[PATCH v2] build/eal: add OS defines for C conditional checks

[Bruce Richardson <bruce.richardson@intel.com>]

Define a set of macros in the build configuration to allow C runtime
code to check the current OS environment. This saves the user having to
use ifdefs for e.g. disabling particular tests on Windows. See included
documentation changes for usage examples.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>

---
V2: Included feedback from Dmitry to expand on documentation to include
guidelines on when to use conditional compilation.
---
 doc/guides/contributing/coding_style.rst | 53 +++++++++++++++++++++++-
 lib/eal/meson.build                      |  7 ++++
 2 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/doc/guides/contributing/coding_style.rst b/doc/guides/contributing/coding_style.rst
index 0ec37c019b..66803dcccd 100644
--- a/doc/guides/contributing/coding_style.rst
+++ b/doc/guides/contributing/coding_style.rst
@@ -136,6 +136,24 @@ For example:
 Conditional Compilation
 ~~~~~~~~~~~~~~~~~~~~~~~
 
+.. note::
+
+ Conditional compilation should be used only when absolutely necessary, as it increases the number of target binaries that need to be built and tested.
+ See below for details of some utility macros/defines available to allow ifdefs/macros to be replaced by C conditional in some cases.
+
+Some high-level guidelines on the use of conditional compilation:
+
+* If code can compile on all platforms/systems, but cannot run on some due to lack of support,
+  then regular C conditionals, as described in the next section, should be used instead of conditional compilation.
+* If the code in question cannot compile on all systems, but constitutes only a small fragment of
+  a file, then conditional complication should be used, as described in this section.
+* If the code for conditional compilation implements an interface in an OS or platform-specific way,
+  then create a file for each OS or platform and select the appropriate file using the meson build system.
+  In most cases, these environment-specific files should be created inside the EAL library,
+  rather than having each library implement its own abstraction layer.
+
+Additional style guidance for the use of conditional compilation macros:
+
 * When code is conditionally compiled using ``#ifdef`` or ``#if``, a comment may be added following the matching
   ``#endif`` or ``#else`` to permit the reader to easily discern where conditionally compiled code regions end.
 * This comment should be used only for (subjectively) long regions, regions greater than 20 lines, or where a series of nested ``#ifdef``'s may be confusing to the reader.
@@ -165,9 +183,40 @@ Conditional Compilation
  /* Or here. */
  #endif /* !COMPAT_43 */
 
-.. note::
+Defines to Avoid Conditional Compilation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In many cases in DPDK, one wants to run code based on the target platform, or runtime environment.
+While this can be done using the conditional compilation directives,
+e.g. ``#ifdef RTE_EXEC_ENV_LINUX``, present in DPDK for many releases,
+this can also be done in many cases using regular ``if`` statements and the following defines:
+
+* ``RTE_ENV_FREEBSD``, ``RTE_ENV_LINUX``, ``RTE_ENV_WINDOWS`` - these define ids for each operating system environment.
+* ``RTE_EXEC_ENV`` - this defines the id of the current environment, i.e. one of the items in list above.
+* ``RTE_EXEC_ENV_IS_FREEBSD``, ``RTE_EXEC_ENV_IS_LINUX``, ``RTE_EXEC_ENV_IS_WINDOWS`` - 0/1 values indicating if the current environment is that specified,
+  shortcuts for checking e.g. ``RTE_EXEC_ENV == RTE_ENV_WINDOWS``
+
+Examples of use:
+
+.. code-block:: c
+
+  /* report a unit tests as unsupported on Windows */
+  if (RTE_EXEC_ENV_IS_WINDOWS)
+     return TEST_SKIPPED;
+
+  /* set different default values depending on OS Environment */
+  switch (RTE_EXEC_ENV) {
+     case RTE_ENV_FREEBSD:
+         default = x;
+         break;
+     case RTE_ENV_LINUX:
+         default = y;
+         break;
+     case RTE_ENV_WINDOWS:
+         default = z;
+         break;
+  }
 
- Conditional compilation should be used only when absolutely necessary, as it increases the number of target binaries that need to be built and tested.
 
 C Types
 -------
diff --git a/lib/eal/meson.build b/lib/eal/meson.build
index 1722924f67..056beb9461 100644
--- a/lib/eal/meson.build
+++ b/lib/eal/meson.build
@@ -10,6 +10,13 @@ if not is_windows
     subdir('unix')
 endif
 
+exec_envs = {'freebsd': 0, 'linux': 1, 'windows': 2}
+foreach env, id:exec_envs
+    dpdk_conf.set('RTE_ENV_' + env.to_upper(), id)
+    dpdk_conf.set10('RTE_EXEC_ENV_IS_' + env.to_upper(), (exec_env == env))
+endforeach
+dpdk_conf.set('RTE_EXEC_ENV', exec_envs[exec_env])
+
 dpdk_conf.set('RTE_EXEC_ENV_' + exec_env.to_upper(), 1)
 subdir(exec_env)
 
-- 
2.32.0