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# Using ILU with ANSI C

## Introduction

This document is for the ANSI C programmer who wishes to use ILU. The following sections will show how ILU is mapped into ANSI C constructs and how both ANSI C clients and servers are generated and built.

Using ILU with ANSI C is intended to be compatible with the OMG CORBA specification. That is, all of the naming and stub generation comply with the Common Object Request Broker Architecture, revision 2.0. (9)

Note that ILU does not support non-ANSI variants of the C language. In particular, it relies on having prototypes, all ANSI C library functions, and the capabilities of the ANSI C pre-processor.

When functions are described in this section, they are sometimes accompanied by locking comments, which describe the locking invariants maintained by ILU on a threaded system. See the file ``ILUHOME/include/iluxport.h'` for more information on this locking scheme, and the types of locking comments used.

A number of macros are used in function descriptions, to indicated optional arguments, and ownership of potentially malloc'ed objects. The macro `OPTIONAL(type)` means that the value is either of the type indicated by type, or the value `NULL`. This macro may only be used with pointer types. The macro `RETAIN(type)` indicates, when used on a parameter, that the caller retains ownership of the value, and when used in the result position, that the called function retains ownership of the value. The macro `PASS(type)` indicates, when used on a parameter, that the caller is passing ownership of the storage to the called function, and when used in the result position, that the called function is passing ownership of the called value to the caller. The macro `GLOBAL(type)` means that neither the caller nor the calling function owns the storage.

## The ISL Mapping to ANSI C

### Names

In general, ILU constructs ANSI C names from ISL names by replacing hyphens with underscores. Type names and class names are prepended with their interface name. For example, for the ISL type `T-1` in interface `I`, the generated name of the ANSI C type would be `I_T_1`.

Enumeration value names are formed by prepending the interface name and "_" to the ISL enumeration value name. Enumeration names and values are then cast into ANSI C `enum` statements.

Constant names are prepended with their interface name. They are implemented with the `const` declaration statements.

Method name prefixes are specified by CORBA to be `module-name_interface-name`. ANSI C function names for ISL methods are composed of the generated class name prepended to the method name. For example, if the interface name is `X` and the class type name is `Y` and the ISL method name is `Z` then the ANSI C callable method name will be `X_Y_Z`. ILU ANSI C servers for this method must implement a function called `server_X_Y_Z`.

For field names within records, hyphens are replaced with underscores.

### Mapping Type Constructs Into ANSI C

#### Basic Types

The following basic ISL types have the corresponding mappings in ANSI C, as specified by the CORBA 2.0 standard mapping for C:

- `BOOLEAN` => `CORBA_boolean`
- `BYTE` => `CORBA_octet`
- `CHARACTER` => `ilu_character`
- `SHORT CHARACTER` => `CORBA_char`
- `CARDINAL` => `CORBA_unsigned_long`
- `SHORT CARDINAL` => `CORBA_unsigned_short`
- `LONG CARDINAL` => `ilu_longcardinal`
- `INTEGER` => `CORBA_long`
- `SHORT INTEGER` => `CORBA_short`
- `LONG INTEGER` => `ilu_longinteger`
- `REAL` => `CORBA_double`
- `SHORT REAL` => `CORBA_float`
- `LONG REAL` => `ilu_longreal`

#### Records

Records map directly into corresponding ANSI C structures.

#### Unions

Because of the somewhat baroque CORBA specification, unions may take one of several forms.

Generally, ILU unions in ANSI C consist of a struct with two members: the type discriminator (a member named "_d"), and a union (a member named "_u") of the possible values. In a simple ISL union that does not name the elements, the union member names are derived from the ISL data types which compose the union. For example, if the ISL type in interface `I` is `TYPE u1 = UNION INTEGER, SHORT REAL END;` the generated ANSI C struct would be

    typedef struct _I_u1_union I_u1;
    enum I_u1_allowableTypes {
      I_u1_integer,
      I_u1_shortreal
    };
    struct _I_u1_union {
      enum I_u1_allowableTypes _d;
      union {
        ilu_integer integer;
        ilu_shortreal shortreal;
      } _u;
    };

Note the discriminator `_d` may take on the values of `I_u1_integer` or `u_u1_shortreal` indicating how to interpret the data in the union. Also note how the enumerated names are formed: with the interface name and the type name prepended to the enumeration element name.

