Portability Library

Squid Portability

Aim

Squid aims to build and run on many modern systems. To do this we have traditionally added small hacks and wrappers all over the code whenever one was needed. The final result of that is a vast amount of code duplication, dodgy licensing on some older hacks, and some cases of obsolete algorithms sitting side by side with their current equivalent.
The Portability library libcompatsquid.la has been created to correct the three issues of stable build portability, code cleanliness, and clearer licensing.

Requirements

The system calls used by Squid are not required to be standard. Often we depend on some non-standard call which can give great performance benefits. But they are required to meet several other criteria:
  • They must be of actual benefit to Squid during its operation.
  • A better alternative must not exist.
  • If not available on all OS an open-source GPLv2+ compatible implementation must be available to be included with the Squid sources and used when required.
To be built into the libcompatsquid.la as a layer below all Squid-bundled binaries. The code must also qualify by being provided natively by some OS where Squid builds. Code and Algorithms which do not meet this final criteria are relegated to the slightly higher level of basic component, rather than portability.

Component Types

Macro re-definition
Where the only difference between systems is their naming scheme. One of the schemes is chosen by the developers for use and mappings are created in the form of Macros.
Inline Functions
Algorithm Templates and Inline functions
Being C++ we are able to use templates in place of inline functions where that is more convenient. Care is taken to provide no additional requirements upon the callers when using the template as to when using the native constructs.
Emulators
As mentioned above if a useful library function calls or global is not available on all operating systems a GPLv2+ alternative may be added to the compat layer. As an emulator it retains the exact naming and definition of the source systems. Thus being able to be used seamlessly across all platforms by the main code.
Wrappers
Sometimes common and useful library functions are not always as safe as they could be. An alternative which wraps the original in extra safety checks is provided under the same name with an 'x' pre-pended. Currently these extra protections are added on string handling and memory allocation.

Layout

The internal code structure of libcompatsquid.la files has a hierarchy. The API has a flat global scope separate from the file layout. The API is pulled in by including compat/compat.h. The strict dependency requirements which exist within the compat API make including an individual part separately a risky operation.

Squid coding guidelines require each .c and .cc file to include squid.h first in their included files. squid.h begins with an order-specific sequence of defines and includes compat/compat.h to incorporate the compat layer appropriately in every source file.
Internally the compat/ directory contains the public API file compat/compat.h which structures order-specific includes as documented inside that file. Included by that is compat/osdetect.h which determines which operating system and in some cases compiler is being used.
The compat/os/ directory contains separate files for each supported system which requires special compat layer handling. Hacks for specific systems should be restricted to these files as much as possible.
compat/compat_shared.h file contains the portability definitions which are shared across a great many systems. These should be written with protective macros to prevent any symbols or code being defined which is not necessary. Protections here must not be system-specific.
Also in compat/ directory are the .h and .c files for emulators detected by autoconf. These are added by autoconf to the build objects as required.

 

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