# Global object handlings of SigScheme

## 1. About

SigScheme has portable mechanisms that control:

- global symbols hiding

- global variable to dynamically allocated variable conversion

This document describes its necessity and usage.

## 2. The problems

### 2.1. Global symbol conflict with other libraries

Since SigScheme uses semi-common scm_ and Scm prefixes for exported symbols, it may conflict with other libraries if the client application is also directly or indirectly linked with another Scheme implementation such as libguile. This is a serious problem when implementing a fundamental library based on libsscm, such as libuim.

To avoid such conflict, many platforms provide symbol exportation control ability. For example, GNU ld and Windows DLL can handle it based on a optional file which contains the symbol information. And libtool provides an useful option `-export-symbols-regex` for such purpose to make the handlings platform-independent. However, unfortunately libtool does not ensure its portability. Currently supported platforms are considerably limited (at least on version 2.1a 2006-03-30) and some platforms seems that can never be supported.

Since primal portability is indispensable value of SigScheme to be being an useful embedded Scheme implementation, we cannot depend on such problematic toolchain-based symbol exportation control.

### 2.2. Platforms that lack writable static data

Some platforms such as BREW and some versions of Symbian OS lack writable static data capability. It means that no writable global variable can be used in SigScheme. But SigScheme do need various global data such as dynamic extent, symbol table, R5RS constant object holder and so on. So an alternative dynamic data store that can globally be accessed is needed.

## 3. Solution

The two problems are resolved with two related mechanisms.

For the symbol conflict problem, an alternative special compilation method called combined-source mode is provided.

It combines all source code of SigScheme into single file, and makes all global objects static. So no symbols will be exposed. A client of libsscm can use this combined version of the library by including the file `sigscheme-combined.c` directly into a C/C++ source of the client. Once it included, all configured SigScheme features can be used as file-local code, and it can also be linked with other arbitrary objects via user-written wrapper.

And the writable static data problem is resolved with aggregated global variables mechanism works on the combined-source mode. When this feature is directed, all of the global variables including static ones are aggregated into a single big struct, and allocated to platform-specific global store such as thread local storage or a member variable of application instance. The accessing method to the variables is abstracted by some macros, and any variable can be accessed as if ordinary variable.

Finally, a variant configuration of the two mechanisms is also available for the platforms lacking writable static data. It is combined, global variables aggregated, but exports SigScheme’s API symbols. This configuration is supposed to provide libsscm on such platforms.

## 4. Combined-source mode usage for libsscm users

### 4.1. Preparation

The combined-source version of the library is optional and not built by default. Instruct as follows to build it.

      $ make -C src combined

It results the file `src/sigscheme-combined.c`. Since the generated code reflects user-configuration, it must be rebuilt every after `configure`.

### 4.2. Compilation

Include the `sigscheme-combined.c` directly into a C/C++ code, as following example. Ordinary separated compilation and linking does not work because of its "all global objects made static" nature.

a C code using combined-source mode of the SigScheme

    #include "sigscheme-combined.c"

    #include  /* client's own config.h */

    #include
    #include

    #include "my-header.h"

    static ScmObj
    my_function(int val)
    {
        return SCM_MAKE_INT(val * 2);
    }

Requirements:

- `sigscheme-combined.c` must be included prior to client’s own `config.h` since many autoconf-defined macros of SigScheme must appropriately be undefined in the file to avoid conflicting with the client-side ones

- Add include path to `sigscheme/include` and `sigscheme/src` since the `sigscheme-combined.c` includes various source files in the directories

Other notes:

- The client code can use all of public SigScheme API defined in `sigscheme.h`, `scmint.h`, `encoding.h`, `global.h` and SigScheme’s own `condig.h`. Any other interfaces should not be used even if accessible

- All of SigScheme’s internal macros are undefined and hidden at the end of `sigscheme-combined.c` inclusion, to reduce namespace pollution

- Many internal variables and functions of SigScheme are exposed to the client code. Be careful of conflict

## 5. Combined-source mode with exported API symbols

To export SigScheme API symbols against the combined-source mode, define `SCM_EXPORT_API` to 1 prior to including the `sigscheme-combined.c`, as follows. See `src/dllentry.c` as real example.

instructs that export SigScheme API

    #define SCM_EXPORT_API 1
    #include "sigscheme-combined.c"

## 6. Non-static functions handling for SigScheme developers

To control global symbol exportation, any non-static function declaration and definition need little modification. For both prototype declaration and actual definition, prepend `SCM_EXPORT` to the function. There is no distinction between public API and library-internal API on the exportation control. Use `SCM_EXPORT` for both type of functions.

