Hello Chen
I found that malloc/free functions have been implemented in at least three different libraries: cxx, mini_c and libc. I assume cxx is used for genode base and core implementations, mini_c for demos and libc for porting. However, I am not able to use any of them in my own
the malloc/free implementation of the cxx library is meant only for internal use of the C++ support library (i.e., needed by libsupc++). This implementation is private to the 'cxx' library and not visible outside this library. This is by intention to prevent Genode components from unknowingly depending on this kind of libc functionality. The implementation provided by 'mini_c' is just there to support libpng and libz that we use for the Scout tutorial browser. It is part of the 'demo' repository. I'd recommend not to use this library outside this repository. Now for the libc, this is the official way to use malloc/free on Genode.
The absence of malloc/free in the Genode API is by intention for two reasons. First, Genode is C++, so using new/delete (on Genode, 'new' takes an 'Allocator' argument, delete is replaced by 'Genode::destroy') is the way to go for dynamic memory allocation. Second, the Genode concept devises the use of non-anonymous memory by explicitly stating an allocator when allocating/freeing memory. This way, resource usage can be tracked (and limited) at a fine granularity. To support this concept, there is an abstract 'Allocator' interface and a number of implementations (e.g., the 'Slab', 'Allocator_avl', and 'Heap' classes). Anyway, in some cases, anonymous memory allocations are needed. The easiest way is:
void *p = Genode::env()->heap()->alloc(size);
application. When using malloc defined in mini_c or libc, I got a compiler error “ reference to ‘size_t’ is ambiguous”. It looks like both stdlib.h in mini_c (or libc) directory and stdint.h in base directory have defined “size_t” type. And I cannot get rid of the use of stdint.h as it has been included in many genode base and core files.
This error occurs because you are including the said C header file in your C++ program and also stating 'using namespace Genode' in your program. To solve this problem, you may prefer stating 'using namespace Genode' locally within selected functions instead of stating this at the global scope of the file. The best would be to avoid 'using namespace Genode' altogether for files that include both libc headers and Genode headers. Instead, use the Genode namespace prefix explicitly, wherever needed.
On the other hand, if I wanted to use the malloc in cxx by putting “cxx” into LIB in my application’s target.mk, malloc cannot be recognized by the compiler. To solve this problem, I currently have to define my own malloc/free by copying the implementation from cxx and it works. However, I really wonder which malloc/free defined in genode should I use and how? By the way, the implementation of heap()->alloc() is thread-safe, right?
Can you please elaborate why you need malloc/free in addition to 'env()->heap()'? As you may have noticed, the 'cxx' version uses the Genode heap as back end anyway. It is just a wrapper to make libsupc++ happy.
Cheers Norman