ALIAS (Modifier)


Modifies the data type name mangling (decoration) of a public symbol

Syntax:
... as [ const ] datatype alias "modifier" [ const [ ptr ... ] ]

Usage:
dim variable as datatype alias "modifier"
type name as datatype alias "modifier"
declare sub name ( param as datatype alias "modifier", ... )
declare function name ( param as datatype alias "modifier", ... ) as datatype alias "modifier"

Parameters:
datatype
Standard data type or user defined data type to modify
modifier
One of the supported modifiers as described in Description section following

Description:
Alias "modifier", when specified following a data type, gives an alternate meaning to the data type, which may be needed for linking with languages other than FreeBASIC.

Public symbol names are mangled (decorated) to encode information about the data type that is used for the symbol. When linking with the c language, the special meaning of the alias modifier is meaningless, since the extra information is not encoded in to the public name. When linking with the c++ language, typically more information is encoded in to the public symbol, and the alias modifier may be required. The public name is written to the compiled object file, and used by the linker to match symbol names from one object module to another.

The same rules for mapping data types is used regardless of which backend (gas or gcc) code emitter is used, And the intent is that FB's compiled code can link consistently with it's own object modules and object modules (or libraries) compiled from other languages.

Supported Modifiers

long alias "long"
ulong alias "long"
On Win 64-bit targets, used to map FB's 32-bit long and ulong types to c/c++'s 32-bit long [int] type, instead of the 32-bit int type.

any alias "char" ptr
Maps any ptr to c/c++'s char *. In c/c++, char, signed char, and unsigned char, are three distinct types.
  • byte ptr maps to signed char *
  • ubyte ptr maps to unsigned char *
  • On some platforms the variable argument list va_list type is a typed as a char *, but FB does not have an equivalent type, therefore any ptr is used instead. Linking with names encoded with this type will fail since, normally, FB encodes void * data type instead of char *.
  • alias "char" keeps the any ptr behaviour in FB but then encodes the public name as char * for linking.

any alias "__builtin_va_list" ptr
Maps the data type to gcc's __builtin_va_list type
  • expected that gcc's built-in type is a pointer type
  • used on dos, win32, win64, linux-x86, targets
  • see cvalist for default usage in the cva_list data type

alias "__builtin_va_list"
Maps the data type to gcc's __builtin_va_list type
  • expected that gcc's built-in type is a struct type
  • used on aarch64 target
  • see cvalist for default usage in the cva_list data type

alias "__builtin_va_list[]"
Maps the data type to gcc's __builtin_va_list type
  • expected that gcc's built-in type is a struct array type
  • used on linux-x86_64 target
  • see cvalist for default usage in the cva_list data type

Data Type Mapping Details

On all targets, FB to c/c++:
Several of FB's data types are consistently mapped across all targets:
  • 8-bit byte maps to signed char
  • 8-bit ubyte maps to unsigned char
  • 16-bit short maps to [signed] short [int]
  • 16-bit ushort maps to unsigned short [int]
  • 32-bit long maps to int
  • 32-bit ulong maps to unsigned int
  • 64-bit longint maps to long long [int]
  • 64-bit ulongint maps to unsigned long long [int]

On Dos/Win/Linux 32-bit targets, FB to c/c++:
integer on 32-bit targets is 32-bits wide
  • 32-bit integer maps to long [int]
  • 32-bit uinteger maps to unsigned long [int]

On Linux 64-bit targets, FB to c/c++:
integer on 64-bit targets is 64-bits wide
  • 64-bit integer maps to long [int]
  • 64-bit uinteger maps to unsigned long [int]

On Win 64-bit targets, FB to c/c++:
integer on 64-bit targets is 64-bits wide. However, on Win target, c/c++'s long int type is 32-bit, not 64-bit, and we can't use the long long int mangling because it's already used by FB's longint type. Reusing the same mangling (decoration) for two different data types would cause function overloading to fail or have duplicate definitions. To preserve FB's behaviour that Integer on 64-bit targets is always 64-bits, we mangle (decorate) the symbol with a custom datatype and keep the size at 64-bit.
  • 64-bit integer maps to custom INTEGER
  • 64-bit uinteger maps to custom UINTEGER
To create a data type in FB that will map to c/c++'s long [int] 32-bit on win, we must use alias modifier.
  • 32-bit long alias "long" maps to long [int]
  • 32-bit ulong alias "long" maps to unsigned long [int]

For example extern c++ : declare sub proc( byval as long alias "long" ) : end extern. This allows FreeBASIC to call external c++ procedures (on win-64) requiring a 32-bit long int type. Usage of Alias in this way affects win-64 targets only, and is ignored on all other targets.

Differences from QB:
See also:
Back to Datatypes

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