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AmiWest Lesson 2

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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

Opening and Closing

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);
	}
}

Special Syntax Support

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 messages 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

AmigaOS has two basic threads of control: Tasks and Processes. The following class diagram illustrates their relationship to each other:

ProcessTaskClassDiagram.png

Tasks are created by Exec (exec.library) while Processes are created by DOS (dos.library). A Process includes more overhead than a Task. This is primarily because a Process can access all the facilities offered by DOS such as file handling and input and output streams.

In general, you should always prefer to use a Process instead of a Task. Although a Process may include more overhead, a Process is also simpler to work with. Tasks are generally relegated to low level activities. I will be focusing on the use of Processes in this lesson.

To create a new Process, DOS provides the following function:

struct Process *proc = CreateNewProcTags(uint32 Tag1, ...);

There are many tags which can be used to customize the way in which the new Process is created which are fully explained in the dos.doc AutoDoc.

Library base pointers (struct Library*) may be global and shared between Processes. Interface pointers must not be shared between Processes unless this is explicitly allowed. The reason for this is because an Exec Interface may allocate resources which are bound to the context of the caller. That means if your parent Process calls GetInterface() then it allocates resources only for that Process. A child Process must call GetInterface() as well to allocate resources for that child Process.

Synchronization Primitives

When two or more Tasks and/or Processes want to share common data with each other then synchronization primitives are likely to be required.

  • Signals are the most basic way for two processes to communicate with each other. A signal is a single bit which is either 1 or 0. There are 16 user signals available and many system signals. See Signals for more details.
  • Message ports are used to transfer messages between two processes. The message can be sent one way or it can be sent and replied. Message data is always shared and not copied so it is critical that message passing protocols are followed. See Messages and Ports for more details.
  • Semaphores provide a rich interface for sharing resources between processes. Sempahores can be shared so that multiple processes can wait on a single semaphore simultaneously. See Semaphores for more details.
  • Mutexes provide a simple high speed interface for sharing resources between processes. Mutexes are exclusive and cannot be shared. Mutexes can also be recursive or not depending on the application. See Mutexes for more details.
Note
If you are sharing memory between two entities that memory must be marked as MEMF_SHARED. Failure to comply with this requirement will result in problems when AmigaOS starts to enforce MEMF_PRIVATE memory semantics.

Shared Objects

Shared objects are an idea directly lifted from the Unix world. The main reason shared objects were added to AmigaOS is to enable much simpler porting of complex applications. For example, Amiga Python was implemented using shared objects which makes it very easy to maintain. It is up to each programmer to determine whether they want to use traditional Amiga shared libraries or shared objects for their particular project. For a discussion of the pros and cons see The Right Tool for the Job.