The Colostate Fortran Convention

This document is inspired from the FORTRAN Coding conventions that could be read in details on the OMS Documentation Wiki page

The purpose of this document is to ensure that new Fortran code will be as portable and robust as possible, as well as consistent throughout the system. It builds upon commonly shared experience to avoid error-prone practices and gathers guidelines that are known to make codes more robust.

This document covers items in order of decreasing importance (see below), deemed to be important for any code. It is recognized in the spirit of this standard that certain suggestions which make code easier to read for some people (e.g. lining up attributes, or using all lower case or mixed case) are subjective and therefore should not have the same weight as techniques and practices that are known to improve code quality. For this reason, the standards within this document are divided into three components; Standards, Guidelines and Recommendations:

  • Required: Aimed at ensuring portability, readability and robustness. Compliance with this category is mandatory.

  • Recommended: Good practices. Compliance with this category is strongly encouraged. The case for deviations will need to be argued by the programmer.

  • Encouraged: Compliance with this category is optional, but is encouraged for consistency purposes.

Depending on the projects, programmer may opt to adhere to all three levels or just the two-first. All projects must adhere at least to the mandatory standards.

Encouraged Features

These usually help in the robustness of the code (by checking interface compatibility for example) and in the readability, maintainability and portability. They are reminded here:

  • Encapsulation: Use of modules for procedures, functions, data.

  • Use Dynamic Memory allocation for optimal memory usage.

  • Derived types or structures which generally lead to stable interfaces, optimal memory usage, compactness, etc.

  • Optional and keyword arguments in using routines.

  • Functions/subroutines/operators overloading capability.

  • Intrinsic functions: bits, arrays manipulations, kinds definitions, etc.

Interoperability and Portability


Source code must conform to the ISO Fortran95 standard. No compiler- or platform-dependent extensions shall be used. No use shall be made of compiler-dependent error specifier values (e.g. IOSTAT or STAT values). Source code must compiled and run under gfortran that is part of the GNU Compiler Collection.


For applications requiring interaction with independently-developed frameworks, the use of KIND type for all variables declaration is encouraged to facilitate the integration.



  • Use free format syntax.

  • Use consistent indentation across the code. Each level of indentation should use at least two spaces.

  • Use modules to organize source code.

  • FORTRAN keywords (e.g., DATA) shall not be used as variable names.

  • Use meaningful, understandable names for variables and parameters. Recognized abbreviations are acceptable as a means of preventing variable names getting too long.

  • Each externally-called function, subroutine, should contain a header. The content and style of the header should be consistent across the system, and should include the functionality of the function, as well as the description of the arguments, the author(s) names. A header could be replaced by a limited number of descriptive comments for small subroutines.

  • Magic numbers should be avoided; physical constants (e.g., pi, gas constants) should never be hardwired into the executable portion of a code; use PARAMETER statements instead.

  • Hard-coded numbers should be avoided when passed through argument lists since a compiler flag, which defines a default precision for constants, cannot be guaranteed.


  • Use construct names to name loops, to increase readability, especially in nested loops.

  • Similarly, use construct names in subroutines, functions, main programs, modules, operator, interface, etc.

  • Include comments to describe the input, output and local variables of all procedures. Grouping comments for similar variables is acceptable when their names are explicit enough.

  • Use comments as required to delineate significant functional sections of code.

  • Do not use FORTRAN statements and intrinsic function names as symbolic names.

  • Use named parameters instead of “magic numbers”; REAL, PARAMETER :: PI=3.14159, ONE=1.0

  • Do not use GOTO statements. These are hard to maintain and complicate understanding the code. If absolutely necessary to use GOTO (if using other constructs complicates the code structure), thoroughly document the use of the GOTO.


  • When writing new code, adhere to the style standards within your own coding style. When modifying an old code, adhere to the style of the existing code to keep consistency.

  • Use the same indentation for comments as for the rest of the code.

  • Functions, procedures, data that are naturally linked should be grouped in modules.

  • Limit to 80 the number of characters per line (maximum allowed under ISO is 132)

  • Use of operators <, >, <=, >=, ==, /= is encouraged (for readability) instead of .lt., .gt., .le., .ge., .eq., .ne.

