Short answer
Software copyright protects expression, not what the program does.
"Protection in accordance with this Directive shall apply to the expression in any form of a computer program. Ideas and principles which underlie any element of a computer program, including those which underlie its interfaces, are not protected by copyright under this Directive." —
In SAS Institute (C-406/10) the Court of Justice applied that to three things a competitor might want to replicate: the functionality of a program, the programming language, and the format of data files used to exploit its functions. None of them constitutes a form of expression of the program, and none is protected by copyright in computer programs.
What is protected is the source code and the object code — the forms of expression that permit the program to be reproduced — together with preparatory design work of a nature capable of leading to a program at a later stage.
The important caveat: "not protected as a computer program" is not the same as "free to copy". Several other rights can apply, and the Court in SAS Institute was careful to leave one of them open.
What the Court actually reasoned
The Court grounded the distinction in the Directive's own recitals and in international law.
Recital 14 of the predecessor Directive confirms that to the extent that logic, algorithms and programming languages comprise ideas and principles, those ideas and principles are not protected. Recital 15 states that the expression of those ideas and principles is protected.
The Court also noted that Article 2 of the WIPO Copyright Treaty and Article 9(2) of the TRIPs Agreement both provide that copyright protection extends to expressions and not to ideas, procedures, methods of operation or mathematical concepts as such, and that Article 10(1) of TRIPs requires computer programs, in source or object code, to be protected as literary works.
Following Bezpečnostní softwarová asociace (C-393/09), the object of protection is the expression in any form of a computer program, such as the source code and the object code, which permits reproduction in different computer languages.
So the boundary is drawn where it is in copyright generally — at the line between idea and expression — and the Directive states expressly that it runs through interfaces too.
What this permits, and what it does not
Permitted in principle:
- Writing an independent program that does the same things.
- Replicating behaviour, outputs and results.
- Reading a manual or specification and implementing the described functionality afresh.
- Using the same programming language.
- Reading and writing the same data-file formats for interoperability.
Not permitted:
- Copying source or object code, in whole or in part.
- Translating, adapting or arranging the code — Article 4(1)(b) covers translation, adaptation, arrangement and any other alteration, and the reproduction of the results.
- Copying preparatory design work capable of leading to the program.
- Reproducing the manual or documentation, which is protected as a literary work in its own right.
That last point is easy to miss. SAS Institute separately addressed whether reproducing keywords and syntax in a manual describing the second program could infringe copyright in the first program's manual. Documentation is a distinct work with distinct protection.
The right to study the program
The Directive backs the idea/expression line with an enforceable user right:
"The person having a right to use a copy of a computer program shall be entitled, without the authorisation of the rightholder, to observe, study or test the functioning of the program in order to determine the ideas and principles which underlie any element of the program if he does so while performing any of the acts of loading, displaying, running, transmitting or storing the program which he is entitled to do." —
Article 8 makes any contractual provision contrary to Article 5(3) null and void. A licence clause banning benchmarking, behavioural analysis or reverse analysis of this kind does not work.
The limit is that the study must be done while performing acts the user is entitled to perform. It does not license decompilation, which has its own regime in Article 6 with narrower conditions — indispensability for interoperability of an independently created program, information not previously readily available, confined to the necessary parts, and restrictions in Article 6(2) on how the information obtained may be used.
Where other rights still bite
This is the part that a bare "functionality is not protected" summary gets wrong.
Copyright as an ordinary work. The Court in SAS Institute considered whether elements falling outside software-specific protection might nonetheless be protected under the general InfoSoc framework where they constitute the author's own intellectual creation. Graphical user interfaces were analysed on that footing in BSA (C-393/09), and the Court has returned to the distinction more recently in C-159/23 (2024). So a data-file format or a set of syntax elements may fail as a "computer program" and still raise a question as an original work.
Patents. Software-implemented inventions can be patentable in Europe where they produce a technical effect, notwithstanding the exclusion of programs for computers "as such". A functional feature outside copyright may be inside a patent claim.
Trade secrets. Directive (EU) 2016/943 protects undisclosed know-how and business information meeting its conditions. Algorithms and internal designs obtained improperly may be actionable even though they are unprotected ideas for copyright purposes.
Database rights. Where the program is bundled with a protected database, the sui generis right under Directive 96/9/EC applies independently.
Trade marks and designs. Names, logos and the registered appearance of an interface are separate rights.
Contract. Licence restrictions bind as a matter of contract, subject to Article 8's nullity of clauses contrary to Articles 5(2), 5(3) and 6. A restriction Article 8 does not reach may still sound in damages for breach.
Unfair competition. National regimes vary and can catch conduct that copyright does not.
Practical guidance for a clean-room reimplementation
- Separate the team that studies the original from the team that writes the new code.
- Record the entitlement to use the original — Article 5(3) requires a right to use.
- Document what was observed at the level of ideas and principles, not code.
- Do not import source code, object code, or preparatory design material.
- Do not copy documentation text; write your own.
- Keep contemporaneous records of the specification handover; the evidential trail is the defence.
- Run a separate patent and trade-secret check before launch.
- Check what the licence actually restricts, and which restrictions Article 8 voids.
Conclusion
EU software copyright protects the way a program is written, not what it does. SAS Institute is explicit that functionality, programming languages and data-file formats are not forms of expression of a computer program, and Article 1(2) says the same about the ideas and principles underlying interfaces.
That gives genuine room for independent reimplementation, reinforced by an Article 5(3) study right that no licence can remove. But "not protected as a computer program" is a narrow finding. Patents, trade secrets, database rights, documentation copyright and general copyright in original elements all remain available, and a reimplementation project should be cleared against each of them rather than against copyright alone.
Sources
This article provides general legal information, not advice for a specific matter. Laws, facts and procedural deadlines may change the result. Follow the linked primary sources and obtain local advice before acting.