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[[ LCB ]] Add varargs support to foreign handlers
This patch adds varargs support to foreign C handlers, allowing binding to a wider range of C functions. The syntax for vararg handlers is similar to that of C: ``` foreign handler myvarargs(in pFirst as Pointer, ...) returns CInt binds to ... ``` Using the token `...` to indicate the start of the variable parameter list. To implement vararg functions, a new parameter mode 'variadic' has been added which causes a handler to be treated as variadic if such a parameter with a mode is present. As C requires any int type with rank less than int to be promoted to int, and float to be promoted to double (when passed to a non-fixed parameter in a variadic function), a 'promotedtype' and 'promote' function have been added to foreign type descriptors to describe the appropriate relation.
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‎docs/guides/LiveCode Builder Language Reference.md‎

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A foreign handler definition binds an identifier to a handler defined in
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foreign code.
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The last parameter in a foreign handler declaration may be '...' to indicate
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that the handler is variadic. This allows binding to C functions such as
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sprintf.
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Note: No bridging of types will occur when passing a parameter in the non-fixed
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section of a variadic argument list. You must ensure the arguments you pass there
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are of the appropriate foreign type (e.g. CInt, CDouble).
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There are a number of types defined in the foreign module which map to
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the appropriate foreign type when used in foreign handler signatures.
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‎docs/lcb/notes/19991.md‎

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# LiveCode Builder Language
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## Variadic foreign C functions
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* It is now possible to bind to variadic C functions:
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foreign handler printf(in pFormat as Pointer, ...) returns CInt binds to "<builtin>"
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In this case, the '...' must be the last parameter, and there must be at
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least one fixed parameter.
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# [19991] Add support for variadic foreign C functions.
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‎libfoundation/include/foundation.h‎

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@@ -1776,6 +1776,14 @@ struct MCForeignTypeDescriptor
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bool (*doimport)(void *contents, bool release, MCValueRef& r_value);
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bool (*doexport)(MCValueRef value, bool release, void *contents);
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bool (*describe)(void *contents, MCStringRef & r_desc);
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/* The promotedtype typeinfo is the type to which this type must be promoted
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* when passed through variadic parameters. The 'promote' method does the
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* promotion. */
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MCTypeInfoRef promotedtype;
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/* Promote the value in contents as necessary. The slot ptr must be big enough
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* to hold the promotedtype. */
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void (*promote)(void *contents);
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};
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MC_DLLEXPORT bool MCForeignTypeInfoCreate(const MCForeignTypeDescriptor *descriptor, MCTypeInfoRef& r_typeinfo);
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kMCHandlerTypeFieldModeIn,
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kMCHandlerTypeFieldModeOut,
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kMCHandlerTypeFieldModeInOut,
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kMCHandlerTypeFieldModeVariadic,
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};
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struct MCHandlerTypeFieldInfo
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// Returns true if the handler is of foreign type.
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MC_DLLEXPORT bool MCHandlerTypeInfoIsForeign(MCTypeInfoRef typeinfo);
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// Returns true if the handler is variadic.
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MC_DLLEXPORT bool MCHandlerTypeInfoIsVariadic(MCTypeInfoRef typeinfo);
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// Get the return type of the handler. A return-type of kMCNullTypeInfo means no
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// value is returned.
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MC_DLLEXPORT MCTypeInfoRef MCHandlerTypeInfoGetReturnType(MCTypeInfoRef typeinfo);
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// Get the number of parameters the handler takes.
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// Get the number of parameters the handler takes. If the handler is variadic,
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// this returns the number of fixed parameters.
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MC_DLLEXPORT uindex_t MCHandlerTypeInfoGetParameterCount(MCTypeInfoRef typeinfo);
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// Return the mode of the index'th parameter.

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