vlibc/docs/compatibility.md

Compatibility

vlibc offers five compatibility profiles, selected at configure time. They are mutually exclusive; the default is vlibc.

Level	Flag	Profile	What it provides
1	--enable-onlyposix	onlyposix	Pure POSIX, nothing more — the lightest possible build.
2	--enable-muslmimic	muslmimic	musl-like, still light, with musl-compatible features.
3	--enable-muslext	muslext	An extension of musl, adding more.
4	--enable-spoof	spoof	A glibc replica: emulates glibc for drop-in compatibility, higher than muslext or vlibc full.
5	(default)	vlibc	glibc-extended: glibc minus its baggage, plus extended standard features. High (but not spoof-level) glibc compatibility.

Profile mechanism

The profiles map onto a single integer level, VLIBC_LEVEL. Levels are cumulative: a profile at level N exposes everything whose minimum level is <= N.

Level	Classification
1	ISO C + POSIX base
2	+ BSD / XSI extensions
3	+ GNU extensions

Public headers gate declarations with #if VLIBC_LEVEL >= N; a function whose minimum level is 2 is only declared when the active profile is level 2 or higher. The build applies the same rule to sources via AM_CONDITIONAL([PROFILE_GE_2]) and AM_CONDITIONAL([PROFILE_GE_3]), so each level's implementation files are only compiled when the selected profile reaches that level.

The generated installed header include/vlibc/features.h exposes the macros the gating is based on:

• VLIBC_LEVEL — the active profile's integer level.
• VLIBC_PROFILE — the selected profile name.
• VLIBC_LEVEL_GE(n) — expands to 1 when VLIBC_LEVEL >= n, 0 otherwise.

Slice manifest

The initial implementation covers eight string/memory functions, classified as follows:

Function	Level	Origin
memcpy	L1	ISO C
memmove	L1	ISO C
memset	L1	ISO C
strlen	L1	ISO C
strcmp	L1	ISO C
strlcpy	L2	BSD
strlcat	L2	BSD
strcasestr	L3	GNU

Profiles 4 and 5 (spoof, vlibc) build on the same level mechanism but are not implemented in this increment beyond their configure declarations.

Differences from glibc

• No legacy baggage. vlibc targets modern, standard-conforming behavior and drops glibc's deprecated and non-standard behaviors unless a profile explicitly restores them.
• Extended standard features. The vlibc profile adds extensions on top of the C and POSIX standards that glibc does not provide.
• Spoofing is opt-in. The --enable-spoof profile re-enables the long-standing legacy behaviors that scripts rely on, for drop-in compatibility with existing binaries and build systems.

Full static linking

Every profile except spoof must be able to link fully statically. The spoof profile is exempt: its glibc-emulation layer depends on dynamic facilities (e.g. dlopen-based compatibility shims) that a static build cannot provide.