jwlodek / IocshDeclWrap

C++ Templates for Easy Wrapping of User Functions for the EPICS IOC-shell

IocshDeclWrapper

Till Straumann, 2020/5

IocshDeclWrapper is hosted on github

Introduction

This module provides a C++ header file which automates the generation of boiler-plate code that is necessary for interfacing a user function to the EPICS iocsh.

Wrapping a user function requires the following steps:

1. allocate and populate a iocshFuncDef struct
2. allocate and populate a iocshArg* array
3. allocate and populate iocshArg structs
4. point the iocshArg* array elements to the structs from step 3.
5. define a wrapper function which extracts function arguments from a iocshArgBuf* array, passes them to the user function and prints its result. The values of mutable arguments (passed by pointers or references) are also printed.
6. register the wrapper function and iocshFuncDef with iocsh. This is often done via a 'registrar' function that must be defined and declared in a 'dbd' file.

The iocshDeclWrapper.h header automates steps 1-5 and optionally (6).

Example

Assume you want to access a user function (e.g., written in 'C' ) from iocsh. The function myFunction shall be declared in a header file:

myHeader.h:

  int myFunction(int debugLevel, const char *comment);

Wrapping requires C++; thus you must create or add to a C++ file:

extern "C" {
#include <myHeader.h>
};
#include <iocshDeclWrapper.h>

IOCSH_FUNC_WRAP_REGISTRAR( myRegistrar,
  IOCSH_FUNC_WRAP( myFunction, "debugLevel(int)", "comment(string)" );
  /* more functions may be registered here */
)

And in a 'dbd' file you add:

registrar( myRegistrar );

IOCSH_FUNC_WRAP_REGISTRAR()

The IOCSH_FUNC_WRAP_REGISTRAR macro is a convenience macro which defines and epicsExports a registrar function:

IOCSH_FUNC_WRAP_REGISTRAR(
   <registrar_name>,
   <any number of IOCSH_FUNC_WRAP() statements>
)

The name of the registrar must also be declared in a dbd file.

Note that multiple IOCSH_FUNC_WRAP() statements may be present in a single registrar. It is also not mandatory to use the IOCSH_FUNC_WRAP_REGISTRAR() macro at all. IOCSH_FUNC_WRAP() may be added to any registrar routine, the IOC's main function or any other appropriate place for executing iocshRegister() (it must be a C++ source, though; plain C does not work). Again: it is perfectly fine to wrap C-functions; just the IOCSH_FUNC_WRAP() macro must be expanded from C++ code.

IOCSH_FUNC_WRAP() and IOCSH_FUNC_WRAP_QUIET()

This is the main macro that does the heavy lifting. It is really easy to use:

IOCSH_FUNC_WRAP( <function_to_wrap> { ',' <arg_help_string> } );

The first argument identifies the function which you want to wrap. The declaration of this function must be available from where you expand the macro (the C++ template needs to know the exact types of the function's return value and arguments).

Additional arguments are optional; they can be used to give explanatory names to the arguments which will show up when the user types

help <function_to_wrap>

in iocsh. By default, a string describing the type of the argument is used.

If you don't want the return value of the user function to be printed to the console then you use

IOCSH_FUNC_WRAP_QUIET( <function_to_wrap>, { ',' <arg_help_string> } );

Use this variant also if no Printer (see below) for the return type of your function is available and you want to suppress the respective notice which is otherwise printed to the console. Note that printing of the values of mutable arguments is also suppressed by this variant.

C++98 Restrictions

When using a C++98 compiler (no C++11 support) the user function must have external linkage. I.e., it is not possible to wrap a static function.

For C++98 the maximal number of arguments the user function may have is limited to IOCSH_FUNC_WRAP_MAX_ARGS (currently 10). Under C++11 there is no static maximum.

Namespace IocshDeclWrapper

All the templates are defined in their own

namespace IocshDeclWrapper

This is important to keep in mind if you intend to write your own specializations for some of the templates.

