C Interface to Scalable Allocator


Low-level interface for scalable memory allocation.

// Defined in header <oneapi/tbb/scalable_allocator.h>

extern "C" {
    // Scalable analogs of C memory allocator
    void* scalable_malloc( size_t size );
    void  scalable_free( void* ptr );
    void* scalable_calloc( size_t nobj, size_t size );
    void* scalable_realloc( void* ptr, size_t size );

    // Analog of _msize/malloc_size/malloc_usable_size.
    size_t scalable_msize( void* ptr );

    // Scalable analog of posix_memalign
    int scalable_posix_memalign( void** memptr, size_t alignment, size_t size );

    // Aligned allocation
    void* scalable_aligned_malloc( size_t size, size_t alignment);
    void scalable_aligned_free( void* ptr );
    void* scalable_aligned_realloc( void* ptr, size_t size, size_t alignment );

    // Return values for scalable_allocation_* functions
    typedef enum {
    } ScalableAllocationResult;

    typedef enum {
        // To turn on/off the use of huge memory pages
        // To set a threshold for the allocator memory usage.
        // Exceeding it will forcefully clean internal memory buffers
        // Lower bound for the size (Bytes), that is interpreted as huge
        // and not released during regular cleanup operations
    } AllocationModeParam;

    // Set allocator-specific allocation modes.
    int scalable_allocation_mode(int param, intptr_t value);

    typedef enum {
        // Clean internal allocator buffers for all threads.
        // Clean internal allocator buffer for current thread only.
    } ScalableAllocationCmd;

    // Call allocator-specific commands.
    int scalable_allocation_command(int cmd, void *param);

These functions provide a C-level interface to the scalable allocator. With the exception of scalable_allocation_mode and scalable_allocation_command, each routine scalable_x behaves analogously to the library function x. The routines form the two families shown in the table below, “C Interface to Scalable Allocator”. Storage allocated by a scalable_x function in one family must be freed or resized by the scalable_x function in the same family, not by a C standard library function. Likewise, storage allocated by a C standard library function should not be freed or resized by a scalable_x function.

C Interface to Scalable Allocator

Allocation Routine

Deallocation Routine

Analogous Library



C standard library







Microsoft* C run-time library


The following functions do not allocate or free memory but allow obtaining useful information or influencing behavior of the memory allocator.

size_t scalable_msize(void *ptr)

Returns: The usable size of the memory block pointed to by ptr if it was allocated by the scalable allocator. Returns zero if ptr does not point to such a block.

int scalable_allocation_mode(int mode, intptr_t value)

Use this function to adjust behavior of the scalable memory allocator.

Returns: TBBMALLOC_OK if the operation succeeded, TBBMALLOC_INVALID_PARAM if mode is not one of the described below, or if value is not valid for the given mode. Other return values are possible, as described below.

scalable_allocation_mode Parameters: Parameter, Description


scalable_allocation_mode(TBBMALLOC_USE_HUGE_PAGES, 1) tells the allocator to use huge pages if enabled by the operating system. scalable_allocation_mode(TBBMALLOC_USE_HUGE_PAGES, 0) disables it. Setting TBB_MALLOC_USE_HUGE_PAGES environment variable to 1 has the same effect as scalable_allocation_mode(TBBMALLOC_USE_HUGE_PAGES, 1). The mode set with scalable_allocation_mode() takes priority over the environment variable.

May return: TBBMALLOC_NO_EFFECT if huge pages are not supported on the platform.

For now, this allocation mode is only supported for Linux* OS. It works with both explicitly configured and transparent huge pages. For information about enabling and configuring huge pages, refer to OS documentation or ask your system administrator.


scalable_allocation_mode(TBBMALLOC_SET_SOFT_HEAP_LIMIT, size) sets a threshold of size bytes on the amount of memory the allocator takes from OS. Exceeding the threshold urges the allocator to release memory from its internal buffers; however it does not prevent from requesting more memory if needed.


scalable_allocation_mode(TBBMALLOC_SET_HUGE_SIZE_THRESHOLD, size) sets a lower bound threshold (with no upper limit) of size bytes. Any object bigger than this threshold becomes huge and does not participate in internal periodic cleanup logic. However, it does not affect the logic of the TBBMALLOC_SET_SOFT_HEAP_LIMIT mode as well as the TBBMALLOC_CLEAN_ALL_BUFFERS operation.

Setting TBB_MALLOC_SET_HUGE_SIZE_THRESHOLD environment variable to the size value has the same effect, but is limited to the LONG_MAX value. The mode set with scalable_allocation_mode takes priority over the environment variable.

int scalable_allocation_command(int cmd, void *reserved)

This function may be used to command the scalable memory allocator to perform an action specified by the first parameter. The second parameter is reserved and must be set to 0.

Returns: TBBMALLOC_OK if the operation succeeded, TBBMALLOC_INVALID_PARAM if cmd is not one of the described below, or if reserved is not equal to 0.

scalable_allocation_command Parameters: Parameter, Description


scalable_allocation_command(TBBMALLOC_CLEAN_ALL_BUFFERS, 0) cleans internal memory buffers of the allocator, and possibly reduces memory footprint. It may result in increased time for subsequent memory allocation requests. The command is not designed for frequent use, and careful evaluation of the performance impact is recommended.

May return: TBBMALLOC_NO_EFFECT if no buffers were released.


It is not guaranteed that the call will release all unused memory.


scalable_allocation_command(TBBMALLOC_CLEAN_THREAD_BUFFERS, 0) cleans internal memory buffers, but only for the calling thread.

May return: TBBMALLOC_NO_EFFECT if no buffers were released.