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  • APR's Version Numbering

    This document covers how the APR projects are versioned. Since the APR projects are libraries, it is very important to define a stable API for users of the libraries. However, we also need to move the libraries forward, technologically. To balance these two needs, a strict policy of versioning is required, which users can rely upon to understand the limitations, restrictions, and the changes that can occur from one release of APR to the next.

    The Basics

    Versions are denoted using a standard triplet of integers: MAJOR.MINOR.PATCH. The basic intent is that MAJOR versions are incompatible, large-scale upgrades of the API. MINOR versions retain source and binary compatibility with older minor versions, and changes in the PATCH level are perfectly compatible, forwards and backwards.

    It is important to note that a library that has not reached 1.0.0 is not subject to the guidelines described in this document. Before a 1.0 release (version 0.x.y), the API can and will be changing freely, without regard to the restrictions detailed below.

    Source Compatibility

    We define "source compatible" to mean that an application will continue to build without error, and that the semantics will remain unchanged.

    Applications that write against a particular version will remain source-compatible against later versions, until the major number changes. However, if an application uses an API which has become available in a particular minor version, it (obviously) will no longer build or operate against previous minor versions.

    Binary Compatibility

    We define "binary compatible" to mean that a compiled application can be linked (possibly dynamically) against the library and continue to function properly.

    Similar to source compatibility, an application that has been compiled against a particular version will continue to be linkable against later versions (unless the major number changes). It is possible that an application will not be able to successfully link against a previous minor version.


    Here are some examples to demonstrate the compatibility:

    Original Version New Version Compatible?
    2.2.3 2.2.4 Yes
    Compatibility across patch versions is guaranteed.
    2.2.3 2.2.1 Yes
    Compatibility across patch versions is guaranteed.
    2.2.3 2.3.1 Yes
    Compatibility with later minor versions is guaranteed.
    2.2.3 2.1.7 No
    Compatibility with prior minor versions is not guaranteed.
    2.2.3 3.0.0 No
    Compatibility with different major versions is not guaranteed.
    2.2.3 1.4.7 No
    Compatibility with different major versions is not guaranteed.

    Note: while some of the cells say "no", it is possible that the versions may be compatible, depending very precisely upon the particular APIs used by the application.


    This section details how we will build the code to meet the above requirements and guidelines.

    Patch Version

    To retain perfect source and binary compatibility, a patch release can only change function implementations. Changes to the API, to the signatures of public functions, or to the interpretation of function parameters is not allowed. Effectively, these releases are pure bug fix releases.

    Minor Versions

    Minor releases can introduce new functions, new symbolic and enumerated constants, and deprecate existing functions.

    New functions

    An application coded against an older minor release will still have all of its functions available with their original signatures. Once an application begins to use a new function, however, they will be unable to work against older minor versions.

    It is tempting to say that introducing new functions might create incompatibility across minor releases. If an application takes advantage of an API that was introduced in version 2.3 of a library, then it is not going to work against version 2.2. However, we have stated that an any application built against version 2.2 will continue to work for all 2.x releases. Thus, an application that states "requires 2.3 or later" is perfectly acceptable -- the user or administrator simply upgrades the installed library to 2.3. This is a safe operation and will not break any other application that was using the 2.2 library.

    In other words, yes an incompatibility arises by mandating that a specific version needs to be installed. But in practice, this will not be a problem since upgrading to newer versions is always safe.

    New constants
    Similar to functions, all of the original (old) constants will be available to an application. An application can then choose to use new constants to pick up new semantics and features.
    Replacing functions
    This gets a bit trickier. The original function must remain available at the link-level so that an application compiled against a minor version will continue to work with later minor versions. Further, if an application is designed to work with an earlier minor version, then we don't want to suddenly change the requirements for that application. This means that the headers cannot silently map an old function into a newer function, as that would turn an application, say, based on 1.2 into an application requiring the 1.4 or later release.

    This means that functions cannot truly be replaced. The new, alternate function can be made available in the header and applications can choose to use it (and become dependent upon the minor release where the function appears).

    It is possible to design a set of headers where a macro will always refer to the "latest" function available. Of course, if an application chooses to use this macro, then the resulting compiled-binary will be dependent upon whatever version it was compiled against. This strategy adds the new functionality for applications, yet retains the necessary source and binary compatibility for applications designed or built against previous minor releases.

    Constants (enumerated values and preprocessor macros) are not allowed to change since an older application will still be using them. Similarly, function signatures at the link-level may not change, so that support for older, compiled applications is maintained.

    Deprecating functions
    Since a function must remain available for applications coded against a previous minor release, it is only possible to "deprecate" a function. It cannot be removed from the headers (so that source compatibility is retained) and it cannot be removed from the library (so that binary compatibility is retained).

