kernel_methods

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ARTICLE

Kernel Methods
For internal use, kernel methods can be implemented in the ABAP kernel instead of in the ABAP language.

Introduction
Kernel methods allow ABAP kernel functions implemented in C or C++ to be called directly. Kernel methods replace the previous concepts of C calls and system calls. No new C calls or system calls need to be introduced.
Kernel methods offer the same checks and security features as normal ABAP methods. Except for the Constructors and the C Destructor, all ABAP methods can be implemented as kernel methods. An ABAP method can still be redefined as a kernel method and a kernel method can still be redefined as an ABAP method within a path of the inheritance hierarchy.
For C developers who want to implement a kernel method, an API is available that allows simple, high-performance, and secure access to arguments. Class-based exceptions also continue to be supported.

Defining Kernel Methods

Declaration in ABAP
A kernel method is declared in the same way as a normal ABAP method, in Class Builder or in the declaration section of a local class. Whether a method is implemented as a kernel method is not important for the declaration. In ABAP, this means that a kernel method can be used just like a normal ABAP method.

Implementation in ABAP
A method is specified as a kernel method in the implementation part of the class using the optional addition BY KERNEL MODULE kmod1 kmod2 ... of the statement METHOD. kmod1, kmod2, ... are the names of kernel modules that implement the method. The ABAP implementation of a kernel method must be empty, which means that there cannot be any ABAP statements between METHOD and ENDMETHOD: METHOD meth BY KERNEL MODULE kmod1 kmod2 ...
ENDMETHOD.
Constructors and the C Destructor cannot be implemented as kernel methods. There is a separate mechanism for the C Destructor.
After ABAP_KEY KERNEL MODULE, a list of kernel modules kmod1 , kmod2, ... can be specified. Currently, only C functions of the kernel can be specified for kmod1, kmod2, ... The list after KERNEL MODULE is evaluated by the compiler from left to right. The first kernel module in the list that is registered in the kernel (see below) is used in the generation.
If no valid kernel module is found in the list, a syntax error occurs. There are still two standard C functions that can appear at the end of the list: FAIL and IGNORE. If one of these functions is specified at the end of the list, then a syntax error is avoided if the previous list does not contain a valid module. IGNORE is used to ignore the call of a kernel method of this type (behavior as in an empty ABAP implementation) and, in the case of FAIL, a handleable exception of the class CX_SY_DYN_CALL_ILLEGAL_METHOD is raised.

Examples METHOD meth BY KERNEL MODULE xx_impl_630 xx_impl_620 xx_impl_610.
First, the kernel is searched for xx_impl_630. The kernel is then searched for xx_impl_620, and finally for xx_impl_610. If none of these functions are found, a syntax error is raised. METHOD meth BY KERNEL MODULE xx_impl_630 xx_impl_620 FAIL.
First, the kernel is searched for xx_impl_630. Then the kernel is searched for xx_impl_620. If neither of these functions are found, a syntax error is not raised; a handleable exception of the class CX_SY_DYN_CALL_ILLEGAL_METHOD is raised instead, when the method is called. METHOD meth BY KERNEL MODULE xx_impl_620 xx_impl_610 IGNORE.
First, the kernel is searched for xx_impl_620. Then the kernel is searched for xx_impl_610. If none of the functions are found, a syntax error is not raised; the empty ABAP implementation is called instead, when the method is called.

Implementation in the Kernel
Currently, only C functions can be used as kernel modules of kernel methods. The C functions can have any position in the kernel. No special includes from the ABAP runtime environment are required for implementing the C function. The C functions must have a specific interface. The interface itself is wrapped by a macro called ARGUMENTS. All required definitions and prototypes are in the include //src/include/abkmeth.h. This is the only include needed for defining C functions for kernel methods.
Since C functions can be defined in C and C++ , externC must be used in C++: #include 'abkmeth.h'
...
externC void name_of_cmodule( ARGUMENTS )
{
...
}
A C function that implements a kernel method must be registered for the kernel method. If, after METHOD meth BY KERNEL MODULE, the name of a C function is specified that was not registered for the kernel method, a syntax error occurs (as mentioned above). Multiple C functions can be registered for one kernel method. The sequence of the kernel modules kmod1, kmod2, ... specified in the list after METHOD meth BY KERNEL MODULE defines which of the registered C functions is used. This allows downward-compatible further development of kernel methods.
To make changes to the registration active, the destination lib of the project krn/runt must be recompiled and the kernel relinked.

