This chapter describes the mapping of OMG IDL constructs to the C programming language for the generation of native C - Erlang communication.
This language mapping defines the following:
The IDL compiler reserves all identifiers starting with
OE_
and oe_
for internal use.
The C programmer must always use the global name for a type, constant
or operation. The C global name corresponding to an OMG IDL global
name is derived by converting occurrences of "::" to underscore,
and eliminating the leading "::". So, for example, an operation op1
defined in interface I1
which is defined in module
M1
would be written as M1::I1::op1
in IDL and as
M1_I1_op1
in C.
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If underscores are used in IDL names it can lead to ambiguities due to the name mapping described above, therefore it is advisable to avoid the use of underscores in identifiers. |
Two files will be generated for each scope. One set of files
will be generated for each module and each interface scope.
An extra set is generated for those definitions at top
level scope. One of the files is a header file(.h
), and the
other file is a C source code file (.c
). In addition to these
files a number of C source files will be generated for type encodings,
they are named according to the following template:
oe_code_<type>.c
.
For example:
// IDL, in the file "spec.idl" module m1 { typedef sequence<long> lseq; interface i1 { ... }; ... };
Will produce the files oe_spec.h
and
oe_spec.c
for the top scope level. Then the files
m1.h
and m1.c
for the module m1
and
files m1_i1.h
and m1_i1.c
for the interface
i1
. The typedef will produce oe_code_m1_lseq.c
.
The header file contains type definitions for all
struct
types and sequences and constants in the IDL file. The
c file contains all operation stubs if the the scope is an interface.
In addition to the scope-related files a C source file will
be generated for encoding operations of all struct
and
sequence types.
The mapping of basic types is flexible to allow type adjustment. This can be used when porting to machines with different architectures.
OMG IDL type | C type | Implementation | Adjustable |
float | CORBA_float | float | yes |
double | CORBA_double | double | yes |
short | CORBA_short | short | yes |
unsigned short | CORBA_unsigned_short | unsigned short | yes |
long | CORBA_long | long | yes |
long long | CORBA_long_long | long | yes |
unsigned long | CORBA_unsigned_long | unsigned long | yes |
unsigned long long | CORBA_unsigned_long_long | unsigned long | yes |
char | CORBA_char | char | yes |
wchar | CORBA_wchar | unsigned long | yes |
boolean | CORBA_boolean | unsigned char | yes |
octet | CORBA_octet | char | yes |
any | Not supported | ||
long double | Not supported | ||
Object | Not supported | ||
void | void | void | no |
Constructed types all have native mappings as shown in the list below.
OMG IDL strings are mapped to C CORBA_char*.
OMG IDL wstrings are mapped to C CORBA_wchar*.
An OMG IDL structure is mapped directly onto a C struct.
An OMG IDL union is mapped directly onto a C discriminated union.
An OMG IDL enum is directly mapped onto a C enum.
OMG IDL sequences are mapped to a C struct that represents the sequence.
Consider the following IDL declaration:
typedef sequence<long> lseq;
Which in C is represented as:
typedef struct { CORBA_unsigned_long _maximum; CORBA_unsigned_long _length; CORBA_long* _buffer; } lseq;
OMG IDL arrays are mapped directly to C arrays.
Constants are mapped to C #define
.
For example:
// IDL module M1 { const long c1 = 99; };
Would result in the following define:
#define M1_c1 99
Operation invocation is achieved through a function call. The function calls have two default parameters, the interface object and the environment parameter. The result of the function is returned the usual way, while in and out parameters lie between the two default parameters in the same order as they appear in the IDL file.
Default parameters:
Example:
// IDL interface i1 { long op1(in long a); long op2(in string s, out long count); };
Is mapped to the following C functions
// C CORBA_long i1_op1(i1 oe_obj, CORBA_long a, CORBA_Environment* oe_env) { ... } CORBA_long i1_op2(i1 oe_obj, CORBA_char* s, CORBA_long *count, CORBA_Environment* oe_env) { ... }
There is no standard CORBA mapping for the C-server side, as it is implementation-dependent but built in a similar way. The current server side mapping is different from the client side mapping in several ways:
While exception mapping is not implemented, the stubs will generate CORBA system exceptions in case of operation failure. Thus, the only exceptions propagated by the system are built in system exceptions.
The user-defined parameters can only be in
or out
parameters, as
inout
parameters are not supported.
This table summarize the types a client passes as arguments to a stub, and receives as a result.
