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.
basetype ABAP Data Type Type in C
C c with specified length SAP_CHAR (*) [Length]
C_GENERIC c without specified length SAP_CHAR*
X x with specified length SAP_RAW (*) [Length]
X_GENERIC x without specified length SAP_RAW*
N n with specified length SAP_CHAR (*) [Length]
N_GENERIC n without specified length SAP_CHAR*
P p with specified length and decimals SAP_BCD (*)
[Length]
P_GENERIC p without specified length and decimals
SAP_BCD*
D d SAP_DATE*
T t SAP_TIME*
INT1 b SAP_INT1*
INT2 s SAP_SHORT*
INT8 s SAP_LLONG*
I i SAP_INT*
F f SAP_DOUBLE*
DECFLOAT16 decfloat16 DecFloat16
DECFLOAT34 decfloat34 DecFloat34
STRING string StrRef*
XSTRING xstring StrRef*
TABLE All table types TABH_REF*
OBJ_REF All object references ObjRef*
DATA_REF All data references FldRef*
STRUCT All structure typesRegistered type ctype*
ANY any void*
DATA data void*
SIMPLE simple void*
CSEQUENCE csequence void*
XSEQUENCE xsequence void*
NUMERIC numeric void*
CLIKE clike SAP_CHAR*
C_POINTER %_c_pointer void**
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 type ctype 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.
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