Overriding Malloc

Overriding the standard `malloc` can be done either dynamically or statically.

## Dynamic override

This is the recommended way to override the standard malloc interface.

### Linux, BSD

On these systems we preload the mimalloc shared library so all calls to the standard `malloc` interface are resolved to the mimalloc library.

- `env LD_PRELOAD=/usr/lib/libmimalloc.so myprogram`

You can set extra environment variables to check that mimalloc is running, like:

env MIMALLOC_VERBOSE=1 LD_PRELOAD=/usr/lib/libmimalloc.so myprogram

or run with the debug version to get detailed statistics:

env MIMALLOC_SHOW_STATS=1 LD_PRELOAD=/usr/lib/libmimalloc-debug.so myprogram

### MacOS

On macOS we can also preload the mimalloc shared library so all calls to the standard `malloc` interface are resolved to the mimalloc library.

- `env DYLD_FORCE_FLAT_NAMESPACE=1 DYLD_INSERT_LIBRARIES=/usr/lib/libmimalloc.dylib myprogram`

Note that certain security restrictions may apply when doing this from the shell.

(Note: macOS support for dynamic overriding is recent, please report any issues.)

### Windows

Overriding on Windows is robust and has the particular advantage to be able to redirect all malloc/free calls that go through the (dynamic) C runtime allocator, including those from other DLL's or libraries.

The overriding on Windows requires that you link your program explicitly with the mimalloc DLL and use the C-runtime library as a DLL (using the `/MD` or `/MDd` switch). Also, the `mimalloc-redirect.dll` (or `mimalloc-redirect32.dll`) must be available in the same folder as the main `mimalloc-override.dll` at runtime (as it is a dependency). The redirection DLL ensures that all calls to the C runtime malloc API get redirected to mimalloc (in `mimalloc-override.dll`).

To ensure the mimalloc DLL is loaded at run-time it is easiest to insert some call to the mimalloc API in the `main` function, like `mi_version()` (or use the `/INCLUDE:mi_version` switch on the linker). See the `mimalloc-override-test` project for an example on how to use this. For best performance on Windows with C++, it is also recommended to also override the `new`/`delete` operations (by including `mimalloc-new-delete.h` a single(!) source file in your project without linking to the mimalloc library).

The environment variable `MIMALLOC_DISABLE_REDIRECT=1` can be used to disable dynamic overriding at run-time. Use `MIMALLOC_VERBOSE=1` to check if mimalloc was successfully redirected.

(Note: in principle, it is possible to even patch existing executables without any recompilation if they are linked with the dynamic C runtime (`ucrtbase.dll`) – just put the `mimalloc-override.dll` into the import table (and put `mimalloc-redirect.dll` in the same folder) Such patching can be done for example with CFF Explorer).

## Static override

On Unix systems, you can also statically link with mimalloc to override the standard malloc interface. The recommended way is to link the final program with the mimalloc single object file (`mimalloc-override.o`). We use an object file instead of a library file as linkers give preference to that over archives to resolve symbols. To ensure that the standard malloc interface resolves to the mimalloc library, link it as the first object file. For example:

gcc -o myprogram mimalloc-override.o myfile1.c ...

## List of Overrides:

The specific functions that get redirected to the mimalloc library are:

// C
void* malloc(size_t size);
void* calloc(size_t size, size_t n);
void* realloc(void* p, size_t newsize);
void free(void* p);
void* aligned_alloc(size_t alignment, size_t size);
char* strdup(const char* s);
char* strndup(const char* s, size_t n);
char* realpath(const char* fname, char* resolved_name);
// C++
void operator delete(void* p);
void operator delete[](void* p);
void* operator new(std::size_t n) noexcept(false);
void* operator new[](std::size_t n) noexcept(false);
void* operator new( std::size_t n, std::align_val_t align) noexcept(false);
void* operator new[]( std::size_t n, std::align_val_t align) noexcept(false);
void* operator new ( std::size_t count, const std::nothrow_t& tag);
void* operator new[]( std::size_t count, const std::nothrow_t& tag);
void* operator new ( std::size_t count, std::align_val_t al, const std::nothrow_t&);
void* operator new[]( std::size_t count, std::align_val_t al, const std::nothrow_t&);
// Posix
int posix_memalign(void** p, size_t alignment, size_t size);
// Linux
void* memalign(size_t alignment, size_t size);
void* valloc(size_t size);
void* pvalloc(size_t size);
size_t malloc_usable_size(void *p);
void* reallocf(void* p, size_t newsize);
// macOS
void vfree(void* p);
size_t malloc_size(const void* p);
size_t malloc_good_size(size_t size);
// BSD
void* reallocarray( void* p, size_t count, size_t size );
void* reallocf(void* p, size_t newsize);
void cfree(void* p);
// NetBSD
int reallocarr(void* p, size_t count, size_t size);
// Windows
void* _expand(void* p, size_t newsize);
size_t _msize(void* p);
void* _malloc_dbg(size_t size, int block_type, const char* fname, int line);
void* _realloc_dbg(void* p, size_t newsize, int block_type, const char* fname, int line);
void* _calloc_dbg(size_t count, size_t size, int block_type, const char* fname, int line);
void* _expand_dbg(void* p, size_t size, int block_type, const char* fname, int line);
size_t _msize_dbg(void* p, int block_type);
void _free_dbg(void* p, int block_type);