In more complex union forms, the user may specify the type of the discriminator as well as the member names and which member corresponds to which discriminator value. Consider the following ISL example:

    INTERFACE I;
    TYPE e1 = ENUMERATION red, blue, green, yellow, orange END;
    TYPE u1 = e1 UNION
     a : INTEGER = red, green END,
     b : SHORT REAL = blue END,
     c : REAL
    END;

The generated union is:

    typedef struct _I_u1_union I_u1;
    typedef enum {
      I_red = 0,
      I_blue = 1,
      I_green = 2,
      I_yellow = 3,
      I_orange = 4
    } I_e1;
    struct _I_u1_union {
      I_e1 _d;
      union {
        ilu_integer a;
        ilu_shortreal b;
        ilu_real c;
      } _u;
    };

This example shows that the discriminator type is to be `I_e1` and that the member names are to be `a`, `b`, and `c`. When the discriminator has the value `I_red` or `I_green` the member `a` has a valid value and the type is interpreted to be integer. When the discriminator has the value `I_green` the member `b` has a valid value and the type is interpreted to be shortreal. If the discriminator has any other value, the member `c` is expected to have a valid value and the type is interpreted to be ilu_real (double).

Discriminator types may be `INTEGER`, `ENUMERATION`, or `SHORT INTEGER`. The default for an unspecified discriminator is `SHORT INTEGER`.

#### Floating Point Values

The ISL `SHORT REAL` primitive type maps to the ANSI C `float` data type while `REAL` maps to `double`. The ISL `LONG REAL` primitive type currently doesn't map to anything real.

#### Sequences

Sequence type names, as most type definitions, are formed with the interface name and the type name. Sequence instances are represented to the ANSI C programmer as a pointer to the sequence descriptor structure. For each sequence type declared in the interface description, a pseudo-object sequence type is defined in ANSI C. These sequence types will hold any number of values of type sequence's primary type. For the sequence
`INTERFACE I;`
`TYPE T2 = SEQUENCE OF T1;`
the following functions are defined:

[ANSI C]: I_T2 * I_T2_Create ( OPTIONAL(unsigned long) length, OPTIONAL(T1 *) initial-values )

This function creates and returns a pointer to a newly allocated instance of T2. If length is specified, but initial-values is not specified, enough space for length values of type T1 is allocated in the sequence. If initial-values is specified, length is assumed to be the number of values pointed to by initial-values, and must be specified. Note that if type T1 is a `character` or `short character` type, a pointer to a NIL-terminated sequence will be returned; otherwise, a normal CORBA sequence structure will be returned by reference.

[ANSI C]: CORBA_unsigned_long I_T2_Length ( I_T2 * s )

Returns the length of s.

[ANSI C]: void I_T2_Append ( I_T2 * s, T1 value )

Appends value to the end of s. This function will reallocate space and copy, if necessary.

[ANSI C]: void I_T2_Push ( I_T2 * s, T1 value )

Pushes value on to the beginning of the sequence. This function will reallocate space and copy, if necessary.

[ANSI C]: void I_T2_Pop ( I_T2 * s, T1 * value-ptr )

Removes the first value from the sequence s, and places it in the location pointed to by value-ptr.

[ANSI C]: void I_T2_Every ( I_T2 * s, void (*func)(T1, void *), void * data )

Calls the function func on each element of s in sequence, passing data as the second argument to func.

[ANSI C]: I_T1 * I_T2_Nth ( I_T2 * s, CORBA_unsigned_long n )

Returns the address of the nth element of the sequence s. Returns `ILU_NIL` if n is out of range.

[ANSI C]: void I_T2_Init ( I_T2 * s, OPTIONAL(CORBA_unsigned_long) length, OPTIONAL(T1 *) initial-values )

This function works like `T2_Create`, except that it takes a the address of an already-existing T2 to initialize. This can be used to initialize instances of T2 that have been stack-allocated.

[ANSI C]: void I_T2__Free ( I_T2 * s )

Frees allocated storage used internally by s. Does not free s itself.

String sequences (`SEQUENCE OF SHORT CHARACTER` or `SEQUENCE OF CHARACTER`) are just arrays of the character codes for the characters, using either Latin-1 codes (for `SEQUENCE OF SHORT CHARACTER`), or ISO 10646 Unicode codes (for `SEQUENCE OF CHARACTER`). These sequences are terminated with a character code of zero. The terminating code is not counted in the length of the sequence. All other sequence types have a record structure, mandated by CORBA:

    typedef struct I_T2 {
      unsigned long _maximum;
      unsigned long _length;
      long *_buffer;
    } I_T2;

The field `_maximum` contains the number of elements pointed to by `_buffer`. The field `_length` indicates the number of valid or useful elements pointed to by `_buffer`.

For example, the ISL specification

    INTERFACE I;

    TYPE iseq = SEQUENCE OF INTEGER;

would have in its ANSI C mapping the type

    typedef struct I_iseq {
      unsigned long _maximum;
      unsigned long _length;
      ilu_integer *_buffer;
    } I_iseq;

In a client program, a pointer to this type would be instantiated and initialized by calling the type specific sequence creation function generated for the sequence, e.g.

