Amiga malloc() function

[Steam Ranger]

How does the malloc() function work on Amiga? Can I make my program not use the inbuilt one and instead write a custom one?

[Samurai_Crow]

AllocMem() is the memory allocation routine on the Amiga. C uses malloc() to wrap it or one of several other wrappers that use it. One of the more common wrappers is the memory pool functions added in AmigaOS 3.x but if you use an older version of the OS, writing a wrapper is recommended.

[Steam Ranger]

Originally Posted by Samurai_Crow

AllocMem() is the memory allocation routine on the Amiga. C uses malloc() to wrap it or one of several other wrappers that use it. One of the more common wrappers is the memory pool functions added in AmigaOS 3.x but if you use an older version of the OS, writing a wrapper is recommended.

I am using Workbench 3.1, and my program is linking in a wrapper automatically. I want to know how I can not link it so I can write my own wrapper to work on WB 1.3 and 3.1.

[Samurai_Crow]

If you link your version of malloc() before the c runtime, it will redirect to your allocator. You will have to wrap AllocMem() from Exec.library though.

[Olaf Barthel]

Originally Posted by Steam Ranger

How does the malloc() function work on Amiga? Can I make my program not use the inbuilt one and instead write a custom one?

Why do you need to replace the malloc() function? Which problem are you trying to solve?

As Samurai_Crow already mentioned, replacing the malloc() function by one of your own design will replace it for the entire 'C' runtime library, not just for the use of your own code.

The startup code for your program is likely to use malloc() and also free() before your own program's main() function is called. This means that your replacements of malloc() and free() have to work before your main() function is called and after exit() has been called (either by your code calling it, or the startup code).

And it gets even more complicated very quickly. You not only have to replace malloc() and free(), you will likely have to replace realloc() and calloc(), too, and find a way to release any memory allocated through realloc(), calloc() and malloc() which has not been freed yet, before your program exits since the startup code no longer takes care of it. This is a huge challenge all by itself.

Reimplementing the 'C' runtime library memory management functions and the infrastructure they need is really tough because you have to make it work exactly like it says in the specifications which describe them. I went through that kind of work almost 20 years ago when I built clib2, because I wanted to port Samba 2.0.4 to the Amiga, without relying upon ixemul.library, etc. Let's say it was educational, and I'm still finding and fixing my own bugs after all these decades

Please keep in mind that malloc() is not easily replaced by AllocMem(). Unlike AllocMem(), malloc() will remember the size of the allocation made and it will remember if you forgot to release any of the memory allocated through it, releasing it automatically when your program exits.


There's an alternative option if your own code makes use of malloc() and free() only, and you do not use calloc() or realloc(). You can write your own my_malloc() and my_free() functions and then use macros to redirect the calls to your own functions, like so:

#include <stddef.h>

extern void * my_malloc(size_t size);
extern void my_free(void * memory);

#define malloc(size) my_malloc(size)
#define free(memory) my_free(memory)

Use atexit() to install a cleanup function for the my_malloc() allocations which will release any memory not freed yet.

[No.3]

Originally Posted by Olaf Barthel

Unlike AllocMem(), malloc() will remember the size of the allocation made...

at least for this case AllocVec() is your friend.

[Thomas Richter]

Originally Posted by Steam Ranger

How does the malloc() function work on Amiga?

That is compiler dependent. It is typically a function in the C standard library that is linked to your code, and it typically uses some kind of local memory pool provided by the compiler library that also includes other memory management functions from ANSI C, such as realloc or calloc. Or rather, the memory pool provided by the C library is made such that it allows realloc.

Originally Posted by Steam Ranger

Can I make my program not use the inbuilt one and instead write a custom one?

Well, if you are using other functions from the C standard library, that is at least not advisable. If you are creating a program solely for the Amiga that does not use any ANSI-C functions, and that does not use the C startup code, then you should probably use AllocMem() and friends instead of replacing malloc.