# 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. |

## 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.