            ...
        I_O h;
        ILU_C_ENVIRONMENT s;
        I_iseq sq;
            ...
        sq = I_iseq_Create (0, NULL);
        I_iseq_Append (&sq, 4);
            ...

### Objects and Methods

As indicated earlier, method names are generated by prepending the interface name and the class name to the method name. The first argument to a method is an object instance. The object instance is an opaque pointer value returned from a class specific constructor function. All object types are subtypes for the type defined by `ILU_C_OBJECT`, a macro which expands to the appropriate CORBA object type for the version of CORBA being used. CORBA also specifies that the type of the handle be called `interface-name_type-name`. A typedef of the CORBA-specified name to the `ILU_C_OBJECT` type is therefore generated for each object type. In the example above, the type of the object instance would be `I_O`.

Two binding procedures are specified for each object type. A binding procedure is a procedure that takes some name for an object instance, and returns the actual instance. Users of a module typically use a surrogate-side binding procedure, which takes the string binding handle of the object, and the most specific type ID of the object's type (if known). Suppliers of a module typically bind objects with a creation procedure, which takes an instance ID, a server on which to maintain the object, and arbitrary user data, and creates and returns the true instance of the object.

In general, for any object type T, the following ANSI C functions are defined:

[ANSI C]: OPTIONAL(T) T__CreateTrue ( OPTIONAL(RETAIN(char *)) instance-id, OPTIONAL(GLOBAL(ilu_Server)) server, OPTIONAL(PASS(void *)) user-data )

Locking: Main Invariant holds

Creates a true instance of type T, exporting it with instance-id instance-id, exporting it via server server, associating the value user-data with it. If instance-id is not specified, a server-relative instance-id will be assigned automatically. If server is not specified, a default server will be created automatically.

[ANSI C]: OPTIONAL(T) T__OTCreateTrue ( RETAIN(char *) instance-id, GLOBAL(ilu_Server) server, OPTIONAL(PASS(void *)) user-data )

Locking: Inside(server, T)

Similar to `T__CreateTrue()`, but designed to be used within the `ot_object_of_ih` function of an object table (section Using C Object Tables). Requires kernel server locks to be held before invocation.

Creates a true instance of type T, exporting it with instance-id instance-id, exporting it via server server, associating the value user-data with it.

[ANSI C]: OPTIONAL(T) T__CreateFromSBH ( RETAIN(char *) sbh, RETAIN(CORBA_Environment *) Env)

Locking: Main Invariant holds

Finds or creates an instance of T, using the given object reference.

[ANSI C]: extern `ilu_Class` T__MSType

A value of type `ilu_Class` which identifies the most specific ILU type of the type T.

In the following example, the ILU definition is:

    INTERFACE I;

    TYPE T = OBJECT
      METHODS
        M ( r : REAL ) : INTEGER
      END;

This definition defines an interface `I`, an object type `T`, and a method `M`. The method `M` takes a `REAL` as an argument and returns an `INTEGER` result. The generated ANSI C header file would include the following statements:

    typedef ILU_C_OBJECT I_T;

    I_T I_T__CreateTrue (ilu_string ih, ilu_Server server, void *user_data);
    I_T I_T__CreateFromSBH (char *sbh, ILU_C_ENVIRONMENT *Env);

    ilu_integer I_T_M (I_T, ilu_real, ILU_C_ENVIRONMENT *);

The functions `I_T__CreateTrue` and `I_T__CreateFromSBH` are used to create instances of the class `I_T`. `I_T__CreateTrue` is used by servers while `I_T__CreateFromSBH` is used by clients. The pointer returned in each case is the object instance and must be passed with each method invocation.

In addition to its specified arguments, the method `I_T_M` takes an instance of the type `I_T` and a reference to a variable of type `ILU_C_ENVIRONMENT *`, which is a macro defined to be the appropriate CORBA environment type, and is used to return exception codes. The environment struct pointed to by the environment argument must be instantiated in a client; its address is passed as the last argument to each method. True procedures must expect a pointer to this structure as the last argument. Finally, the ANSI C client calling the method for `M` might be as follows:

    #include "I.h"

    int main (int ac, char **av)
    {
      double atof( );
      I_T inst;
      int xx;
      double f;
      ILU_C_ENVIRONMENT ev;

      I__Initialize( );
      f = atof (av[1]);
      inst = I_T__CreateFromSBH (av[2], &ev);
      if (!ILU_C_SUCCESSFUL(&ev)) {
        printf( "CreateFromSBH raised exception <%s>\n",
          ILU_C_EXCEPTION_ID(&ev));
        return(1);
      }
      xx = I_T_M (inst, f, &ev);
      if (!ILU_C_SUCCESSFUL(&ev)) {
        printf( "exception <%s> signalled on call to I_T_M\n",
          ILU_C_EXCEPTION_ID(&ev));
        return(2);
      }
      printf( "result is %d\n", xx );
      return(0);
    }

Note the call on the interface-specific client initialization procdedure `I__Initialize`; these are described in a later section.

