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The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
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Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
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the License for the specific language governing rights and limitations
under the License.
IDL to C mapping
2002-08-06
PB1
ch_c_mapping.xml
Introduction
The IC C mapping (used by the C client and C server back-ends) follows
the OMG C Language Mapping Specification.
The C mapping supports the following:
-
All OMG IDL basic types except long double and any.
-
All OMG IDL constructed types.
-
OMG IDL constants.
-
Operations with passing of parameters and receiving of
results. inout parameters are not supported.
The following is not supported:
-
Access to attributes.
-
User defined exceptions.
-
User defined objects.
C Mapping Characteristics
Reserved Names
The IDL compiler reserves all identifiers starting with
OE_ and oe_ for internal use.
Scoped Names
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.
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 underscores in
identifiers.
Generated Files
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:
.c]]>.
For example:
lseq;
interface i1 {
...
};
...
};
]]>
XXX This is C client specific.
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.
Basic OMG IDL Types
The mapping of basic types is as follows.
| OMG IDL type |
C type |
Mapped to C type |
| float |
CORBA_float |
float |
| double |
CORBA_double |
double |
| short |
CORBA_short |
short |
| unsigned short |
CORBA_unsigned_short |
unsigned short |
| long |
CORBA_long |
long |
| long long |
CORBA_long_long |
long |
| unsigned long |
CORBA_unsigned_long |
unsigned long |
| unsigned long long |
CORBA_unsigned_long_long |
unsigned long |
| char |
CORBA_char |
char |
| wchar |
CORBA_wchar |
unsigned long |
| boolean |
CORBA_boolean |
unsigned char |
| octet |
CORBA_octet |
char |
| any |
Not supported |
|
| long double |
Not supported |
|
| Object |
Not supported |
|
| void |
void |
void |
OMG IDL Basic Types
XXX Note that several mappings are not according to OMG C Language
mapping.
Constructed OMG IDL Types
Constructed types have mappings as shown in the following table.
| OMG IDL type |
Mapped to C type |
| string |
CORBA_char* |
| wstring |
CORBA_wchar* |
| struct |
struct |
| union |
union |
| enum |
enum |
| sequence |
struct (see below) |
| array |
array |
OMG IDL Constructed Types
An OMG IDL sequence (an array of variable length),
NAME;
]]>
is mapped to a C struct as follows:
/* C */
typedef struct {
CORBA_unsigned_long _maximum;
CORBA_unsigned_long _length;
C_TYPE* _buffer;
} C_NAME;
where C_TYPE is the mapping of IDL_TYPE, and where
C_NAME is the scoped name of NAME.
OMG IDL Constants
An IDL constant is mapped to a C constant through a C
#define macro, where the name of the macro is scoped.
Example:
// IDL
module M1 {
const long c1 = 99;
};
results in the following:
/* C */
#define M1_c1 99
OMG IDL Operations
An OMG IDL operation is mapped to C function. Each C operation
function has two mandatory parameters: a first parameter of
interface object type, and a last parameter of
environment type.
In a C operation function the the in and out
parameters are located between the first and last parameters
described above, and they appear in the same order as in the IDL
operation declaration.
Notice that inout parameters are not supported.
The return value of an OMG IDL operation is mapped to a
corresponding return value of the C operation function.
Mandatory C operation function parameters:
- CORBA_Object oe_obj - the first parameter of a C
operation function. This parameter is required by the OMG C Language Mapping Specification, but in the current
implementation there is no particular use for it.
-
CORBA_Environment* oe_env - the last parameter of a C
operation function. The parameter is defined in the C header
file ic.h and has the following public fields:
-
CORBA_Exception_type _major - indicates if an
operation invocation was successful which will be one of
the following:
- CORBA_NO_EXCEPTION
- CORBA_SYSTEM_EXCEPTION
- int _fd - a file descriptor returned from
erl_connect function.
- int _inbufsz - size of input buffer.
- char* _inbuf - pointer to a buffer used for
input.
- int _outbufsz - size of output buffer.
- char* _outbuf - pointer to a buffer used for
output.
-
int _memchunk - expansion unit size for the
output buffer. This is the size of memory chunks in
bytes used for increasing the output in case of buffer
expansion. The value of this field must be always set
to >= 32, should be at least 1024 for performance
reasons.
- char regname[256] - a registered name for a
process.
- erlang_pid* _to_pid - an Erlang process
identifier, is only used if the registered_name parameter
is the empty string.
- erlang_pid* _from_pid - your own process id so
the answer can be returned
Beside the public fields, other private fields
are internally used but are not mentioned here.
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)
{
...
}
Operation Implementation
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:
- Argument mappings
- Result values
- Structure
- Usage
- Exception handling
Exceptions
Although 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.
Access to Attributes
Not Supported
Summary of Argument/Result Passing for the C-client
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* |
Basic Argument and Result passing
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 |
Client argument storage responsibility
| 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. |
Argument passing cases
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.
Supported Memory Allocation Functions
-
CORBA_Environment can be allocated from the user by calling
CORBA_Environment_alloc().
The interface for this function is
CORBA_Environment *CORBA_Environment_alloc(int inbufsz, int outbufsz);
where :
-
inbufsz is the desired size of input buffer
-
outbufsz is the desired size of output buffer
-
return value is a pointer to an allocated and initialized
CORBA_Environment structure
-
Strings can be allocated from the user by calling CORBA_string_alloc().
The interface for this function is
CORBA_char *CORBA_string_alloc(CORBA_unsigned_long len);
where :
-
len is the length of the string to be allocated.
Thus far, no other type allocation function is supported.
Special Memory Deallocation Functions
-
void CORBA_free(void *storage)
This function will free storage allocated by the stub.
-
void CORBA_exception_free(CORBA_environment *ev)
This function will free storage allocated under exception propagation.
Exception Access Functions
-
CORBA_char *CORBA_exception_id(CORBA_Environment *ev)
This function will return raised exception identity.
-
void *CORBA_exception_value(CORBA_Environment *ev)
This function will return the value of a raised exception.
Special Types
-
The erlang binary type has some special features.
While the erlang::binary idl type has the same C-definition as
a generated sequence of octets :
binary;
};
]]>
it provides a way on sending trasparent data between C and Erlang.
The C-definition (ic.h) for an erlang binary is :
typedef struct {
CORBA_unsigned_long _maximum;
CORBA_unsigned_long _length;
CORBA_octet* _buffer;
} erlang_binary; /* ERLANG BINARY */
The differences (between erlang::binary and ]]>) are :
-
on the erlang side the user is sending/receiving typical
built in erlang binaries, using term_to_binary() / binary_to_term()
to create / extract binary structures.
-
no encoding/decoding functions are generated
-
the underlying protocol is more efficient than usual sequences of
octets
The erlang binary IDL type is defined in erlang.idl, while its
C definition is located in the ic.h header file, both in the
/include]]> directory.
The user will have to include the file erlang.idl in order to use the
erlang::binary type.
A Mapping Example
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.