_ISOCxx_SOURCE

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Project / Subsystem

gcc / gcc

Date

2026-06-13

Proposer

Alejandro Colomar <alx@kernel.org>

Source type

public_inbox

Consensus

Proposed

Sentiment

—/10

Technical tradeoffs

• Simpler, clearer macro naming scheme vs. disruption to existing build systems using current `_ISOCxx_SOURCE` macros.
• Improved accuracy of feature availability vs. potential for introducing new compatibility issues during transition.

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All attributes

project: gcc
subsystem: gcc
patch_id: —
discussion_id: ai1DVieT3gTr4J5S@devuan
source_type: public_inbox
title: _ISOCxx_SOURCE
headline: C Standard library feature test macros need redesign and documentation fixes
tldr: The current design and documentation for C standard library feature test macros like `_ISOC23_SOURCE` are inaccurate and lead to confusion, requiring a new app…
proposer: Alejandro Colomar <alx@kernel.org>
consensus: Proposed
outcome: proposed
sentiment_score: —
technical_tradeoffs: • Simpler, clearer macro naming scheme vs. disruption to existing build systems using current `_ISOCxx_SOURCE` macros.
• Improved accuracy of feature availability vs. potential for introducing new compatibility issues during transition.
series_id: —
series_role: standalone
series_parts: []
tags: • c-standard
• glibc
• feature-macros
• documentation
• abi
url: https://inbox.sourceware.org/gcc/ai1DVieT3gTr4J5S@devuan
bugzilla_url: —
date: 2026-06-13T00:00:00.000Z

_ISOCxx_SOURCE

Alejandro Colomar proposes a redesign of the _ISOCxx_SOURCE feature test macros in glibc, arguing their current documentation is incorrect and the design is suboptimal. The existing macros incorrectly claim to enable older C features, leading to situations where functions are unexpectedly disabled. Colomar suggests a unified macro, _ISO_C_SOURCE, with a version-specifying value, mirroring POSIX’s approach for clearer and more accurate feature management.