In this example, the string binding handle is obtained from standard input along with some floating-point value.

The class specific function `I_T__CreateFromSBH` is called to obtain the object instance. This function was passed the string binding handle, and a CORBA environment in which to report exceptions. The returned object instance is then passed as the first argument to the method `I_T_M`, along with the environment `ev`, and the single actual `ilu_real` argument `f`. `I_T_M` returns an `ilu_integer` value which is placed in `xx`.

The true implementation of the method `M` might be as follows:

    ilu_integer server_I_T_M ( I_T h, ilu_real u, ILU_C_ENVIRONMENT *s )
    {
      return( (ilu_integer) (u + 1) );
    }

In this simple example, the corresponding server, or true, method computes some value to be returned. In this case it adds one to its `ilu_real` argument `u`, converts the value to an integer, and returns that value. Note that the server method, if not signalling any exceptions, may ignore the environment parameter.

#### Interface Inheritance

Through interface inheritance, an object type may participate in the behaviors of several different types that it inherits from. These types are called

ancestors of the object type. In ANSI C, an object type supplies all methods either defined directly on that type, or on any of its ancestor types.

Consider the following example:

    INTERFACE I2;

    EXCEPTION E1;

    TYPE T1 = OBJECT
      METHODS
        M1 (a : ilu.CString) : REAL RAISES E1 END
      END;

    TYPE T2 = OBJECT
      METHODS
        M2 ( a : INTEGER, Out b : INTEGER )
      END;

    TYPE T3 = OBJECT SUPERTYPES T1, T2 END
      METHODS
        M3 ( a : INTEGER )
      END;

The object type `T3` inherits from the object type `T2`. Thus, five ANSI C methods are relevant to the interface `I2`: `server_I2_T1_M1`, `server_I2_T2_M2`, `server_I2_T3_M1`, `server_I2_T3_M2`, and `server_I2_T3_M3`. A module that implements true instances of `T3` would have to define the last three true methods. A client uses only three generic functions: `I2_T1_M1`, `I2_T2_M2`, and `I2_T3_M3`.

#### Object Implementation

This information is provided for those interested in the implementation of the ANSI C object system. It is not guaranteed to remain the same from release to release.

Each object type is represented by a TypeVector, which is a vector of pointers to MethodBlock structs, one for each component type of the object type, ordered in the proper class precedence for that object type. Each MethodBlock struct contains a `ilu_Class` value, followed by a vector of pointers to the methods directly defined by that `ilu_Class`. There are two different TypeVectors for each object type, one for the surrogate class of the type, and the other for the true class of the type. The TypeVector for the surrogate class uses the MethodBlocks of its supertypes; the TypeVector for the true class uses its own MethodBlocks for both direct and inherited methods, as true classes in the ANSI C implementation override all of their methods. The TypeVectors, and MethodBlocks for true classes, are not exported; the MethodBlocks for surrogate classes are, as they are used by their subclasses.

For each method directly defined in the type, a generic function is defined in the common code for its interface, which dispatches to the appropriate method. It does this by walking down the TypeVector for the object, till it finds a MethodBlock which contains the appropriate ilu_Class on which this method is directly defined), then calling the method pointer which is indexed in the MethodBlock's vector of method pointers by the index of the method. The generic functions have the correct type signature for the method. They can be referenced with the `&` operator.

### Exceptions

ANSI C has no defined exception mechanism. As already indicated, exceptions are passed in ILU ANSI C by adding to the end of each method an additional status argument that can convey an exception code and a value of a type associated with that exception. To signal an exception, a method implementation sets the exception code and supplies the parameter value (if any).

An exception parameter is conveyed in the status argument as a C pointer; the parameter-conveying member is declared to be a `void *`. In particular, this pointer is a pointer to a value of the type that is the ANSI C mapping of the exception's ISL parameter. For an exception that has no parameter, the parameter-conveying member is not meaningful.

In the following example, the `div` method can raise the exception `DivideByZero`:

    INTERFACE calc;

    TYPE numerator = INTEGER;

    EXCEPTION DivideByZero : numerator;

    TYPE self = OBJECT
      METHODS
        Div( v1 : INTEGER, v2 : INTEGER ) : INTEGER RAISES DivideByZero END
      END;

The generated include file ``calc.h'` contains the exception definitions:

    #ifndef __calc_h_
    #define __calc_h_
    /*
    ** this file was automatically generated for C
    ** from the interface spec calc.isl.
    */

