reporting using error code\n";

// Create a new error_code instance.
hpx::error_code ec;

// If an instance of an error_code is passed as the last argument while
// invoking the action, the function will not throw in case of an error
// but store the error information in this error_code instance instead.
raise_exception_action do_it;
do_it(hpx::find_here(), ec);

if (ec) {
// Print just the essential error information.
hpx::cout << "returned error: " << ec.get_message() << "\n";

// Print all of the available diagnostic information as stored with
// the exception.
hpx::cout << "diagnostic information:"
<< hpx::diagnostic_information(ec) << "\n";
}


Note

The error information is transferred back to the invocation site even if it is executed on a different locality.

This example show how an error can be handled without having to resolve to exceptions and that the returned hpx::error_code instance can be used in a very similar way as the hpx::exception type above. Simply pass it to the hpx::diagnostic_information, which retrieves all available diagnostic information from the error code instance as a formatted string.

As for handling exceptions, when working with error codes, under certain circumstances it is desirable to output only some of the diagnostics, or to output those using different formatting. For this case, HPX exposes a set of lower-level functions usable with error codes as demonstrated in the following code snippet:

        {
//[error_handling_diagnostic_elements
hpx::cout << "Detailed error reporting using error code\n";

// Create a new error_code instance.
hpx::error_code ec;

// If an instance of an error_code is passed as the last argument while
// invoking the action, the function will not throw in case of an error
// but store the error information in this error_code instance instead.
raise_exception_action do_it;
do_it(hpx::find_here(), ec);

if (ec) {
// Print the elements of the diagnostic information separately.
hpx::cout << "{what}: "        << hpx::get_error_what(ec) << "\n";
hpx::cout << "{locality-id}: " << hpx::get_error_locality_id(ec) << "\n";
hpx::cout << "{hostname}: "    << hpx::get_error_host_name(ec) << "\n";
hpx::cout << "{pid}: "         << hpx::get_error_process_id(ec) << "\n";
hpx::cout << "{function}: "    << hpx::get_error_function_name(ec)
<< "\n";
hpx::cout << "{file}: "        << hpx::get_error_file_name(ec) << "\n";
hpx::cout << "{line}: "        << hpx::get_error_line_number(ec) << "\n";
hpx::cout << "{thread-id}: "   << std::hex
hpx::cout << "{state}: "       << std::hex << hpx::get_error_state(ec)
<< "\n";
hpx::cout << "{stack-trace}: " << hpx::get_error_backtrace(ec) << "\n";
hpx::cout << "{env}: "         << hpx::get_error_env(ec) << "\n";
}


For more information please refer to the documentation of hpx::get_error_what, hpx::get_error_locality_id, hpx::get_error_host_name, hpx::get_error_process_id, hpx::get_error_function_name, hpx::get_error_file_name, hpx::get_error_line_number, hpx::get_error_os_thread, hpx::get_error_thread_id, hpx::get_error_thread_description, hpx::get_error_backtrace, hpx::get_error_env, and hpx::get_error_state.

### Lightweight error codes¶

Sometimes it is not desirable to collect all the ambient information about the error at the point where it happened as this might impose too much overhead for simple scenarios. In this case, HPX provides a lightweight error code facility that will hold the error code only. The following snippet demonstrates its use:

        {
//[lightweight_error_handling_diagnostic_information
hpx::cout << "Error reporting using an lightweight error code\n";

// Create a new error_code instance.
hpx::error_code ec(hpx::lightweight);

// If an instance of an error_code is passed as the last argument while
// invoking the action, the function will not throw in case of an error
// but store the error information in this error_code instance instead.
raise_exception_action do_it;
do_it(hpx::find_here(), ec);

if (ec) {
// Print just the essential error information.
hpx::cout << "returned error: " << ec.get_message() << "\n";

// Print all of the available diagnostic information as stored with
// the exception.
hpx::cout << "error code:" << ec.value() << "\n";
}


All functions that retrieve other diagnostic elements from the hpx::error_code will fail if called with a lightweight error_code instance.

## Utilities in HPX¶

In order to ease the burden of programming, HPX provides several utilities to users. The following section documents those facilies.

See checkpoint.

## The HPX I/O-streams component¶

The HPX I/O-streams subsystem extends the standard C++ output streams std::cout and std::cerr to work in the distributed setting of an HPX application. All of the output streamed to hpx::cout will be dispatched to std::cout on the console locality. Likewise, all output generated from hpx::cerr will be dispatched to std::cerr on the console locality.

Note

All existing standard manipulators can be used in conjunction with hpx::cout and hpx::cerr Historically, HPX also defines hpx::endl and hpx::flush but those are just aliases for the corresponding standard manipulators.

In order to use either hpx::cout or hpx::cerr, application codes need to #include <hpx/include/iostreams.hpp>. For an example, please see the following ‘Hello world’ program:

//  Copyright (c) 2007-2012 Hartmut Kaiser
//

///////////////////////////////////////////////////////////////////////////////
// The purpose of this example is to execute a HPX-thread printing
// "Hello World!" once. That's all.

//[hello_world_1_getting_started
// Including 'hpx/hpx_main.hpp' instead of the usual 'hpx/hpx_init.hpp' enables
// to use the plain C-main below as the direct main HPX entry point.
#include <hpx/hpx_main.hpp>
#include <hpx/include/iostreams.hpp>

int main()
{
// Say hello to the world!
hpx::cout << "Hello World!\n" << hpx::flush;
return 0;
}
//]


Additionally, those applications need to link with the iostreams component. When using CMake this can be achieved by using the COMPONENT_DEPENDENCIES parameter; for instance:

include(HPX_AddExecutable)


The hpx::cout and hpx::cerr streams buffer all output locally until a std::endl or std::flush is encountered. That means that no output will appear on the console as long as either of these is explicitly used.