# Programming rules¶

Here we provide some guidelines for DIRAC programmers.

## Use of languages¶

DIRAC is for historical reasons largely written in Fortran 77 (F77), but in the newer parts of the code you may also see parts in Fortran 90 (F90), C and C++. Fortran 90 programming style with modules and other usefull features is required.

Do not introduce a new language before consulting the authors forum. New languages might require compilers that may be proprietary and consequently not be available to all authors and users.

Likewise do not introduce compiler-dependent extensions, though they are inviting and so convenient. Do not use them. You will regret it later when you have to modify the code for other compilers.

### Fortran 90¶

We require a F90 compiler by default. Therefore all files with .F, .F90, .f, and .f90 suffixes can contain F90 syntax. In .F and .f you can use F77 syntax, not in .F90 and .f90 files. In .F and .f files you can use F90 syntax however respecting the F77 line limits. Files with .F and .F90 suffixes are preprocessed, files with .f and .f90 are not preprocessed. If you introduce new source code file, always use .F90 suffix.

F90 is case insensitive and allows both uppercase and lowercase. It is not necessary to program with the caps lock. Use uppercase and lowercase in a way that makes your code more readable.

“C” at the beginning of empty lines is unnecessary.

## Do not read keywords with table/goto lookups¶

Perhaps 98% of the input reading is done using tables and gotos.

Please do not introduce new keyword sections like that for at least three reasons:

1. It is a nightmare to read and maintain.
2. There is no type checking so the user gets useless error messages or unexpected results.
3. New scheme minimizes conflicts when merging branches.

Rather have a look in src/input. This is the modern way to do it.

## Never allocate memory with MEMGET/MEMREL¶

Large parts of the DIRAC code use MEMGET/MEMREL. Please do not allocate memory using MEMGET/MEMREL. We are trying to get rid of MEMGET/MEMREL.

Memory allocation should be done either using Andre’s alloc/dealloc or using the native allocate/deallocate routines. Alloc/dealloc are frontends to the native allocate/deallocate. The advantage of alloc/dealloc is that the memory allocation gets tracked and shows up in the high water mark. Do not use other home-cooked memory allocation schemes unless you have absolutely no other choice.

Here is an example for alloc/dealloc:

use memory_allocator

real(8), allocatable :: one_dim_array(:)
integer, allocatable :: two_dim_array(:, :)

call alloc(one_dim_array, 1000)
call alloc(two_dim_array, 1000, 500)

...

call dealloc(one_dim_array)
call dealloc(two_dim_array)

Here is an example for allocate/deallocate:

real(8), allocatable :: one_dim_array(:)
integer, allocatable :: two_dim_array(:, :)

allocate(one_dim_array(1000))
allocate(two_dim_array(1000, 500))

...

deallocate(one_dim_array)
deallocate(two_dim_array)
When allocating memory please consider the following:
• The memory is not a bottomless pit. Try to be economic.
• Do not allocate and deallocate inside of a loop that is traversed a billion times. Allocate/deallocate outside.
• If you allocate arrays, clean up after yourself and deallocate data that is not needed anymore as soon as possible to keep the peak allocation as low as possible. Ideally at the end of the run we should have zero allocations. We are very far away from that.

## Never use implicit.h¶

Plating the implicit.h into the code is bad programing practice and I strongly recommend to always use the implicit none statement.

Reasons: 1. With implicit.h it is easy to use variables that are undefined. Example:

call daxpy(size(array1), D1, array1, 1, array2, 2)

if one forgets to define the parameter D1, the code will compile and D1 can be any number so the result is undefined. With “implicit none” this will not compile.

Another example is a typo: istead of

NDIM = 12  ! correct code

you could type

NDIN = 12  ! typo

With implicit none such mistake won’t compile, with the implicit.h the code will compile and behave unexpectedly. In this case also the know compiler flag “-Wuninitialized” won’t help!

In the code we had (and unfortunatelly still have( many bugs like this and some of us have spent several long afternoons chasing them. The insertions of “implicit.h” is too dangerous and also experienced programmers make typos.

2. The insertion of implicit none is nice for other people reading your code. They can see which variables are local, which are global (from common blocks). Without implicit none you have no idea whether this is a local or a global variable if you don’t know the common blocks by heart.

3. The command implicit none makes it easier to identify include statements that are really used and includes that are just included but not used. With implicit.h you can comment out includes and the code may still compile. Contrary, the implicit none will warn you. If you remove an include file using implicit none and the code compiles, then this include was useless.

The time that you gain with implicit.h, you will spend it in debugging bugs that implicit none would have detected. Implicit none makes it easier for other people to reuse your routines. With implicit none they know exactly which variables are local to the routine and which are passed via header files or modules. With implicit.h they are completely in the blind.

## Integer kinds¶

integer :: variable

instead of KIND restricted integer variables

integer(KIND=4) :: variable
integer(KIND=8) :: variable

This is because we need flexibility for integer variables: compilation of the DIRAC suite goes also with predefined integer*8 variables (through -i8 Fortranflag), for what all integer variables in the code must be declared without KIND restriction.

Radovan: I don’t agree. I think we should do the exact oposite. We would avoid a lot of trouble if we would use well defined kinds.

## Interface Fortran and C using iso_c_binding¶

This is robust and portable. The iso_c_binding is supported by all modern compilers. Matching types at configure time with fortran_interface.h is painful and not robust.

## If you replace a code remove the old code¶

Do not keep old code “just to be sure”. We have the well preserved Git history. Cut dead wood.

Don’t check in commented out lines of code. It will make it harder for other developers to modify or improve your code because they cannot know whether the commented code is important or not. If you really have to leave commented code you should document exactly why you feel that the old code has to stay as a comment inside the source file.

## Never commit new functionality without tests¶

Functionality without tests will break sooner or later and nobody will notice for long time.

Also with tests you make it easier to understand what your code does and you make it easier for other people to improve your code. If there are no tests, then others will be afraid to modify untested code.

## Make sure that new code compiles with and without MPI¶

Before you push new code to the main line development, test whether it compiles with and without MPI.