    #ifndef __ilu_c_h_
    #include "ilu-c.h"
    #endif

    extern ILU_C_ExceptionCode    _calc__Exception_DivideByZero;
    #define ex_calc_DivideByZero _calc__Exception_DivideByZero

    typedef ilu_integer calc_numerator;
    typedef calc_numerator calc_DivideByZero;

    typedef ILU_C_OBJECT calc_self;

    calc_self calc_self__CreateTrue ( char *id, ilu_Server server,
       void * user_data);
    calc_self calc_self__CreateFromSBH ( char * sbh, ILU_C_ENVIRONMENT *Env );

    ilu_integer calc_self_Div( calc_self, ilu_integer, ilu_integer,
       ILU_C_ENVIRONMENT *Env );

    extern void calc__BindExceptionValue (ILU_C_ENVIRONMENT *, ilu_Exception, ...);

    #endif

The method implementation for `Div` in the true module must detect the divide-by-zero condition and raise the exception:

    long server_calc_self_Div (calc_self h, ilu_integer u, ilu_integer v,
                               ILU_C_ENVIRONMENT *s)
    {
      calc_numerator n = 9;

      if ( v == 0 )
        {
          s->_major = ILU_C_USER_EXCEPTION;
          s->returnCode = ex_calc_DivideByZero;
          s->ptr = (void *) malloc(sizeof(calc_numerator));
          *((calc_numerator *) (s->ptr)) = n;
          s->freeRoutine = (void (*) (void *)) 0;
          return( u );
        }
      else
        return( u / v );
    }

When freeing the parameter requires more than just freeing `s->ptr`, a non-NULL `s->freeRoutine` is provided that does the additional freeing; `s->freeRoutine` is given one argument, `s->ptr`, and returns `void`.

The generated stubs offer as a convenience a variadic procedure (`calc__BindExceptionValue`) that can be used to raise any exception declared in the interface. For an exception that has no parameter, this procedure takes just two actual arguments. For an exception with a parameter, the parameter value is given as the third actual argument, using the usual calling convention for passing `IN` arguments of its type. Using this procedure, the above code would be:

    long server_calc_self_Div (calc_self h, ilu_integer u, ilu_integer v,
                               ILU_C_ENVIRONMENT *s)
    {
      calc_numerator n = 9;

      if ( v == 0 )
        {
          calc__BindExceptionValue(s, ex_calc_DivideByZero, n);
          return( u );
        }
      else
        return( u / v );
    }

The exception is sent back to the client, which can detect it thusly:

      ...
      calc_self instance;
      ILU_C_ENVIRONMENT s;
      ilu_integer i, j;
      ilu_integer val;
      ...
      instance = calc_self__CreateFromSBH (sbh, &s);

      if (! ILU_C_SUCCESSFUL(&s)) {
        fprintf (stderr, "CreateFromSBH(%s) raised %s\n",
          sbh, ILU_C_EXCEPTION_ID (&s) );
        exit(1);
      }

      val = calc_self_Div (instance, i, j, &s);

      /* check to see if an exception occured */

      if (! ILU_C_SUCCESSFUL(&s)) {
        /* report exception to user */
        char *p;

        p = ILU_C_EXCEPTION_ID (&s);

        if (p == ex_calc_DivideByZero) {
          calc_numerator *ip;
          ip = (calc_numerator *) ILU_C_EXCEPTION_VALUE (&s);
          fprintf (stderr, "%s signaled:  numerator = %d\n", p, *ip);
          }
        else {
          /* odd exception at this point */
          fprintf (stderr, "Unexpected <%s> on call to Div.\n", p);
          }
        /* free up any transient exception data */
        ILU_C_EXCEPTION_FREE (&s);
        }
      else {
        /* no exception - print the result */
        printf( "result is %d \n", val );
        }
      ...

### Parameter Passing Considerations

Here is ILU's version of table 20 from the CORBA 2.0 spec.

    DataType        In      InOut   Out        Return    Exn
    --------        --      -----   --        ------    ---
    scalar          T       T*      T*         T         T*
    optional        T       T*      T*         T         T*
    object          T       T*      T*         T         T*
    record, fixed   T*      T*      T*         T         T*
    record, var     T*      T*      T**        T*        T*
    union, fixed    T*      T*      T*         T         T*
    union, var      T*      T*      T**        T*        T*
    string          T       T*      T*         T         T*
    other sequence  T*      T*      T**        T*        T*
    array, fixed    T       T       T          T_slice*  T*
    array, var      T       T       T_slice**  T_slice*  T*

Here `T` is the ANSI C mapping of the type in question.

The `Exn` column describes how exception parameters appear in the parameter-conveying member of a status struct.

### True Module Construction

This section will outline the construction of a true module exported by an address space. For the example, we will demonstrate the calculator interface described above. We will also use the CORBA 2.0 names for standard types and exceptions, to show that it can be done.

