# 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.