Dynamic Allocation

Data objects can be static or dynamic. If a data object is static, a fixed amount of memory storage is created for it at compile time and is not freed until the program exits. If a data object is dynamic, memory storage for the object can be created (allocated), altered, or freed (deallocated) as a program executes.

In Fortran 95/90, pointers, allocatable arrays, and automatic arrays are dynamic data objects.

No storage space is created for a pointer until it is allocated with an ALLOCATE statement or until it is assigned to a allocated target. A pointer can be dynamically disassociated from a target by using a NULLIFY statement.

An ALLOCATE statement can also be used to create storage for an allocatable array. A DEALLOCATE statement is used to free the storage space reserved in a previous ALLOCATE statement.

Automatic arrays differ from allocatable arrays in that they are automatically allocated and deallocated whenever you enter or leave a procedure, respectively.

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Note: Dynamic memory allocation is limited by several factors, including swap file size and memory requirements of other applications that are running. Dynamic allocations that are too large or otherwise attempt to use the protected memory of other applications result in General Protection Fault errors. If you encounter an unexpectedly low limit, you might need to reset your virtual memory size through the Control Panel or redefine the swap file size.

Some programming techniques can help minimize memory requirements, such as using one large array instead of two or more individual arrays. Allocated arrays that are no longer needed should be deallocated.

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This chapter contains information on the following topics:

• The ALLOCATE Statement
• The DEALLOCATE Statement
• The NULLIFY Statement

For More Information:

• See Pointer Assignments.
• See Automatic arrays.
• See the NULL intrinsic function, which can also be used to disassociate a pointer.