First, some runtime initialization of the server stubs must be done. Call `Foo__InitializeServer` for every ISL interface Foo containing an object type implemented by the address space. Also call any client initialization procedures needed (see next section). These server and client initialization calls can be made in any order, and each initialization procedure can be called more than once. However, no two calls may be done concurrently (this is an issue only for those using some sort of multi-threading package).

Then we create an instance of `calc_self`. We then make the string binding handle of the object available by printing it to stdout. Finally the `ILU_C_Run` procedure is called. This procedure listens for connections and dispatches server methods.

The main program for the server is as follows:

    #include "I2.h"

    CORBA_long
      server_calc_self_Div (calc_self h,
                            CORBA_long u,
                            CORBA_long v,
                            CORBA_Environment *s)
    {
      calc_numerator n = 9;

      if ( v == 0 )
        {
          calc__BindExceptionValue(s, ex_calc_DivideByZero, n);
          return( u );
        }
      else
        return( u / v );
    }

    main ()
    {
      calc_self s;
      char * sbh;
      CORBA_Environment ev;

      calc__InitializeServer( );

      s = calc_self__CreateTrue (NULL, NULL, NULL);
      if (s == NULL)
        {
          fprintf (stderr, "Unable to create instance of calc_self.\n");
          exit(1);
        }
      else
        {
          sbh = CORBA_ORB_object_to_string (ILU_C_ORB, &ev, s);
          if (ev._major == CORBA_NO_EXCEPTION)
            {
              printf ("%s\n", sbh);
              ILU_C_Run (); /* enter main loop; hang processing requests */
            }
          else
            {
              fprintf (stderr,
                       "Attempt to obtain sbh of object %p signalled <%s>.\n",
                       s, CORBA_exception_id(&ev));
              exit(1);
            }
        }
    }

### Using ILU Modules

Before manipulating surrogate objects, a client module must first call a runtime initialization procedure `Foo__Initialize` for each ISL interface Foo that declares object types whose surrogates are to be manipulated. Additionally, server modules must also call server initialization procedures (see previous section). These initialization calls may be made in any order, and each procedure may be called more than once. However, no two calls may be done concurrently (this is an issue only for those using some sort of multi-threading package).

A client of an exported module may obtain an object instance either by calling a method which returns the instance, or by calling `TYPE__CreateFromSBH()` on the string binding handle of an instance. Once the object instance, which is typically a surrogate instance, but may in fact be a true instance, is held by the client, it can be used simply by making method calls on it, as shown above.

### Stub Generation

To generate ANSI C stubs from an ISL file, use the program c-stubber. Four files are generated from the ``.isl'` file:

- ``interface-name.h'` contains the definitions for the types and procedures defined by the interface and used by the generated stubs.
- ``interface-name-common.c'` contains the general code used by both client and server; and
- ``interface-name-surrogate.c'` contains the client-side and general code for the interface; and
- ``interface-name-true.c'` contains the server-side stubs and code for the interface.

Typically, clients of a module never have a need for the ``interface-name-true.c'` file.

    % c-stubber foo.isl
    header file interface foo to ./foo.h...
    code for interface foo to ./foo-common.c...
    code for interface foo to ./foo-surrogate.c...
    code for server stubs of interface foo to ./foo-true.c...
    %

### Tailoring Identifier Names

The option `-renames renames-filename` may be used with `c-stubber` to specify particular ANSI C names for ISL types.

It is sometimes necessary to have the ANSI C names of an ILU interface match some other naming scheme. A mechanism is provided to allow the programmer to specify the names of ANSI C language artifacts directly, and thus override the automatic ISL to ANSI C name mappings.

To do this, you place a set of synonyms for ISL names in a

renames-file, and invoke the `c-stubber` program with the switch `-renames`, specifying the name of the renames-file. The lines in the file are of the form

    construct ISL-name ANSI C-name

where construct is one of `method`, `exception`, `type`, `interface`, or `constant`; ISL-name is the name of the construct, expressed either as the simple name, for interface names, the concatenation `interface-name.construct-name` for exceptions, types, and constants, or `interface-name.type-name.method-name` for methods; and ANSI C-name is the name the construct should have in the generated ANSI C code. For example:

    # change "foo_r1" to plain "R1"
    type foo_r1 r1
    # change name of method "m1" to "method1"
    method foo_o1_m1 method1

Lines beginning with the `sharp' character ``#'` are treated as comment lines, and ignored, in the renames-file.

This feature of the `c-stubber` should be used as little and as carefully as possible, as it can cause confusion for readers of the ISL interface, in trying to follow the ANSI C code. It can also create name conflicts between different modules, unless names are carefully chosen.

## Libraries and Linking

For clients of an ILU module, it is only necessary to link with the ``interface-name-surrogate.o'` and ``interface-name-common.o'` files generated from the ANSI C files generated for the interface or interfaces being used, and with the two libraries ``ILUHOME/lib/libilu-c.a'` and ``ILUHOME/lib/libilu.a'` (in this order, as ``libilu-c.a'` uses functions in ``libilu.a'`).