The `SCM_EXPORT` macro is replaced with `static` when the combined-source mode without `SCM_EXPORT_API`.

symbol exportation control for functions

    /* declaration */
    SCM_EXPORT void scm_set_lib_path(const char *path);
    SCM_EXPORT ScmObj scm_p_load(ScmObj filename);

    /* definition */
    SCM_EXPORT void
    scm_set_lib_path(const char *path)
    {
        ...
    }

    SCM_EXPORT ScmObj
    scm_p_load(ScmObj filename)
    {
        ...
    }

## 7. Global variables handling for SigScheme developers

The aggregated variables mechanism is mainly provided for SigScheme developers and used to write libsscm. All global variables used in SigScheme must be written with this mechanism to keep portable to the embedded platforms. Although it tries to keep original usage of the global variables, some rewrite of source codes declaring and defining the variables are needed because of the limitation of C macro ability.

### 7.1. Constant global variables

Constant global variables can safely be used in all platforms regardless of whether static or not. But a little modification of `extern` declaration is needed for the combined-source mode treatment.

Be careful about all part of the variables are certainly qualified as const.

a constant global variables declaration and definition

    /* declaration */
    extern const char *const names[];

    /* definition */
    const char *const names[] = {
      "foo", "bar", NULL
    };

    static const char *const other_names[] = {
      "baz", "quux", NULL
    };

rewritten declaration and definition

    /* declaration */
    SCM_EXTERN(const char *const names[]);

    /* definition */
    const char *const names[] = {
      "foo", "bar", NULL
    };

    static const char *const other_names[] = {
      "baz", "quux", NULL
    };

### 7.2. Writable extern variables

Writable and exported global variables need more complex rewriting.

a writable extern variables declaration and definition

    /* declaration */
    extern int foo bar;
    extern ScmObj obj_a, obj_b;
    extern void (*func)(void);

    /* definition */
    int foo bar;
    ScmObj obj_a, obj_b;
    void (*func)(void);

rewritten declaration and definition

    /* declaration */
    SCM_GLOBAL_VARS_BEGIN(srfi99);
    int foo bar;
    ScmObj obj_a, obj_b;
    void (*func)(void);
    SCM_GLOBAVARS_END(srfi99);
    #define foo   SCM_GLOBAL_VAR(srfi99, foo)
    #define bar   SCM_GLOBAL_VAR(srfi99, bar)
    #define obj_a SCM_GLOBAL_VAR(srfi99, obj_a)
    #define obj_b SCM_GLOBAL_VAR(srfi99, obj_b)
    #define func  SCM_GLOBAL_VAR(srfi99, func)

    /* definition */
    SCM_DEFINE_EXPORTED_VARS(srfi99);

The identifier srfi99 in above example specifies a namespace and aggregational unit which the variables are placed into. It is recommended that the name is taken from the filename which the definition is placed.

The macros defined immediately after the SCM_GLOBAVARS_END() are conventional accessors for the variables. The proper accessing method for the variables is SCM_GLOBAL_VAR(namespace, varname), but it unacceptably bothers code writing. So such macros should be defined. The macros make rewriting of codes operate on the variables unneeded. The SCM_GLOBAL_VAR() allows being accessed as lvalue.

But since it may obviously cause unexpected replacement if any of the names are appeared as a non-global-variable object in a code, such as local variable or struct member. The macro definition may affect to all source files even if the header defines the macro is not included by a file, because of the unified translation unit formed by the combined-source. To avoid such unwanted replacement, The variable names should be prefixed to be unique over all sources.

prefixing each variables is recommended

    /* declaration */
    SCM_GLOBAL_VARS_BEGIN(srfi99);
    int scm_foo scm_bar;
    ScmObj scm_obj_a, scm_obj_b;
    void (*scm_func)(void);
    SCM_GLOBAVARS_END(srfi99);
    #define scm_foo   SCM_GLOBAL_VAR(srfi99, scm_foo)
    #define scm_bar   SCM_GLOBAL_VAR(srfi99, scm_bar)
    #define scm_obj_a SCM_GLOBAL_VAR(srfi99, scm_obj_a)
    #define scm_obj_b SCM_GLOBAL_VAR(srfi99, scm_obj_b)
    #define scm_func  SCM_GLOBAL_VAR(srfi99, scm_func)