Registration
C functions are registered in the signature file //src/krn/runt/abkmeth.sig using the following syntax for kernel methods (all ABAP IDs must be specified in uppercase letters): KERNEL_METHOD('CLASS','METH', cfunc,argcnt)
This definition registers the C function cfunc for the kernel method meth of a global class class. The C function expects a number of argcnt arguments.
Kernel methods of local classes in class pools or other ABAP programs are registered using the following macros: KERNEL_METHOD_CLASS_LOCAL('GCLASS','CLASS','METH',cmodule,argcnt) KERNEL_METHOD_PROGRAM_LOCAL('PROG','CLASS','METH',cmodule,argcnt)
The technique is the same as with KERNEL_METHOD, except that the global class gclass must be specified for local classes in class pools and the program prog for program-local classes.

Registering Arguments
All ABAP data objects (such as parameters, attributes, or global data) that are to be accessed in C functions for kernel methods, are treated as arguments of the C function.
The argument list of a C function for a kernel method is not limited to the interface parameters of the ABAP method and does not have to contain these completely. Before arguments within C functions are accessed for kernel methods, these arguments must be registered.
The argcnt arguments must be registered immediately after the C function is registered using KERNEL_METHOD. A single argument is defined (registered) using one of the following macros: ARGUMENT_basetype(index,'name',type_kind,'type',read_write) ARGUMENT_[C|N|X](index,'name',type_kind,'type',read_write,length) ARGUMENT_P(index,'name',type_kind,'type',read_write,length,decimals) ARGUMENT_STRUCT(index,'name',type_kind,'type',read_write,ctype)
These macros define an argument with the name name and an index index.
basetype must be used to assign the type of the ABAP data object in accordance with the following table. If the basetype is C , N, X, P, or STRUCT, more parameters must be specified than for other types. basetypeABAP Data TypeType in C Cc with specified lengthSAP_CHAR (*) [Length] C_GENERICc without specified lengthSAP_CHAR* Xx with specified lengthSAP_RAW (*) [Length] X_GENERICx without specified lengthSAP_RAW* Nn with specified lengthSAP_CHAR (*) [Length] N_GENERICn without specified lengthSAP_CHAR* Pp with specified length and decimalsSAP_BCD (*) [Length] P_GENERICp without specified length and decimals SAP_BCD* DdSAP_DATE* TtSAP_TIME* INT1bSAP_INT1* INT2sSAP_SHORT*
INT8sSAP_LLONG* IiSAP_INT* FfSAP_DOUBLE* DECFLOAT16decfloat16DecFloat16 DECFLOAT34decfloat34DecFloat34 STRINGstringStrRef* XSTRINGxstringStrRef* TABLEAll table typesTABH_REF* OBJ_REFAll object referencesObjRef* DATA_REFAll data referencesFldRef* STRUCTAll structure typesRegistered type ctype* ANYanyvoid* DATAdatavoid* SIMPLEsimplevoid* CSEQUENCEcsequencevoid* XSEQUENCExsequencevoid* NUMERICnumericvoid* CLIKEclikeSAP_CHAR* C_POINTER%_c_pointervoid**
The macro parameters have the following meanings:
name is the ID for any ABAP data object in uppercase letters that could also be used in an ABAP implementation of the kernel method. In particular, the ID can contain links with component selectors, for example me->attr or struc-comp.
index is a sequential number from 1 to argcnt. The arguments are accessed using this index.
For type_kind, either TYPE or TYPE_REF_TO can be specified.
type is the ID for any ABAP data type in uppercase letters that could also be used in an ABAP implementation of the kernel method. type_kind and type are used to check the interface of the kernel method in ABAP.
For read_write, either READ or WRITE can be specified. This defines whether reads or writes can be performed on the argument and this setting is evaluated in the access macros.
length is used to specify the length of all ABAP data types with a generic length for ARGUMENT_[C|N|X|P]. In characters for c and n and in bytes for x and p.
When using ARGUMENT_P, decimals must be used to specify the number of decimal places.
When using ARGUMENT_STRUCT, ctype must be used to specify a suitable C type. This type should be generated from an ABAP type definition using saphfile.