OMG IDL type | In | Out | Return |
short | CORBA_short | CORBA_short* | CORBA_short |
long | CORBA_long | CORBA_long* | CORBA_long |
long long | CORBA_long_long | CORBA_long_long* | CORBA_long_long |
unsigned short | CORBA_unsigned_short | CORBA_unsigned_short* | CORBA_unsigned_short |
unsigned long | CORBA_unsigned_long | CORBA_unsigned_long* | CORBA_unsigned_long |
unsigned long long | CORBA_unsigned_long_long | CORBA_unsigned_long_long* | CORBA_unsigned_long_long |
float | CORBA_float | CORBA_float* | CORBA_float |
double | CORBA_double | CORBA_double* | CORBA_double |
boolean | CORBA_boolean | CORBA_boolean* | CORBA_boolean |
char | CORBA_char | CORBA_char* | CORBA_char |
wchar | CORBA_wchar | CORBA_wchar* | CORBA_wchar |
octet | CORBA_octet | CORBA_octet* | CORBA_octet |
enum | CORBA_enum | CORBA_enum* | CORBA_enum |
struct, fixed | struct* | struct* | struct |
struct, variable | struct* | struct** | struct* |
union, fixed | union* | union* | union |
union, variable | union* | union** | union* |
string | CORBA_char* | CORBA_char** | CORBA_char* |
wstring | CORBA_wchar* | CORBA_wchar** | CORBA_wchar* |
sequence | sequence* | sequence** | sequence* |
array, fixed | array | array | array_slice* |
array, variable | array | array_slice** | array_slice* |
A client is responsible for providing storage of all arguments passed as in arguments.
OMG IDL type | Out | Return |
short | 1 | 1 |
long | 1 | 1 |
long long | 1 | 1 |
unsigned short | 1 | 1 |
unsigned long | 1 | 1 |
unsigned long long | 1 | 1 |
float | 1 | 1 |
double | 1 | 1 |
boolean | 1 | 1 |
char | 1 | 1 |
wchar | 1 | 1 |
octet | 1 | 1 |
enum | 1 | 1 |
struct, fixed | 1 | 1 |
struct, variable | 2 | 2 |
string | 2 | 2 |
wstring | 2 | 2 |
sequence | 2 | 2 |
array, fixed | 1 | 3 |
array, variable | 3 | 3 |
Case | Description |
1 | Caller allocates all necessary storage, except that which may be encapsulated and managed within the parameter itself. |
2 | The caller allocates a pointer and passes it by reference to the callee. The callee sets the pointer to point to a valid instance of the parameter's type. The caller is responsible for releasing the returned storage. Following completion of a request, the caller is not allowed to modify any values in the returned storage. To do so the caller must first copy the returned instance into a new instance, then modify the new instance. |
3 | The caller allocates a pointer to an array slice which has all the same dimensions of the original array except the first, and passes it by reference to the callee. The callee sets the pointer to point to a valid instance of the array. The caller is responsible for releasing the returned storage. Following completion of a request, the caller is not allowed to modify any values in the returned storage. To do so the caller must first copy the returned instance into a new instance, then modify the new instance. |
The returned storage in case 2 and 3 is allocated as one block of memory so it is possible to deallocate it with one call of CORBA_free.
CORBA_Environment *CORBA_Environment_alloc(int inbufsz, int outbufsz);
where :CORBA_char *CORBA_string_alloc(CORBA_unsigned_long len);
Thus far, no other type allocation function is supported.
void CORBA_free(void *storage)
void CORBA_exception_free(CORBA_environment *ev)
CORBA_char *CORBA_exception_id(CORBA_Environment *ev)
void *CORBA_exception_value(CORBA_Environment *ev)
erlang::binary
idl type has the same C-definition as
a generated sequence of octets :
module erlang { .... // an erlang binary typedef sequence<octet> binary; };it provides a way on sending trasparent data between C and Erlang.
typedef struct { CORBA_unsigned_long _maximum; CORBA_unsigned_long _length; CORBA_octet* _buffer; } erlang_binary; /* ERLANG BINARY */The differences (between
erlang::binary
and sequence< octet >
) are :
term_to_binary() / binary_to_term()
to create / extract binary structures.
erlang.idl
, while it's
C definition is located in the ic.h
header file, both in the
IC-< vsn >/include
directory.
The user will have to include the file erlang.idl
in order to use the
erlang::binary
type.
This is a small example of a simple stack. There are two operations on the stack, push and pop. The example shows all generated files as well as conceptual usage of the stack.
// The source IDL file: stack.idl struct s { long l; string s; }; interface stack { void push(in s val); s pop(); };
When this file is compiled it produces four files, two for the top scope and two for the stack interface scope. The important parts of the generated C code for the stack API is shown below.
stack.c
void push(stack oe_obj, s val, CORBA_Environment* oe_env) { ... } s* pop(stack oe_obj, CORBA_Environment* oe_env) { ... }
oe_stack.h
#ifndef OE_STACK_H #define OE_STACK_H /*------------------------------------------------------------ * Struct definition: s */ typedef struct { long l; char *s; } s; #endif
stack.h just contains an include statement of oe_stack.h
.
oe_code_s.c
int oe_sizecalc_s(CORBA_Environment *oe_env, int* oe_size_count_index, int* oe_size) { ... } int oe_encode_s(CORBA_Environment *oe_env, s* oe_rec) { ... } int oe_decode_s(CORBA_Environment *oe_env, char *oe_first, int* oe_outindex, s *oe_out) { ... }
The only files that are really important are the .h
files and the stack.c file.