AmiWest Lesson 2

AmiWest Lesson 2: Fundamentals

Basic Types

It is important to under at least the basic types when programming. The following table summarizes the basic types used in AmigaOS as compared to standard C and C++ types:

AmigaOS Types

Type	Deprecated Type(s)	C	C++
uint64	none	uint64_t	uint64_t
int64	none	int64_t	int64_t
uint32	ULONG or LONGBITS or CPTR	uint32_t	uint32_t
int32	LONG	int32_t	int32_t
uint16	UWORD or WORDBITS or USHORT or UCOUNT or RPTR	uint16_t	uint16_t
int16	WORD or SHORT or COUNT	int16_t	int16_t
uint8	UBYTE or BYTEBITS	char or unsigned char	unsigned char
int8	BYTE	signed char	signed char
STRPTR	none	char*	char*
CONST STRPTR	n/a	char* const x	char* const x
CONST_STRPTR	n/a	const char*	const char*
CONST CONST_STRPTR	n/a	const char* const	const char* const
APTR	none	void*	void*
CONST APTR	none	void* const x	void* const x
CONST_APTR	none	const void*	const void*
CONST CONST_APTR	none	const void* const	const void* const
float32	FLOAT	float	float
float64	DOUBLE	double	double
BOOL	none	int16	int16
TEXT	none	char	char
NULL	none	0L	(void*)0L
BPTR	none	int32_t	int32_t
BSTR	none	int32_t	int32_t
ZERO	none	(BPTR)0	(BPTR)0

Static versus Dynamic Linking

The different between static and dynamic linking can best be explained with an example.

#include <stdio.h>
 
int main()
{
 printf("Hello, world\n");
 return 0;
}

Here is "hello" created using static linking:

gcc -mcrt=clib2 -N -o hello hello.c -Wl,--cref,-M,-Map=hello.map
strip hello

The "hello" executable is roughly 34964 bytes in size. The "hello.map" file contains the linker map which shows you exactly what pieces of code have been pulled in from where to create that executable.

Here is "hello" created using dynamic linking:

gcc -mcrt=newlib -N -o hello hello.c -Wl,--cref,-M,-Map=hello.map
strip hello

The "hello" executable is now 5488 bytes in size and the "hello.map" file is substantially simpler as well.

Note
The -N switch is used to work around a feature in the current GCC toolset. The problem has been fixed but a new compiler is not yet generally available. See Myth #2: AmigaOS binaries are fat for more details.

It is important to remember that each program is still essentially the same size. In fact, the dynamically linked program may even be larger. The reason is that the same amount of code is still used. The difference is that the dynamically linked executable is sharing code with other executables. The statically linked executable is not sharing code and thus occupies more disk space.

A statically linked program is a self-contained unit which generally has no extraneous external dependencies. Any dependency problems will show up when linking the executable.

A dynamically linked program requires external libraries in order to function. If any of those libraries are missing or the wrong version, your program will fail at runtime and not when you compile it.

For programmers, it is very important to know where your libraries are coming from and what their limitations are. Just assuming you can mix and match static and dynamic libraries at will is foolish. Always try to understand what each library does and what caveats apply to its usage.

Libraries and Interfaces

AmigaOS provides hundreds of functions which are split up into different shared libraries. Each shared library is further split up into different interfaces. Much more detailed information about Libraries and Interfaces can be found in Exec Libraries.

The following code can be used to open interfaces:

struct Interface* try_open_iface_name(CONST_STRPTR libname, uint32 libver, CONST_STRPTR ifacename)
{
	struct Library* base = IExec->OpenLibrary(libname, libver);
	if ( base != 0 )  {
		struct Interface* iface = IExec->GetInterface(base, ifacename, 1, 0);
 
		if ( iface != 0 )  {
			return iface;
		}
 
		IExec->CloseLibrary(base);
	}
 
	return 0;
}
 
struct Interface* open_iface_name(CONST_STRPTR libname, uint32 libver, CONST_STRPTR ifacename)
{
	struct Interface* iface = try_open_iface_name(libname, libver, ifacename);
 
	if ( iface == 0 )  {
		IDOS->Printf("Can't open %s version %lu interface %s\n", libname, libver, ifacename);
	}
 
	return iface;
}
 
struct Interface* open_iface(CONST_STRPTR libname, uint32 libver)
{
	return open_iface_name(libname, libver, "main");
}

The following code can be used to close an interface and the corresponding library:

void close_iface(struct Interface* iface)
{
	if ( iface != 0 )  {
		struct Library* lib = iface->Data.LibBase;
		IExec->DropInterface(iface);
		IExec->CloseLibrary(lib);
	}
}

Exec Interfaces are called using a special syntax supported by the GNU GCC compiler included with the SDK:

IExec->DebugPrintF("Hello, world\n");

Using a regular compiler this would translate into:

IExec->DebugPrintF(IExec, "Hello, world\n");

Notice how the first argument to the function call is 'hidden' when using the modified GNU GCC compiler. This becomes especially important when trying to understand error message from the compiler. For example, the following code is invalid:

#include <proto/exec.h>
 
int main()
{
	IExec->DebugPrintF(42);
	return 0;
}

gcc -o args args.c
args.c: In function 'main':
args.c:5: warning: passing argument 2 of 'IExec->DebugPrintF' makes pointer from integer without a cast

Notice how the compiler is referring to "argument 2" even though there is only one argument in the call to IExec->DebugPrintF(). This is a side effect of the 'hidden' IExec argument referred to earlier.

Tasks and Processes

Code to display Tasks and Processes.