Accessing Arguments
After registering the arguments, the following macros can be used to access them within the C function. With the exception of the direct access to the data control block, the access macros do not require any includes from the ABAP runtime environment. ARGUMENT_basetype_READ(index,'name');
This macro returns the read address of an argument with the type const ctype, where ctype is defined by basetype according to the above table. The index and name of the argument must be passed. Additional parameters must be specified for the generic types (see below). Only the index is needed to access the argument. However, to make the C function more legible and to ensure that additional consistency checks can be executed, the name must also be specified. If the kernel is compiled in debugging mode, the system executes a consistency check between index and name; the specified C type and ABAP type of the argument are also checked. In the case of an error, an appropriate ABAP runtime error is triggered ( KMETH_INVALID_ARGUMENT_ID, KMETH_INVALID_ARGUMENT_NAME, or KMETH_INVALID_CTYPE_LENG). No checks are made in the optimized kernel. ARGUMENT_basetype_WRITE(index,'name');
This macro has the same semantics as ARGUMENT_basetype_READ. However, the system returns the write address. The system also checks whether the argument was defined as a write argument. If a write is performed on a read-only argument (for example, a constant), this triggers the ABAP runtime error KMETH_ARGUMENT_READ_ONLY. ARGUMENT_[C|N]_READ(index,'name',lengthU);
ARGUMENT_[C|N]_WRITE(index,'name',lengthU);
ARGUMENT_X_READ(index,'name',lengthR);
ARGUMENT_X_WRITE(index,'name',lengthR);
If these macros are used, the expected length in bytes lengthR or in characters lengthU must be specified for the generic types c, x, and n. ARGUMENT_P_READ(index,'name',lengthR,decimals);
ARGUMENT_P_WRITE(index,'name',lengthR,decimals);
If these macros are used, the expected length in bytes (lengthR) and the number of decimal places (decimals) must be specified for the generic type p. ARGUMENT_[C_GENERIC|N_GENERIC|CLIKE]_READ(index,'name',size_tU);
ARGUMENT_[C_GENERIC|N_GENERIC|CLIKE]_WRITE(index,'name',size_tU);
ARGUMENT_X_GENRIC_READ(index,'name',size_tR);
ARGUMENT_X_GENERIC_WRITE(index,'name',size_tR);
When using these macros, a variable of the type size_tU or size_tR, containing the length in bytes or characters, must be specified for the types C_GENERIC, X_GENERIC, N_GENERIC, and CLIKE. ARGUMENT_P_GENERIC_READ(index,'name',size_tR,decimals);
ARGUMENT_P_GENERIC_WRITE(index,'name',size_tR,decimals);
If these macros are used, a variable decimals (for the decimal places) as well as the length size_tR must be specified for the type P_GENERIC. ARGUMENT_STRUCT_READ(index,'name',ctype);
ARGUMENT_STRUCT_READ(index,'name',ctype);
If these macros are used, a suitable C typectype must be specified for all structured types STRUCT. ARGUMENT_C_POINTER(index,'name');
This macro is available specifically for the type %_c_pointer. This type is a special internal ABAP type that has exactly the byte length of a C pointer (4, 8, or 16 bytes, depending on platform). The type is always mapped to the predefined ABAP type x. The macros for the type X or X_GENERIC are not used due to the variable length and platform-dependency. ARGUMENT_IS_SUPPLIED(index,'name');
This macro has the same semantics as the predicate expression IS SUPPLIED in ABAP. The same consistency checks are executed as for ARGUMENT_READ. ARGUMENT_DATA(index,'name',ctype);
This macro returns the data control block with the C type const DATA *. The same consistency checks are executed as for ARGUMENT_READ. The macro is only active if the include //src/include/abdata.h of the ABAP runtime environment was included.

Raising Exceptions
C functions that implement kernel method can raise class-based exceptions.

Registering Exceptions
The relevant global exception classes must be registered with an extension of //src/include/abexcpc.h. Local exception classes cannot be registered.
The exception class is declared in //src/include/abexcpc.h and any text IDs are defined: //src/include/abexcpc.h
...
CX_ABSTR (CX_..., 'CX_...')
CX_TXTID (CX_..._bar, CX_..., 'BAR') /* special text for class */
...
Classes can also be declared with their standard text only: //src/include/abexcpc.h
...
CX_CLASS (CX_..., 'CX_...') /* class with standard text */
...
The exact documentation is in the file //src/include/abexcpc.h.
The file //src/include/abexcpa.h must be extended so that any attributes of an exception class in a C function can be populated. The name, internal type (according to //src/include/abtypes.h), and the byte length must be specified: //src/include/abexcpa.h
...
CX_ATTR (CX_..._attr1, CX_..., 'ATTR1', TYPCSTRING, sizeofR(StrRef))
CX_ATTR (CX_..._attr2, CX_..., 'ATTR2', TYPC, LEN_UC2RAW(30))
...
Finally, both exceptions and arguments must be registered in the file //src/krn/runt/abkmeth.sig. This is not forced but, during the syntax check, only registered exceptions are checked for their existence: //src/krn/runt/abkmeth.sig
...
EXCEPTION(CX_...)
...