For implementors of servers, the code for the server-side stubs, in the file ``interface-name-true.o'` compiled from ``interface-name-true.c'`, and in the file ``interface-name-common.o'` compiled from ``interface-name-common.c'`, should be included along with the other files and libraries.

## ILU C API

In addition to the functions defined by the CORBA mapping, the ILU ANSI C mapping provides some other functions, chiefly for type manipulation, object manipulation, and server manipulation. There are also a number of macros provided for compatibility with both versions of CORBA (revision 2.0).

### Type Manipulation

[ILU C API]: OPTIONAL(ilu_Class) ILU_C_FindILUClassByTypeName ( RETAIN(ilu_string) type-name )

Locking: L1_sup < otmu, L2, Main unconstrained.

Given the type-name of an ILU object type, of the form `"Interface.Typename"`, returns the `ilu_Class` value for it. This value can be used to compare types for equality.

[ILU C API]: OPTIONAL(ilu_Class) ILU_C_FindILUClassByTypeID ( RETAIN(ilu_string) type-id)

Locking: L1_sup < otmu; L2, Main unconstrained.

Given the type-id of an ILU object type, of the form `"ilu:gfbSCM7tsK9vVYjKfLole1HOBDc"`, returns the `ilu_Class` value for it. This value can be used to compare types for equality.

[ILU C API]: GLOBAL(OPTIONAL(ilu_string)) ILU_C_ClassName ( RETAIN(CORBA_Object) )

Locking: unconstrained.

Returns the ILU name for the most specific type of an object instance.

[ILU C API]: GLOBAL(OPTIONAL(ilu_string)) ILU_C_ClassID ( RETAIN(CORBA_Object) )

Locking: unconstrained.

Returns the ILU type ID for the most specific type of an object instance.

[ILU C API]: ilu_Class ILU_C_ClassRecordOfInstance (CORBA_Object)

Locking: unconstrained.

Returns the `ilu_Class` value for the most specific type of an object instance.

### Object Manipulation

[ILU C API]: ilu_string ILU_C_SBHOfObject ( CORBA_Object instance )

Locking: Main invariant holds.

Given an instance, returns a reference to that instance. The CORBA-specified routine `CORBA_ORB_object_to_string()` should typically be used instead.

[ILU C API]: OPTIONAL(CORBA_Object) ILU_C_SBHToObject (char * sbh, ilu_Class static_type, RETAIN(CORBA_Environment *) Env)

Locking: Main invariant holds.

Takes an object reference and returns the object. static_type is a type the caller knows the object to have.

[ILU C API]: OPTIONAL(PASS(char*)) ILU_C_PublishObject ( CORBA_Object instance )

Locking: Main invariant holds.

Publishes the OID of the instance in a domain-wide registry. This is an experimental interface, and may change in the future.

[ILU C API]: ilu_boolean ILU_C_WithdrawObject ( CORBA_Object instance, PASS(char *) proof)

Locking: Main invariant holds.

Removes the OID of the instance from the domain-wide registry. proof is the string returned from the call to `ILU_C_PublishObject()`.

[ILU C API]: OPTIONAL(GLOBAL(CORBA_Object)) ILU_C_LookupObject ( RETAIN(char *) sid, RETAIN(char *) ih, ilu_Class static-class )

Locking: Main invariant holds.

Using the local registry, find and return the object specified by the given Server ID and server-relative Instance Handle. static_type is one you know the actual object must have; it may also have more refined types. For an already-reified surrogate this procedure will reconsider what contact info to use for reaching the server.

[ILU C API]: OPTIONAL(GLOBAL(CORBA_Object)) ILU_C_CreateSurrogateObject ( ilu_Class type, RETAIN(char *) ih, ilu_Server server, ILU_C_ENVIRONMENT *env )

Locking: Main invariant holds.

Create a new object instance of the specified type on the specified server, with the specified ih. If unable to create such an object, return `ILU_NIL`, and signal the error in env.

This procedure can be used to create new client-side objects for which no true object yet exists. This is the way a client using a server with an object table causes the server to create new instances `on the fly'. When used in this way, the ih must contain all information necessary to allow the server to create the proper true object, as it is the only information passed to the object table's object creation procedure.

### Server Manipulation

[ILU C API]: ilu_boolean ILU_C_ENABLE_THREADS

Locking: Main invariant holds.

This macro expands to a function call. If ILU has been configured with os-level thread support, calling this routine will `turn on' that thread support for use with C. This means that a new thread will be forked to handle each incoming connection, in servers, and if the wire protocol being used permits it, a thread will be forked to handle each incoming request. This routine returns FALSE, and emits an error message, if something goes wrong with enabling thread support. It must be called before making any other ILU calls, and before initializing any interfaces via calls to `interface__Initialize` or `interface__InitializeServer`.

