Add RFC95 text: Use standard C/C++ integer types

doc/source/development/rfc/index.rst

@@ -101,3 +101,4 @@ RFC list
     rfc92_wkb_only_geometries
     rfc93_update_feature
     rfc94_field_precision_width_metadata
+    rfc95_standard_int_types

doc/source/development/rfc/rfc95_standard_int_types.rst

@@ -0,0 +1,216 @@
+.. _rfc-95:
+
+=============================================================
+RFC 95: Use standard C/C++ integer types
+=============================================================
+
+============== =============================================
+Author:        Even Rouault
+Contact:       even.rouault @ spatialys.com
+Started:       2023-Sep-15
+Status:        Draft
+Target:        GDAL 3.8
+============== =============================================
+
+Summary
+-------
+
+This RFC replaces the use of the custom integer types defined in cpl_port.h
+(G[U]Int[8/16/32/64/Big]) by their standard [u]int[8/16/32/64]_t C99/C++11
+counterparts.
+
+Motivation
+----------
+
+- The existing Gxxxxx typedefs have been defined at a time that predates
+  C99 and C++11 which introduced those fixed with integer types.
+  If GDAL would be created today, we would use C99 data types.
+
+- Using those aliases, particularly GIntBig/GUIntBig which are not
+  self-explanatory on their actual width (64 bit), adds cognitive load to
+  developers.
+
+- Due to their poor namespacing, those short typenames may occasionaly clash
+  with other libraries. At a time, Poppler used a ``GBool`` data type as well,
+  which was clashing with GDAL's one.
+
+- Other projects (e.g. libtiff), have recently switched from the use of
+  similar custom types to C99 ones.
+
+Details
+-------
+
+The following data types replacement are done in the whole code base:
+
+- ``GBool``     --> ``bool``  (``GBool`` was aliased to ``int``)
+- ``GInt8``     --> ``int8_t``
+- ``GInt16``    --> ``int16_t``
+- ``GUInt16``   --> ``uint16_t``
+- ``GInt32``    --> ``int32_t``
+- ``GUInt32``   --> ``uint32_t``
+- ``GInt64``    --> ``int64_t``
+- ``GUInt64``   --> ``uint64_t``
+- ``GIntBig``   --> ``int64_t``
+- ``GUIntBig``  --> ``uint64_t``
+
+Not yet done in the candidate implementation, but candidates:
+
+- ``GSpacing``    -->  ``int64_t``   (affects mostly C++ raster drivers)
+- ``GPtrDiff_t``  -->  ``ptrdiff_t`` (affects the multidimensional C and C++ API)
+
+The following macro replacement are done in the whole code base:
+
+- ``CPL_FRMT_GIB``  --> ``"%" PRId64``
+- ``CPL_FRMT_GUIB`` --> ``"%" PRIu64``
+- ``GINT64_MAX``    --> ``std::numeric_limits<int64_t>::max()``
+- ``GINT64_MIN``    --> ``std::numeric_limits<int64_t>::min()``
+- ``GUINT64_MAX``   --> ``std::numeric_limits<uint64_t>::max()``
+- ``GINTBIG_MAX``   --> ``std::numeric_limits<int64_t>::max()``
+- ``GINTBIG_MIN``   --> ``std::numeric_limits<int64_t>::min()``
+- ``GUINTBIG_MAX``  --> ``std::numeric_limits<uint64_t>::max()``
+
+The old types are no longer used, and usable, in the GDAL code base since
+their definition is protected by ``#if !defined(GDAL_COMPILATION)``, which
+means they are still usable by external code.
+
+Impacts in the code base
+------------------------
+
+Significant part of the code base (735 files changed).
+Most changes have been done in a automated way, with manual changes
+specifically for the CPL_FRMT_GIB/CPL_FRMT_GUIB replacement
+which was harder to automate.
+
+SWIG bindings
+-------------
+
+While .i files are impacted to cope with the C type changes, the language
+specific API of SWIG bindings is not impacted.
+
+Backward compatibility
+----------------------
+
+C and C++ API and ABI are impacted.
+
+Main impacts are:
+
+* GBool was aliased to ``int``: changing to ``bool`` in C++ methods of the
+  OGRStyleXXXX classes affect the API and ABI. Impact should be modest as this
+  functionality is thought to be marginally used, and this only impacts the C++ API
+  (the C API for those methods uses ``int`` and not ``GBool``)
+
+* GIntBig/GInt64 was aliased to ``long long`` and GUIntBig/GUInt64 to
+  ```unsigned long long``. While ``int64_t`` and ``long long`` have in practice