Raising Exceptions
A C function can raise an exception by calling the following macros consecutively: EXCEPTION_CREATE(CX_..._bar);
EXCEPTION_SET_CSTRING(CX_..._attr1, value, valueLength);
EXCEPTION_SET_C (CX_..._attr2, value, valueLength);
EXCEPTION_RAISE();
Within the macros EXCEPTION_CREATE or EXCEPTION_RAISE, a long jump to Extri always takes place, which means that the C function that implements the kernel method is exited in a long jump and the ABAP runtime environment takes control. Therefore, the C function should release its temporary memory before raising an exception. If the exception is caught in ABAP using CATCH without the INTO addition, the long jump takes place in EXCEPTION_CREATE. If the exception is caught with the INTO addition (the exception object is used) or not at all, the long jump takes place in EXCEPTION_RAISE.
The exceptions are processed in the runtime environment, as if they were raised in ABAP and the same dynamic checks are executed.
Currently, the following macros, which can be extended if necessary, are available for setting exception attributes. Strings, integer and C fields are supported. See the above sequence for use. EXCEPTION_SET_CSTRING_UC
EXCEPTION_SET_C
Value with length specified EXCEPTION_SET_C_UC
EXCEPTION_SET_INT
Value with null termination

Auxiliary Program for Kernel Methods
The ABAP program RSKMETH serves as a browser for the registration of kernel modules. This can be used to ascertain which C functions are registered for which kernel methods and which arguments/exceptions are registered for these functions. This is helpful when analyzing syntax errors, since kernel methods process information that only exists in the kernel modules.

Example
The following example is a simplified calculation class for floating decimal place numbers. The class has an instance attribute in which the last result of each calculation is stored. A method executes a division and is implemented as a kernel method. If the divisor is zero, the method triggers a class-based exception.

Declaration Section of the Class in ABAP CLASS cl_my_calculation DEFINITION ...
...
DATA last_result TYPE decfloat16.
...
METHODS div
IMPORTING p_dividend TYPE decfloat16 p_divisor TYPE decfloat16
RETURNING VALUE(p_result) TYPE decfloat16.
...
ENDCLASS.

Signature File //src/krn/runt/abkmeth.sig in the kernel ...
KERNEL_METHOD(CL_MY_CALCULATION, DIV, xx_myDiv,4)
ARGUMENT_F(1, 'P_DIVIDEND', TYPE, 'F', READ)
ARGUMENT_F(2, 'P_DIVISOR', TYPE, 'F', READ)
ARGUMENT_F(3, 'P_RESULT', TYPE, 'F', WRITE)
ARGUMENT_F(4, 'ME->LAST_RESULT',TYPE, 'F', WRITE)
EXCEPTION('CX_MY_DIV_BY_ZERO')
...

C++ source code //src/krn/.../mycalc.cpp in the kernel #include 'abkmeth.h'
...
externC void xx_myDiv( ARGUMENTS ){

const SAP_DOUBLE *const dividend = ARGUMENT_F_READ(1,'P_DIVIDEND');
const SAP_DOUBLE *const divisor = ARGUMENT_F_READ(2,'P_DIVISOR');
SAP_DOUBLE *result = ARGUMENT_F_WRITE(3,'P_RESULT');
SAP_DOUBLE *last_result = ARGUMENT_F_WRITE(4 ,'ME->LAST_RESULT');

if( 0 == *divisor )
{
EXCEPTION_CREATE(CX_MY_DIV_BY_ZERO);
EXCEPTION_RAISE();
}

*result = *dividend / *divisor;
*last_result = *result;

}

Implementation Section of the Class in ABAP CLASS cl_my_calculation IMPLEMENTATION.
...
METHOD div BY KERNEL MODULE xx_myDiv.
ENDMETHOD.
...
ENDCLASS.