[ILU C API]: void ILU_C_FINISH_MAIN_THREAD ( int returnvalue )

Locking: Main invariant holds.

This routine will return from the `main' thread with the specified value. If the main thread cannot be terminated until the program ends, the call will block appropriately.

[ILU C API]: void ILU_C_Run (void)

Locking: Main invariant holds.

Called to animate a server and/or other parts of the program. Used only in single-threaded mode. Invokes the event handling loop. Never returns.

[ILU C API]: OPTIONAL(ilu_Server) ILU_C_InitializeServer (OPTIONAL(RETAIN(char *)) serverID, OPTIONAL(GLOBAL(ILU_C_ObjectTable)) obj_tab, OPTIONAL(RETAIN(char *)) protocol, OPTIONAL(RETAIN(ilu_TransportInfo)) transport, OPTIONAL(RETAIN(ilu_Passport)) identity, ilu_boolean setdefaultport)

Locking: Main invariant holds.

Creates and returns an `ilu_Server` with ID serverID, object mapping table obj_tab, using protocol protocol over a transport stack specified by transport. If serverID is specified as `NULL`, a unique string is generated automatically for the server ID. If obj_tab is specified as `NULL`, the default hash table object table is used.

If either protocol or transport is specified, or if setdefaultport, an `ilu_Port` will automatically be created and added to the `ilu_Server`. protocol, if not `NULL`, is a string that specifies which RPC protocol to use on the port; `NULL` causes use of `Sun RPC`. transport, if not `NULL`, is a sequence of strings that specifies the transport stack to use below the RPC protocol; `NULL` signifies use of SunRPC Record Marking over TCP to/from one of the IP addresses of this host. Chapter 8 gives details on protocol and transport strings. If an identity is specified, it may be used for communications security purposes. If setdefaultport is true, the newly created `ilu_Port` will become the default port of the `ilu_Server`.

#### Using C Object Tables

It is sometimes useful to have a server create true objects only when they are mentioned by a client's actual invocation of a method on them. This is allowed in ILU by an interface called an object table. A value of type `ILU_C_ObjectTable` may be created by a call on

[ILU C API]: ILU_C_ObjectTable ILU_C_CreateObjectTable (CORBA_Object (*object_of_ih)(ilu_string instance-handle, ilu_private user-data), void (*free_user_data)(ilu_private user-data), ilu_private user-data )

Locking: Main invariant holds. @br Locking for object_of_ih: L1 >= {server}, L1 >= {gcmu} if result is true and collectible; L2, Main unconstrained. @br Locking for free_user_data: L1 >= {server}; L2, Main unconstrained.

Creates and returns a value of type `ILU_C_ObjectTable` encapsulating the two procedures object_of_ih and free_user_data, and the user-specified data element user-data. When object_of_ih is called, it should create an appropriate `CORBA_Object` with the specified instance handle, and return it. When free_user_data is called, it indicates the end of the object table, and free_user_data should free up any storage associated with user-data.

An object table is associated with a kernel server by passing the object table as a parameter to the function `ILU_C_InitializeServer`. A single object table may be used with multiple different `ilu_Server` instances.

### CORBA Compatibility Macros

ILU supports CORBA 2.0, and formerly supported either 1.1 or 1.2, depending on how it was installed at your site. A number of macros are defined to make programs less dependent on which version they use.

Macro: ILU_C_OBJECT

Expands to `CORBA_Object`.

Macro: ILU_C_ENVIRONMENT

Expands to `CORBA_Environment`.

Macro: ILU_C_NO_EXCEPTION

Expands to `CORBA_NO_EXCEPTION`.

Macro: ILU_C_USER_EXCEPTION

Expands to `CORBA_USER_EXCEPTION`.

Macro: ILU_C_SYSTEM_EXCEPTION

Expands to `CORBA_SYSTEM_EXCEPTION`.

Macro: ILU_C_SUCCESSFUL ( ILU_C_ENVIRONMENT * ev )

Evaluates to true if no exception has been raised.

Macro: ILU_C_SET_SUCCESSFUL ( ILU_C_ENVIRONMENT * ev )

Sets ev to a successful result.

Macro: ILU_C_EXCEPTION_ID ( ILU_C_ENVIRONMENT * ev )

Returns the `char *` value that is the exception's ID.

Macro: ILU_C_EXCEPTION_VALUE ( ILU_C_ENVIRONMENT * ev )

Expands to `CORBA_exception_value(ev)`.

Macro: ILU_C_EXCEPTION_FREE ( ILU_C_ENVIRONMENT * ev )

Expands to `CORBA_exception_free(ev)`.

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