  • Modules should be named the same name as the files they reside in: To simplify the makefiles that compile them. Consequently, multiple modules in a single file are to be avoided where possible.

  • Use blanks to improve the appearance of the code, to separate syntactic elements (on either side of equal signs, etc) in type declaration statements

  • Always use the :: notation, even if there are no attributes.

  • Line up vertically: attributes, variables, comments within the variables declaration section.

  • Remove unused variables

  • Remove code that was used for debugging once this is complete.



  • Use Implicit NONE in all codes: main programs, modules, etc. To ensure correct size and type declarations of variables/arrays.

  • Use PRIVATE in modules before explicitly listing data, functions, procedures to be PUBLIC. This ensures encapsulation of modules and avoids potential naming conflicts. Exception to previous statement is when a module is entirely dedicated to PUBLIC data/functions (e.g. a module dedicated to constants).

  • Initialize all variables. Do not assume machine default value assignments.

  • Do not initialize variables of one type with values of another.


  • Do not use the operators == and /= with floating-point expressions as operands. Check instead the departure of the difference from a pre-defined numerical accuracy threshold (e.g. epsilon comparison).

  • In mixed mode expressions and assignments (where variables of different types are mixed), the type conversions should be written explicitly (not assumed). Do not compare expressions of different types for instance. Explicitly perform the type conversion first.

  • No include files should be used. Use modules instead, with USE statements in calling programs.

  • Structures (derived types) should be defined within their own module. Procedures, Functions to manipulate these structures should also be defined within this module, to form an object-like entity.

  • Procedures should be logically flat (should focus on a particular functionality, not several ones)

  • Module PUBLIC variables (global variables) should be used with care and mostly for static or infrequently varying data.


  • Use parentheses at all times to control evaluation order in expressions.

  • Use of structures is encouraged for a more stable interface and a more compact design. Refer to structure contents with the % sign (e.g. Mixture%Enthalpy).



  • Subscript expressions should be of type integer only.

  • When arrays are passed as arguments, code should not assume any particular passing mechanism.


  • Use of arrays is encouraged as well as intrinsinc functions to manipulate them.

  • Use of assumed shapes is fine in passing vectors/arrays to functions/arrays.


  • Declare DIMENSION for all non-scalars

Dynamic Memory Allocation / Pointers


  • Use of allocatable arrays is preferred to using pointers, when possible. To minimize risks of memory leaks and heap fragmentation.

  • Use of pointers is allowed when declaring an array in a subroutine and making it available to a calling program.

  • Always initialize pointer variables in their declaration statement using the NULL() intrinsic. INTEGER, POINTER :: x=> NULL()

  • The preferable mechanism for dynamic memory allocation is automatic arrays, as opposed to ALLOCATABLE or POINTER arrays for which memory must be explicitly allocated and deallocated; space allocated using ALLOCATABLE or POINTER must be explicitly freed using the DEALLOCATE statement.


  • Always deallocate allocated pointers and arrays. This is especially important inside subroutines and inside loops.

  • Always test the success of a dynamic memory allocation and deallocation - the ALLOCATE and DEALLOCATE statements have an optional argument to allow this.

  • In a given program unit do not repeatedly ALLOCATE space, DEALLOCATE it and then ALLOCATE a larger block of space - this will almost certainly generate large amounts of unusable memory.


  • Use of dynamic memory allocation is encouraged. It makes code generic and avoids declaring with maximum dimensions.

  • For simplicity, use Automatic arrays in subroutines whenever possible, instead of allocatable arrays.



  • Do not use GOTO to exit/cycle loops, use instead EXIT or CYCLE statements.


  • No numbered DO loops such as (DO 10 ...10 CONTINUE).



  • The SAVE statement is discouraged; use module variables for state saving.

  • Do not use an entry in a function subprogram.

  • Functions must not have pointer results.

  • The names of intrinsic functions (e.g., SUM) shall not be used for user-defined functions.

  • Procedures that return a single value should be functions; note that single values could also be user-defined types.

