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/*
* Asterisk -- An open source telephony toolkit.
*
* Copyright (C) 2008, Digium, Inc.
*
* Kevin P. Fleming <kpfleming@digium.com>
*
* See http://www.asterisk.org for more information about
* the Asterisk project. Please do not directly contact
* any of the maintainers of this project for assistance;
* the project provides a web site, mailing lists and IRC
* channels for your use.
*
* This program is free software, distributed under the terms of
* the GNU General Public License Version 2. See the LICENSE file
* at the top of the source tree.
*/
#ifndef __ASTERISK_OPTIONAL_API_H
#define __ASTERISK_OPTIONAL_API_H
/*!
* \file
* \brief Optional API function macros
*
* Some Asterisk API functions are provided by loadable modules, thus,
* they may or may not be available at run time depending on whether the
* providing module has been loaded or not. In addition, there are some
* modules that are consumers of these APIs that *optionally* use them; they
* have only a part of their functionality dependent on the APIs, and can
* provide the remainder even if the APIs are not available.
*
* To accomodate this situation, the AST_OPTIONAL_API macro allows an API
* function to be declared in a special way, if Asterisk being built on a
* platform that supports the GCC 'weak' and 'alias' attributes. If so,
* the API function is actually a weak symbol, which means if the provider
* of the API is not loaded, the symbol can still be referenced (unlike a
* strong symbol, which would cause an immediate fault if not defined when
* referenced), but it will return NULL signifying the linker/loader was
* not able to resolve the symbol. In addition, the macro defines a hidden
* 'stub' version of the API call, using a provided function body, and uses
* the alias attribute to make the API function symbol actually resolve to
* that hidden stub, but only when the *real* provider of the symbol has
* not been found.
*
* An example can be found in agi.h:
*
* \code
* AST_OPTIONAL_API(int, ast_agi_register, (struct ast_module *mod, agi_command *cmd),
* { return AST_OPTIONAL_API_UNAVAILABLE; });
* \endcode
*
* This defines the 'ast_agi_register' function as an optional API; if a
* consumer of this API is loaded when there is no provider of it, then
* calling this function will actually call the hidden stub, and return
* the value AST_OPTIONAL_API_UNAVAILABLE. This allows the consumer to
* safely know that the API is not available, and to avoid using any
* other APIs from the not-present provider.
*
* In the module providing the API, the AST_OPTIONAL_API macro must
* be informed that it should not build the hidden stub function or
* apply special aliases to the function prototype; this can be done
* by defining AST_API_MODULE just before including the header file
* containing the AST_OPTIONAL_API macro calls.
*
* \note If the GCC 'weak' and 'alias' attributes are not available,
* then the AST_OPTIONAL_API macro will result in a non-optional function
* definition; this means that any consumers of the API functions so
* defined will require that the provider of the API functions be
* loaded before they can reference the symbols.
*/
#define __stringify_1(x) #x
#define __stringify(x) __stringify_1(x)
/*!
* \brief A common value for optional API stub functions to return
*
* This value is defined as INT_MIN, the minimum value for an integer
* (maximum negative value), which can be used by any optional API
* functions that return a signed integer value and would not be
* able to return such a value under normal circumstances.
*/
#define AST_OPTIONAL_API_UNAVAILABLE INT_MIN
#if defined(HAVE_ATTRIBUTE_weak) && defined(HAVE_ATTRIBUTE_alias) && !defined(AST_API_MODULE)
#define AST_OPTIONAL_API(result, name, proto, stub) \
static result __##name proto stub; \
result __attribute__((weak, alias("__" __stringify(name)))) name proto;
#define AST_OPTIONAL_API_ATTR(result, attr, name, proto, stub) \
static result __attribute__((attr)) __##name proto stub; \
result __attribute__((weak, alias("__" __stringify(name)), attr)) name proto;
#else
/*!
* \brief Define an optional API function
*
* \param result The type of result the function returns
* \param name The name of the function
* \param proto The prototype (arguments) of the function
* \param stub The code block that will be used by the hidden stub when needed
*
* Example usage:
* \code
* AST_OPTIONAL_API(int, ast_agi_register, (struct ast_module *mod, agi_command *cmd),
* { return AST_OPTIONAL_API_UNAVAILABLE; });
* \endcode
*/
#define AST_OPTIONAL_API(result, name, proto, stub) result name proto;
/*!
* \brief Define an optional API function with compiler attributes
*
* \param result The type of result the function returns
* \param attr Any compiler attributes to be applied to the function (without the __attribute__ wrapper)
* \param name The name of the function
* \param proto The prototype (arguments) of the function
* \param stub The code block that will be used by the hidden stub when needed
*/
#define AST_OPTIONAL_API_ATTR(result, attr, name, proto, stub) result __attribute__((attr)) name proto;
#endif
#undef AST_API_MODULE
#endif /* __ASTERISK_OPTIONAL_API_H */
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