Template Specializations

Converting function arguments and printing of function results is handled by Convert and Printer templates, respectively.

These templates are specialized for specific types. The user may easily add specializations for types that are not implemented yet or in order to override an existing implementation.

Note that a suitable declaration of the template specialization must be visible to the compiler from where IOCSH_WRAP_FUNC() is expanded.

Argument Conversions

The Convert templates handle the passing of arguments between iocshArgBuf and the user function. This is implemented by template specializations for specific types. The standard integral types as well as char* (C-strings), std::string, double, float and std::complex are already implemented.

Specializations for dealing with pointers and references to standard integral types, floats and strings are also provided.

If a particular type cannot be handled by the existing templates then the you can easily define your own specialization:

template <> IocshDeclWrapper::Convert< MYTYPE, MYTYPE > {
{
  static void setArg( iocshArg *a )
  {
    a->name = "MYTYPE"; /* set default name/help for arg;
                         * shown by iocsh help; this may
                         * be overridden by IOCSH_WRAP_FUNC()
                         * arguments for individual functions.
                         */
    a->type = <iocsh type that is most appropriate>;
  }

  MYTYPE getArg( const iocshArgBuf         *arg,
                 IocshDeclWrapper::Context *ctx,
                 int                        argNo)
  {
     /* extract function argument from *arg, convert to MYTYPE and return */
     return convert_to_MYTYPE( arg );
  }
};

Handling Conversion Errors

In some cases getArg cannot convert a value. E.g., when converting from a string argument and the string is NULL or ill-formatted.

In this case getArg may throw a IocshDeclWrapper::ConversionError which is caught and results in an error message being printed to the console. The user function is not executed if such an error is thrown.

Preserving Converted Values in Convert::getArg Context

In more complex cases it is necessary to create an intermediate object during the conversion process. Such an object may need to be passed to the user function and therefore remain available outside of the scope of Context::getArg. This is what the context is used for. It lets you allocate something new and attach it to the context. The context is eventually destroyed after the user function returns. This is always necessary if you cannot pass an argument by value.

Here is the example where the user function expects a C++ std::string * pointer argument; since the iocshArgBuf only holds a const char * pointer we have to create a new std::string which must persist until the user function returns:

template <> IocshDeclWrapper::Convert< string *, string *> {
  static void setArg(iocshArg *a)
  {
    a->name = "<string>";
    a->type = iocshArgString;
  }

  static string * getArg( const iocshArgBuf         *arg,
                          IocshDeclWrapper::Context *ctx,
                          int                        argNo )
  {
     /*
      * 'make' creates a new string and attaches it to the context
      * 'argNo' tells us which argument we are converting.
      *  This can be used for printing arguments that were
      *  possibly modified by the user function (pointer/reference
      *  arguments to non-const objects).
      */
     return ctx->make<string, const char*>( arg->sval, argNo );
  }
};

The make<typename T, typename I> works automatically as long as type T has a constructor that takes an argument of type I.

Convert<> Template Arguments

The Convert<typename T, typename R, int USER> template expects up to three arguments. The R type argument allows for SFINAE-style specializations that can handle multiple related types. Consult the header for examples/guidance.

The USER argument can be used to add an even more specific specialization in order to override a default converter.

If the user wishes, for example, to provide his/her own conversion for integral types then the template may be specialized for the explicit USER value 0:

template <typename T> struct Convert<T, typename is_int<T>::type, 0> {
}

Note: The templates are always instantiated with USER=0`, thus a specialization for a different value would never been selected; the USER argument simply provides the possibility of an additional level of specialization.)

Mutable Arguments

Non-const arguments that are passed as pointers or references to the user function can potentially be modified by the function.

Such arguments should always be created in the Context and initialized from the values passed in by the user.

The wrapper prints the values of mutable arguments after the user function returns.

Printing User Function Results

The return value of a user function (provided that the function does not return void) is passed to the print static member of the Printer template.