  • All communication with the module should be through the argument list or it should access its module variables.


  • All dummy arguments, except pointers, should include the INTENT clause in their declaration

  • Limit use of type specific intrinsic functions (e.g., AMAX, DMAX - use MAX in all cases).

  • Avoid statically dimensioned array arguments in a function/subroutine.

  • Check for invalid argument values.


  • Error conditions. When an error condition occurs inside a function/procedure, a message describing what went wrong should be printed. The name of the routine in which the error occurred must be included. It is acceptable to terminate execution within a package, but the developer may instead wish to return an error flag through the argument list.

  • Functions/procedures that perform the same function but for different types/sizes of arguments, should be overloaded, to minimize duplication and ease the maintainability.

  • When explicit interfaces are needed, use modules, or contain the subroutines in the calling programs (through CONTAINS statement), for simplicity.

  • Do not use external routines as these need interface blocks that would need to be updated each time the interface of the external routine is changed.



  • I/O statements on external files should contain the status specifier parameters err=, end=, iostat=, as appropriate.

  • All global variables, if present, should be set at the initialization stage.


  • Avoid using NAMELIST I/O if possible.

  • Use write rather than print statements for non-terminal I/O.

  • Use Character parameters or explicit format specifiers inside the Read or Write statement. Do not use labeled format statements (outdated).

Fortran Features that are obsolescent and/or discouraged:


  • No Common blocks. Modules are a better way to declare/store static data, with the added ability to mix data of various types, and to limit access to contained variables through use of the ONLY and PRIVATE clauses.

  • No assigned and computed GOTOs - use the CASE construct instead

  • No arithmetic IF statements - use the block IF construct instead

  • Use REAL instead of DOUBLE PRECISION


  • Branch to END IF outside the block IF

  • DO non-integer Control

  • Hollerith Constants


  • multiple RETURN

  • Alternate RETURN


  • Do not make use of the equivalence statement, especially for variables of different types. Use pointers or derived types instead.


  • No implicitly changing the shape of an array when passing it into a subroutine. Although actually forbidden in the standard it was very common practice in FORTRAN 77 to pass N-dimensional arrays into a subroutine where they would, say, be treated as a 1-dimensional array. This practice, though banned in Fortran 90, is still possible possible with external routines for which no Interface block has been supplied. This only works because of assumptions made about how the data is stored.

Source Files


  • Document the function interface: argument name, type, unit, description, constraint, defaults.

  • The INCLUDE statement shall not be used; use the USE statement instead.


  • Try to limit source column length, including comments, to 80 columns (or follow language specific limits).

  • A component should not exceed 300-500 effective lines of code, be efficient with your coding.

  • Use blank lines (or lines with a standard character in column 1) to separate statement blocks to improve code readability.

  • Apply consistent indentation method for code.

  • Module/subprogram names shall be lower case; the name of a file containing a module/subprogram shall be the module/subprogram name with the suffix *.f90.


  • Clearly separate declaration of argument variables from declaration of local variables.

  • Use descriptive and unique names for variables and subprograms (so as to improve the code readability and facilitate global string search);

  • try to limit name lengths to 12-15 characters.

  • Indent continuation lines to ensure that, for example, parts of a multi-line equation line up in a readable manner.

  • Start comment text with a standard character (e.g. !, C, etc.); if a stand-alone line then start comment character in the first column.

General Coding Guidelines

  • Reduce or eliminate global variable usage.

  • Attempt to limit the number of arguments in argument list - long lists make it hard to reuse.

  • Limit of only one return point per component.

  • Use exceptions as error indicators if supported.

  • Components should be specific to one and only one purpose.

  • Components with side effects are not allowed (e.g. don’t mix I/O code with computational code).

  • Program against a standard (e.g., ANSI C, C++, Java, FORTRAN 77/90/95)

  • Make sure your code compiles under different compilers and platforms.

  • Use preprocessor directives for adaptation to different architectures/compilers/OS.

  • Make I/O specific components separate from computatonal components.

  • Avoid static allocation of data (compile time allocation).

  • Be most specific with your data types.

  • Avoid using custom data types for argument types.fp