Solving Constraint Integer Programs

Using the memory functions of SCIP

SCIP provides three ways for allocating memory.

Standard memory

SCIP provides an access to the standard C functions malloc and free with the additional feature of tracking memory in debug mode. In this way, memory leaks can be easily detected. This feature is automatically activated in debug mode.

The most important functions are

Block memory

SCIP offers its own block memory handling, which allows efficient handling of smaller blocks of memory in cases in which many blocks of the same (small) size appear. This is adaquate for branch-and-cut codes in which small blocks of the same size are allocated and freed very often (for data structures used to store rows or branch-and-bound nodes). Actually, most blocks allocated within SCIP have small sizes like 8, 16, 30, 32, 64. The idea is simple: There is a separate list of memory blocks for each interesting small size. When allocating memory, the list is checked for a free spot in the list; if no such spot exists the list is enlarged. Freeing just sets the block to be available. Very large blocks are handled separatedly. See the dissertation of Tobias Achterberg for more details.

One important comment is that freeing block memory requires the size of the block in order to find the right list.

The most important functions are

An example code is:

SCIP_RETCODE dosomething(
SCIP* scip
int nvars;
int* array;
nvars = SCIPgetNVars(scip);
SCIP_CALL( SCIPallocBlockMemoryArray(scip, &array, nvars) );
do something ...
SCIPfreeBlockMemoryArray(scip, &array, nvars);

Buffer memory

In addition to block memory, SCIP offers buffer memory. This should be used if memory is locally used within a function and freed within the same function. For this purpose, SCIP has a list of memory buffers that are reused for this purpose. In this way, a very efficient allocation/freeing is possible.

The most important functions are

SCIP 3.2 introduced a new type of buffer memory, the clean buffer. It provides memory which is initialized to zero and requires the user to reset the memory to zero before freeing it. This can be used at performance-critical places where only few nonzeros are added to a dense array and removing these nonzeros individually is much faster than clearing the whole array. Same as the normal buffer array, the clean buffer should be used for temporary memory allocated and freed within the same function.

The most important functions are

General notes

The following holds for all three types of memory functions:

  • In debug mode the arguments are checked for overly large allocations (negative sizes are converted into very large values of type size_t).
  • The functions always allocate at least one byte, so that freeing is always possible.
  • The freeing methods set the pointer to the memory to NULL.
  • For maximum speed you should free memory in the reverse order in which it was allocated. For block and buffer memory this significantly speeds up the code.