Note that printing of results is optional. The templates support a parameter that causes the user function's return value to be discarded. In order to deactivate printing of its return value register your function with the

IOCSH_FUNC_WRAP_QUIET()

macro (see above).

The PrinterBase Template

PrinterBase is specialized for particular types of function return values and handles printing for individual types.

template <typename T, typename R, int USER = 0> class PrinterBase {
  public:
    /* The Reference<R>::const_type idiom is for C++98
     * and resolves to 'const R &' -- even if R is already
     * a reference type.
     */
    static void print( typename Reference<R>::const_type r );
};

You can implement a PrinterBase class specialization for your own types.

Specializing for USER=0 you can override an existing implementation of PrinterBase.

The PrintFmts Template

The default version of PrinterBase can handle any data type which can be printed by errlogPrintf using a suitable format.

The purpose of template <typename T> struct PrintFmts template is supplying a suitable errlogPrintf format for type T.

The get() member returns a NULL terminated array of formats and PrinterBase prints the function result for each one of these formats. This can be used, e.g., to print decimal as well as hexadecimal versions of a result.

As usual, the default specializations for the standard types may be overridden by providing a more specific version for the explicit USER=0 argument.

The Printer Template

The purpose of Printer is adding the possibility for providing a specialization for a specific user function.

The Printer template normally derives from its base class PrinterBase and simply uses the base classes' print function:

template <typename R, typename SIG, SIG *sig> class Printer
: public PrinterBase<R, R>
{
};

However, you may define a specialization that is specific to a particular user function, e.g.

int mySpecialFunction();

template <> class Printer<int, int(), mySpecialFunction> :
: public PrinterBase<int, int> {
  public:

  static void print(const int &v) {
     /* Example: only a particular bit is of interest: */
     errlogPrintf( "mySpecialFunction returned: %s\n", v & (1<<8) ? "TRUE" : "FALSE" );
  }

};

While you are not using PrinterBase::print in this case you should still derive your Printer class from PrinterBase. This ensures forward-compatibility if the PrinterBase class is ever enhanced with additional members.

The ArgPrinter Template

This template is used to print the values of mutable arguments after the user function returns. It can also be specialized for a particular function signature. Consult the Printer template section for more information.

Printing Summary

Summarizing the purpose of the multiple print-related templates:

• The default implementation of PrinterBase can handle types which can be printed using standard printf-style formats.
• The default implementation of PrinterBase uses a suitable specialization of PrintFmts to obtain a printf-style format.
• In order to print a specific data type:
  • specialize PrintFmts if possible
  • if the type cannot be displayed by printf or you don't like the default version then specialize PrinterBase to handle the specific type.
• If you need a special print method specifically for your user- function then you should specialize Printer for your function.

Overloaded Functions

The templates support overloaded functions. However, you cannot use the simple IOCSH_FUNC_WRAP() macro but you must use

 IOCSH_FUNC_WRAP_OVLD( func, signature, name, argHelps... )

instead. func is the name of the function you want to wrap and signature is the list of argument types that identifies the overloaded variant you want to wrap. Since this list may contain commas it must be enclosed in parenthesis. E.g.:

 IOCSH_FUNC_WRAP( myFunction, (int, const char*), "myFunction" );

(the parenthesis must be present even if your function has only one or no argument.) Because iocsh does not support overloading functions you must always specify the name that iocsh shall use to identify your wrapped function. If the signature is empty then it is automatically resolved (which fails if the function is overloaded). In fact, the IOCSH_FUNC_WRAP(func,argHelps..) macro is just a shorthand for

 IOCSH_FUNC_WRAP( func, , #func, argHelps )

and even IOCSH_FUNC_WRAP_OVLD() is defined in terms of a yet more general macro

 IOCSH_FUNC_REGISTER_WRAPPER( func, signature, name, doPrintResult, argHelps... )

which takes an additional argument doPrintResult which may be true or false and which defines whether iocsh should print the return value (and modified arguments) of your functions.

Examples

Examples can be found in the test source file

test/wrapper.cc