    /* definition */
    SCM_DEFINE_EXPORTED_VARS(srfi99);

Finally, conditional compilation is allowed even if in the declaration.

conditional compilation is allowed

    /* declaration */
    SCM_GLOBAL_VARS_BEGIN(srfi99);
    int scm_foo scm_bar;
    #if SCM_USE_FOO
    ScmObj scm_obj_a, scm_obj_b;
    void (*scm_func)(void);
    #endif
    SCM_GLOBAVARS_END(srfi99);
    #define scm_foo   SCM_GLOBAL_VAR(srfi99, scm_foo)
    #define scm_bar   SCM_GLOBAL_VAR(srfi99, scm_bar)
    #define scm_obj_a SCM_GLOBAL_VAR(srfi99, scm_obj_a)
    #define scm_obj_b SCM_GLOBAL_VAR(srfi99, scm_obj_b)
    #define scm_func  SCM_GLOBAL_VAR(srfi99, scm_func)

    /* definition */
    SCM_DEFINE_EXPORTED_VARS(srfi99);

### 7.3. Static variables

Static variables handling is similar to external one, but some additional treatment is needed.

a static variables definition

    static int foo bar;
    static ScmObj obj_a, obj_b;
    static void (*func)(void);

rewritten definition

    SCM_GLOBAL_VARS_BEGIN(static_srfi99);
    #define static
    static int l_foo l_bar;
    static ScmObj l_obj_a, l_obj_b;
    static void (*l_func)(void);
    #undef static
    SCM_GLOBAL_VARS_END(static_srfi99);
    #define l_foo   SCM_GLOBAL_VAR(static_srfi99, l_foo)
    #define l_bar   SCM_GLOBAL_VAR(static_srfi99, l_bar)
    #define l_obj_a SCM_GLOBAL_VAR(static_srfi99, l_obj_a)
    #define l_obj_b SCM_GLOBAL_VAR(static_srfi99, l_obj_b)
    #define l_func  SCM_GLOBAL_VAR(static_srfi99, l_func)
    SCM_DEFINE_STATIC_VARS(static_srfi99);

The technical difference to external one is:

- Use SCM_DEFINE_STATIC_VARS() instead of SCM_DEFINE_EXPORTED_VARS() to define declared variables

And some more conventions:

- Enclose the declaration with `#define static` and `#undef static` to keep indicating that the variables are static

  - The `static` specifier is technically not allowed here

  - The `#define` and `#undef` must be placed inside `SCM_GLOBAL_VARS_{BEGIN,END}()`. Placing outside of them makes the declaration broken

- The namespace should be prefixed with `static_`

- The variable names should be prefixed with `l_` (stands for local)

### 7.4. Common restrictions

- Reserved namespaces

  - `dummy`

  - `dummy_*`

  - `instance`

  - `instance_*`

  - `aggregated`

  - `aggregated_*`

### 7.5. Initialization and finalization

The aggregated variables MUST be initialized with `SCM_GLOBAL_VARS_INIT()` prior to being used. Before the initialization, accessing the variables is completely invalid. It is not only containing unspecified value but may cause crash on some platforms since the storage is not allocated yet.

initialization of a set of variables

    void
    scm_srfi99_init(void)
    {
        SCM_GLOBAL_VARS_INIT(srfi99);
        SCM_GLOBAL_VARS_INIT(static_srfi99);
    }

After the initialization, the variables contained in the namespace specified by the `SCM_GLOBAL_VARS_INIT()` are allocated and zero-cleared. If you want that an variable is initialized, assign the value by hand after the initialization. Load-time initialization is not allowed and technically does not work.

initialization is not allowed (and does not work)

    SCM_GLOBAL_VARS_BEGIN(static_srfi99);
    #define static
    static int l_foo = 100;
    #undef static
    SCM_GLOBAL_VARS_END(static_srfi99);

The by-function initialization ensures that cyclic finalization → re-initialization loop works properly without careful initial value treatment of global variables. It is helpful to ensure that SigScheme is re-initialization safe.

Finalization macro is also provided and usable. But since it is currently empty expression on all platforms, and most code modules do not have finalization function, no finalization is performed for global variables to keep footprint shrunk. But the macro should be added if a code module has finalization function.

finalization of the variables

    void
    scm_srfi99_fin(void)
    {
        SCM_GLOBAL_VARS_FIN(srfi99);
        SCM_GLOBAL_VARS_FIN(static_srfi99);
    }