# Fortran/memory management
## Introduction and historical background

Most Fortran programs prior to the Fortran90 standard used self-contained data, without structures, and without much in the way of shared, structured data. However, it was possible to share data, in structured and unstructured ways, using common blocks. Furthermore, there used to be little memory management going on in a Fortran program. Until Fortran90 allocated storage wasn't even possible, except via certain extensions (e.g. Cray pointers). Modern Fortran, however, supports many modern programming paradigms, has full support for allocatable data (including allocatable types), and allows for the use of pointers.

## Shared variables in modules

Since Fortran90, shared variables are conveniently managed by the use of modules. Common blocks were used to define global memory prior to the Fortran90 standard; their use in modern Fortran is discouraged. A Fortran module can also contain subroutines and functions, but we shall leave the discussion of these features for later. As for the management of shared variables, they may be defined in a module:
Note that it is considered good practice to declare any module private, even if it contains only public variables. Although save is the default for a variable in a module, meaning that it retains its previous value whenever the variables within the modules are used, it is sometimes considered good practice to make this explicit. The module can then be used in the main program:

## Common blocks

Common blocks have been replaced by the use of public variables in modules in modern Fortran standards (Fortran90 and later). They are, however, historically important due to their use in older Fortran standards (77 and prior). A common block was Fortran's way of using shared, common storage for standards prior to Fortran90. In its simplest form, a common block is a way of defining global memory. Be careful, though. In most languages, each item in common memory is shared as a globally known name separately. In Fortran, however, the common block is a shared thing. I'll show several examples, but each example will share i and another_integer, and my_array, a 10x10 array of real numbers.
In C, for instance, I can define the shared memory using:
and use these data elsewhere with:
Note that one module declares the storage, and another uses the storage. Also note that the definitions and usages are not in the same order. This is because in C, as in most languages, i, another_integer, and my_array are all shared items. Not so in Fortran. In Fortran, all routines sharing this storage would have a definition something like this:
This common block is stored as a block of data, as a linkable named structure. The only problem is that we don't know its name. Various compilers will give various names to this block. In some systems, the block actually doesn't have a name. We can avoid this problem by giving the structure a name. For instance,
Using this form, two different Fortran programs can identify the same area of storage and share it, without having to know the structure of all shared storage. Also using this format, a C or other program could share the storage. For instance, a C program wanting to share this storage would declare the same storage as follows:
In the above example, having the my_block names match is critical, as well as having the types, sizes, and order match. However, having the names internally match is not since these names are known only locally. Also note that in the above example, Fortran's my_array(i,j) matches C's my_block.my_aArray[j][i].

## Byte alignment

Byte alignment of intrinsic data types can mostly be ensured simply by using the appropriate kind. Fortran does not have any way of automatically ensuring derived data types are byte aligned. However, it is quite simple for the programmer to ensure that appropriate padding for data is inserted. For example, let's say we have a derived type that contains a character and an integer
Arrays of this type will have elements of size 5 bytes. If we want the elements of an array of this type to align every 8 bytes we need to add 3 more bytes of padding. We can do this by adding characters that serve no other purpose than as padding.

## Memory management with pointers

In Fortran one can use pointers as some kind of alias for other data, e.g. such as a row in a matrix.

### Pointer states

Each pointer is in one of the following states
* undefined: right after definition if it has not been initialized
* defined
** null/not associated: not the alias of any data
** associated: alias of some data.
The intrinsic function associated distinguished between the second and third states.

### Assignments
#### Overview

We will use the following example: Let a pointer ptr be the alias of some real value x.
For the next example we will use a real matrix matr as target and the pointer ptr should alias a specific row.
Pointers can also be appointed to other pointers. This causes them to be an alias of the same data that the first pointer is. See the example below.

#### Ordinary vs. pointer assignments

The difference between ordinary and pointer assignments of pointers can be explained by the following equalities. Assume this setup
Ordinary assignments of pointers lead to assignments of the data they point to. One can see this by the following two statements which are equal.
In contrast, pointer assignments changes the alias of one of the pointers and no change on the underlying data. See the equal example statements.

### Memory allocation

After definition of pointers one can allocate memory for it using the allocate command. The memory pointed to by a pointer is given free again by the deallocate command. See the following example.

### Examples
#### Allocatable vs. pointer

You can declare an array to have a known number of dimensions, but an unknown size using allocation:
You can also declare something as a pointer:
In archaic versions of FORTRAN (77 and before), you'd just have a big static array and use whatever portion of it you need.