Source-code compatible usually means that source written for a defined API or interface can be compiled for another supported implementation or version, often after recompilation and sometimes with limited edits. It does not, by itself, promise that an already-compiled program will run, that behavior will be identical, or that performance and build files will carry over.
The phrase has no single universal scope. To know what a compatibility claim covers, check the project’s definition, supported versions and targets, and any listed exceptions.
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What does source-code compatible mean?
Source code is the human-readable program text before it is compiled. A source-compatibility claim says that text written against a specified interface—typically an API or standard—can be built for another supported implementation or environment. A rebuild is normally allowed or required; the compiled output need not be interchangeable.
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For example, Open MPI describes source-code compatibility as API compatibility for compliant applications using supported MPI standard versions. That is a scoped promise, not a guarantee that every application or every version combination will work. See Open MPI’s version-numbering and compatibility documentation.
Compatibility claims can also be narrower than they sound. NXP’s C-Ware API User Guide attributes to C-Port Corporation a definition centered on recompiling source for a target chip and tools. The guide says a compile-time flag may be needed and excludes guarantees about performance, memory consumption, microcode, Makefiles, directory structure, and bug-for-bug behavior. This is an example from that guide, not a statement of NXP’s current policy. See the C-Ware API User Guide.
How is source compatibility different from binary compatibility?
Source compatibility concerns whether source can be compiled against an interface. Binary compatibility concerns whether compiled components can link or run together under the relevant binary interface, or ABI. These are separate properties: source may compile after a rebuild even when previously compiled objects cannot be reused.
Coin3D illustrates the distinction: it documents source compatibility across the Open Inventor 2.1 API, with implementations selectable at build time, while warning that those implementations are not ABI compatible. See Coin3D’s documentation. Do not infer binary compatibility from a source-compatibility label.
Does source compatibility mean the program behaves the same?
Not necessarily. A program can compile successfully and still produce different results, encounter different runtime conditions, or perform differently. Behavioral compatibility concerns user-observable behavior; performance and memory use are additional concerns that may have their own guarantees or exclusions.
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Open MPI’s documentation treats user-observable behavior as part of its broader description of backward compatibility, distinct from its source/API compatibility discussion. The C-Ware guide, by contrast, expressly excludes performance and memory guarantees. Read the claim’s own boundaries rather than treating successful compilation as proof of equivalent operation.
What does a standards-based compatibility claim cover?
A standard can define a shared interface that helps software compile across implementations, but the usefulness of the claim depends on the standard version, conformance, and the features the application actually uses. Debian’s FAQ identifies POSIX as a major basis for source-code compatibility among Unix-like systems while making clear that broad compatibility does not establish complete compatibility for every application. See the Debian FAQ.
Compatibility can also be defined for a particular software layer rather than an entire system. AUTOSAR Classic Platform R23-11 requires source-code compatibility among RTE operating modes at the software-component level when source is available. Its specification distinguishes that case from object-code components, which can have additional constraints related to the RTE generator mode. See the AUTOSAR Classic Platform R23-11 RTE specification.
How to evaluate a source-compatibility promise
Before relying on the phrase, find the project’s compatibility policy and pin down what it actually covers. Check each dimension separately:
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- Interface and versions: Which API or standard, versions, and feature subset are supported?
- Targets and tools: Which platforms, chips, compilers, and toolchains are in scope?
- Required changes: Must the source compile unchanged, or are edits, flags, generated files, or configuration changes required?
- Build process: Is recompilation expected for each target? Are build scripts, Makefiles, generated code, or directory layouts included in the promise?
- Compiled artifacts: Is ABI compatibility separately promised, including any release-specific or language-specific exceptions?
- Runtime guarantees: Does the claim cover observable behavior, performance, memory use, or only successful compilation?
- Exclusions: Which cases, features, versions, or artifacts does the policy explicitly leave out?
The practical interpretation is simple: source-code compatible means compatible at the source/interface level within a stated scope. Treat everything beyond that—especially binary reuse and identical runtime behavior—as a separate claim that needs separate evidence.
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