From a5c16e97d721dad1c66053a7bd78f341c4c861f1 Mon Sep 17 00:00:00 2001
From: huntedbytheirs <huntedbyth3irs@gmail.com>
Date: Mon, 31 Aug 2026 19:49:49 -0400
Subject: [PATCH] feat(profile): generate installed features.h exposing
 VLIBC_LEVEL

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
---
 .gitignore                  |  3 ++
 configure.ac                |  2 ++
 include/vlibc/features.h.in | 58 +++++++++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+)
 create mode 100644 include/vlibc/features.h.in

diff --git a/.gitignore b/.gitignore
index 9297fff..9d3cdcf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -85,6 +85,9 @@ Makefile.in
 /build-aux/
 /m4/
 
+# Generated feature header (from include/vlibc/features.h.in); the template is tracked.
+/include/vlibc/features.h
+
 # Libtool build directories and artifacts.
 .deps/
 .libs/
diff --git a/configure.ac b/configure.ac
index 2897664..fc3c996 100644
--- a/configure.ac
+++ b/configure.ac
@@ -116,6 +116,7 @@ AS_CASE([$vlibc_profile],
         [vlibc],     [AC_DEFINE([VLIBC_PROFILE_VLIBC],     [1], [vlibc (glibc-ext) profile])])
 
 AC_SUBST([vlibc_profile])
+AC_SUBST([vlibc_level])
 
 # ---- Install method ------------------------------------------------------
 # "alongside" installs vlibc next to the system libc: headers and libraries go
@@ -168,6 +169,7 @@ LT_INIT
 
 AC_CONFIG_FILES([Makefile
                  benchmarks/Makefile
+                 include/vlibc/features.h
                  tools/vlibc-gcc
                  tools/vlibc-clang])
 
diff --git a/include/vlibc/features.h.in b/include/vlibc/features.h.in
new file mode 100644
index 0000000..1837cc3
--- /dev/null
+++ b/include/vlibc/features.h.in
@@ -0,0 +1,58 @@
+#ifndef VLIBC_FEATURES_H
+#define VLIBC_FEATURES_H
+
+/*
+ * vlibc — compatibility feature gate.
+ *
+ * This header is generated by configure from include/vlibc/features.h.in; do
+ * not edit the generated file. It exposes the active compatibility profile as
+ * build-time constants, so headers and sources can gate declarations on the
+ * profile level (see configure.ac and docs/compatibility.md):
+ *
+ *   1  onlyposix   pure POSIX, nothing more
+ *   2  muslmimic   musl-like, light
+ *   3  muslext     musl-extended
+ *   4  spoof       glibc replica (drop-in compatibility)
+ *   5  vlibc       glibc-extended, the default
+ *
+ * VLIBC_LEVEL is the numeric level (1..5); VLIBC_PROFILE is the profile name.
+ * Both are emitted only when not already defined: the library's own sources
+ * include config.h first (via HAVE_CONFIG_H), whose values win, and this
+ * header skips the redefinition. Consumers of the installed header get the
+ * values baked in at configure time.
+ */
+
+#ifndef VLIBC_LEVEL
+#define VLIBC_LEVEL @vlibc_level@
+#endif
+
+#ifndef VLIBC_PROFILE
+#define VLIBC_PROFILE "@vlibc_profile@"
+#endif
+
+/* True when the active profile is at least level n. */
+#define VLIBC_LEVEL_GE(n) (VLIBC_LEVEL >= (n))
+
+/*
+ * Per-profile feature booleans, derived from VLIBC_LEVEL. Levels are
+ * cumulative: level 3 implies the POSIX and musl feature sets as well. A
+ * macro is defined (as 1) when its feature set is present and left undefined
+ * otherwise, so `#if VLIBC_HAS_MUSL` works in both cases.
+ */
+#if VLIBC_LEVEL >= 1
+#define VLIBC_HAS_POSIX 1
+#endif
+#if VLIBC_LEVEL >= 2
+#define VLIBC_HAS_MUSL 1
+#endif
+#if VLIBC_LEVEL >= 3
+#define VLIBC_HAS_MUSLEXT 1
+#endif
+#if VLIBC_LEVEL >= 4
+#define VLIBC_HAS_GLIBC 1
+#endif
+#if VLIBC_LEVEL >= 5
+#define VLIBC_HAS_VLIBC 1
+#endif
+
+#endif /* VLIBC_FEATURES_H */