+  same width and signedness, they are formally different data types.
+  For scalar usage of those types, compilers shouldn't warn.
+  But for usage of those types as pointers, compilers warn in C
+  (``-Wincompatible-pointer-types`` with gcc) and error outs in C++
+  (unless ``-fpermissive`` is passed), when mixing a
+  ``int64_t`` pointer and a ``long long`` pointer (similarly for their unsigned
+  counterparts).
+
+  C Raster API impacts (and equivalent C++ methods):
+
+  - :cpp:func:`GDALGetDefaultHistogramEx` (e.g. used by QGIS)
+  - :cpp:func:`GDALSetDefaultHistogramEx`
+  - :cpp:func:`GDALGetRasterHistogramEx` (e.g. used by QGIS)
+  - :cpp:func:`GDALGetVirtualMemAuto`
+
+  C Vector API impacts (and equivalent C++ methods):
+
+  - :cpp:func:`OGR_F_GetFieldAsInteger64List`
+  - :cpp:func:`OGR_F_SetFieldInteger64List`
+
+  C Multidimension API impacts (and equivalent C++ methods):
+
+  - :cpp:func:`GDALGroupCreateDimension`
+  - :cpp:func:`GDALGroupCreateAttribute`
+  - :cpp:func:`GDALMDArrayRead`
+  - :cpp:func:`GDALMDArrayWrite`
+  - :cpp:func:`GDALMDArrayAdviseRead`
+  - :cpp:func:`GDALMDArrayAdviseReadEx`
+  - :cpp:func:`GDALMDArrayCreateAttribute`
+  - :cpp:func:`GDALMDArrayResize`
+  - :cpp:func:`GDALMDArrayGetBlockSize`
+  - :cpp:func:`GDALMDArrayGetStatistics`
+  - :cpp:func:`GDALMDArrayComputeStatistics`
+  - :cpp:func:`GDALMDArrayComputeStatisticsEx`
+  - :cpp:func:`GDALAttributeGetDimensionsSize`
+
+* Out-of-tree drivers are also impacted:
+
+  - the vector ones that implement :cpp:func:`OGRLayer::ISetFeature`,
+    :cpp:func:`OGRLayer::SetNextByIndex`, :cpp:func:`OGRLayer::DeleteFeature`,
+    :cpp:func:`OGRLayer::GetFeatureCount`
+
+  - the ones that implement the multidimensional API (no publicly ones known by us)
+
+Questions to answer before adoption
+-----------------------------------
+
+Q1: What to do with the 64-bit integer changes?
+
+A1: Potential alternatives:
+
+1) Proceed with them, keep GDAL 3.8 as version number. External code will have
+   to use #ifdef if support for GDAL < 3.8 and >= 3.8 is needed.
+
+2) Same as 1), but bump GDAL version number to 4.0. Cleaner way
+
+3) Revert all 64-bit integer related changes
+
+4) Partial revert of 64-bit integer related changes, to reintroduce
+   GIntBig/GUIntBig for the above mentionned methods of the C API
+   (i.e. GDALGetDefaultHistogramEx, etc), but not the C++ one.
+   This preserves C API.
+
+5) Variant of 4) where the revert is done only for the C raster and vector API,
+   but the thought-to-be-marginally-used multidimensional one uses the
+   int64_t/uint64_t types.
+
+
+Q2: Should the ``#if !defined(GDAL_COMPILATION)`` in cpl_port.h that controls
+whether the old types are accessible be changed to
+``#if !defined(GDAL_COMPILATION) && defined(GDAL_USE_OLD_INT_TYPES)``,
+so that users have to opt-in to use the legacy types?
+
+
+Q3: Should ``vsi_l_offset`` be replaced by ``uint64_t``? (they are now aliased
+through a typedef)
+
+A3: I'm 50% 50% on this. The name captures a semantic, which can be interesting
+to preserve (like they is a ``off_t`` type in Unix file API)
+
+Risks
+-----
+
+The changes of this RFC are somewhat risky, particularly the replacement of the
+formatting macros CPL_FRMT_GIB/CPL_FRMT_GUIB with the PRId64/PRIu64, which
+requires to add a ``%`` formatting character. While compiler caught most of the
+mismatches, there were remaining ones undetected at compilation tmie. Manual
+corrections have been done to make the regression test suite pass.
+Additional "grep" searches in the code bases have been done to find faulty
+patterns, but we cannot exclude that some might have been missed.
+
+Documentation
+-------------
+
+MIGRATION_GUIDE.TXT will be updated to point to this RFC.
+
+Testing
+-------
+
+No changes in Python tests. Updates in the C++ test suite.`
+
+Related issues and PRs
+----------------------
+
+- https://github.com/OSGeo/gdal/issues/8288: Consider using standard C/C++
+  integer types
+
+- https://github.com/OSGeo/gdal/pull/8396: candidate implementation
+
+Voting history
+--------------
+
+TBD