    If you deprecate a function in APR, please mark it as such in the function documentation, using the doxygen "\deprecated" tag. Deprecated functions can only be removed in major releases.

    A deprecated function should remain available through the original header. The function prototype should remain in the same header, or if moved to a "deprecated functions" header, then the alternate header should be included by the original header. This requirement is to ensure that source compatibility is retained.

    Finally, if you are deprecating a function so that you can change the name of the function, please use the method described above under "Replacing functions", so that projects which use APR can retain binary compatibility.

    Note that all deprecated functions will be removed at the next major version bump.

    Major Versions

    Any kind of change can be made during a major version release. Particular types of changes that might occur:

    • remove or change constants
    • remove (deprecated) functions
    • fold together macro-ized function replacements
    Version Checking

    In many cases, the user of a library will need to check the version that they are compiling against, or that is being used at runtime. Because of the strict rules of source and binary compatibility, these checks can be simpler and more complicated depending on what is needed.

    Compile-time Checks

    Libraries should make their version number available as compile-time constants. For example:

    #define FOO_MAJOR_VERSION 1
    #define FOO_MINOR_VERSION 4
    #define FOO_PATCH_VERSION 0

    The above symbols are the minimum required for this specification.

    An application that desires, at compile-time, to decide on whether and how to use a particular library feature needs to only check two values: the major and the minor version. Since, by definition, there are no API changes across patch versions, that symbol can be safely ignored. Note that any kind of a check for a minimum version will then pin that application to at least that version. The application's installation mechanism should then ensure that that minimal version has been installed (for example, using RPM dependency checks).

    If the feature changes across minor versions are source compatible, but are (say) simply different choices of values to pass into the library, then an application can support a wider variety of installed libraries if it avoids compile-time checks.

    Run-time Checks

    A library meeting this specification should support a way for an application to determine the library's version at run-time. This will usually be emboded as a simple function which returns the MAJOR, MINOR, and PATCH triplet in some form.

    Run-time checks are preferable in all cases. This type of check enables an application to run against a wider variety of minor releases of a library (the application is "less coupled" to a particular library release). Of course, if an application requires a function that was introduced in a later, minor release, then the application will require that, at least, that release is installed on the target system.

    Run-time checks are particurly important if the application is trying to determine if the library has a particular bug that may need to be worked around, but has been fixed in a later release. If the bug is fixed in a patch release, then the only avenue for an application is to perform a runtime check. This is because an application cannot require a specific patch level of the library to be installed -- those libraries are perfectly forward and backwards compatible, and the administrator is free to choose any patch release, knowing that all applications will continue to function properly. If the bug was fixed in a minor release, then it is possible to use a compile-time check, but that would create a tighter coupling to the library.

    Parallel Installation

    Parallel installation refers to the ability to install multiple versions of a library simultaneously -- they exist in parallel. This document will not discuss the full rationale for why this is important, but will instead detail how this versioning specification maps onto those concepts. Please refer to Havoc Pennington's "Parallel Installation" essay for futher details and the rationale behind this form of parallel installation.

    Library Naming

    On Unix-ish platforms, the library name should include the MAJOR version number:

    This strategy allows an application to explicitly state which version of the library that it wants to link against. If the application was built for version 2 of the API, then it can link against If another application was built against version 3 of the API, then it links against Since both libraries can reside on the system at the same time, both applications' needs can be satisfied.

    Typically, shared libraries on Unix-ish platforms will set up symlinks from the .so library to specific versions of that library. For example: -> ->

    In this configuration, applications will be bound to the .so.0 library. The minor version does not come into play here because we want applications to dynamically load and link to the new library when a new minor version is installed. Thus, the MINOR and the PATCH values are relegated to the library name after the .so.0 portion.

    The implication here is that build systems for libraries should arrange to generate .so libraries matching the above pattern.

    Include Directories

    The default installation directory for a library's include files should specify the MAJOR version number, and should normally be installed as a subdirectory in some standard location. For example:


    An application can place the FOO-MAJOR directory on its include path and include the files normally:

    #include <FOO-stuff.h>
    #include <FOO-more.h>

    Depending upon the API that the application is designed to work against, it can simply include different versions of the include directory.

    Other Files

    NOTE: There is no recommendation at this time for the best and proper handling of, say, FOO-config types of files. Or non-code types of files (e.g. things that typically get installed into areas like /usr/shared).

    Further thought and exploration is needed here.

    Other Notes

    It is expected that other libraries, besides those in the APR project, will want to use the above definitions of versioning. This is quite fine, and those libraries can simply reference this document. Its canonical location is:

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