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.

Unix, BSD, macOS

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 (on Linux, BSD, etc.)

• env DYLD_INSERT_LIBRARIES=usr/lib/libmimalloc.dylib myprogram (On macOS)

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

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

Windows

On Windows you need to link your program explicitly with the mimalloc DLL, and use the C-runtime library as a DLL (the /MD or /MDd switch). To ensure the mimalloc DLL gets loaded it is easiest to insert some call to the mimalloc API in the main function, like mi_version().

Due to the way mimalloc intercepts the standard malloc at runtime, it is best to link to the mimalloc import library first on the command line so it gets loaded right after the universal C runtime DLL (ucrtbase). See the mimalloc-override-test project for an example.

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);

// 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*  aligned_alloc(size_t alignment, size_t size);
void*  valloc(size_t size);
void*  pvalloc(size_t size);
size_t malloc_usable_size(void *p);

// BSD
void*  reallocarray( void* p, size_t count, size_t size );
void*  reallocf(void* p, size_t newsize);
void   cfree(void* p);

// 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);