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inflateGetHeader

Source snapshot · ecafc184cdf44b7f · Source at the matching commit ↗

Read the function’s preconditions and results first. Expand the shared rules below for buffer and ownership requirements used throughout this API group.

int ZEXPORT inflateGetHeader(z_streamp strm, gz_headerp head);

Domain Restricted domain: live gzip-capable streams after init/reset and before decoding.

Effects Registers borrowed storage and writes head->done.

Pre head is non-NULL and writable. Its extra, name, comment pointers are NULL or have writable capacities extra_max, name_max, comm_max. Keep all registered storage valid until header parsing ends or the request is cleared. The header record and its destination buffers stay separate from each other, the stream and its owned storage, and the input/output ranges of calls using them.

Post Returns Z_OK, retains the pointer and sets done to 0. Later inflate calls fill scalar fields and available buffer prefixes. With an extra field, extra_len is its actual length; without one, extra becomes NULL and extra_len is unchanged. Names/comments are terminated only if the terminator fits. Absent optional fields set the corresponding pointer to NULL. Completion sets done to 1; automatic detection of zlib sets it to -1.

Edges No allocation or ownership transfer. Reset clears registration. A failed stream check or request without gzip support returns Z_STREAM_ERROR unchanged. NULL head on an accepted request is outside this contract and is dereferenced by the C; it is not a clean-error case.

Shared conditions and scope

API manual · Shared stream rules · Stream compression

Use an inflate stream to decode input in pieces while controlling output space. Select zlib, gzip or raw DEFLATE at initialization, then continue with more input or room until completion, a dictionary request or an error. End releases the decoder after any of those outcomes. For whole-buffer zlib decoding, see uncompress.

Task	Entries
Start or reuse	Init, configured init, reset, new configuration, keep history
Decode or provide metadata	Inflate, set dictionary, read history, gzip header
Recover or inspect	Sync search, sync point, mark, table use
Adjust or branch	Prime bits, validation, distance option, copy
Release storage	End

The focused contracts follow the pinned inflate.c and zlib.h in the default LP64 Linux build, with gzip enabled and without optional invalid-distance or strict-distance modes. Source readings are to confirm by proof. The broader registered public goals retain their domains. See contract conventions for terminology and progress for proof status and remaining work.

Common conditions

All contracts below have a Restricted domain. A C pointer type alone does not supply the storage, lifetime or call history required here.

A live stream was successfully initialised or copied, has not been ended, retains its library-owned state, and is used without concurrent calls. Use the default allocator and free function. Init takes a writable, non-live stream with allocator fields set to NULL or the default functions.
A readable or writable range has the stated capacity for the whole call. Caller ranges are separate from the stream and its owned storage. Input and output ranges do not overlap. Dictionary destinations, the registered header record and its destination buffers are separate from each other and from the stream. Registered header storage is also separate from input/output ranges on calls using it. These are explicit focused-domain restrictions.
The caller owns its stream object and buffers. The library owns the state and optional window. A call changes only its stream, owned storage, stated output ranges and any registered header destination. It never frees caller buffers.
A rejected stream check returns Z_STREAM_ERROR without changes, unless a query or copy contract says otherwise. This covers NULL and safely readable records rejected by the check; it does not cover dangling or arbitrary pointers. The check requires allocators, state, the state's back-pointer and a valid mode.
Counters use the C unsigned widths and wrap. No contract assumes allocation succeeds. End remains available after decoding, dictionary or allocation errors.

flowchart LR
    I["Init"] --> F["inflate"]
    F -->|"Z_NEED_DICT"| D["SetDictionary"] --> F
    F -->|"more room or input"| F
    F -->|"end or abandon"| E["End"]
    F -->|"data error"| R["Reset or Sync"] --> F

Formal contract

ZlibF.Spec.Public.Goal.inflateGetHeader

Proof status: Registered proof · Evidence and remaining work

/-- `int inflateGetHeader(z_streamp strm, gz_headerp head)`.
Only gzip-capable wrappers accept the header. Set `done` to zero and retain all other fields. -/
def Goal.inflateGetHeader : Prop :=
  ∀ h r i z s head, InflateView h r i z s →
    ((s.wrap &&& 2) ≠ 0 → ∃ g gh, head = some (.tab g) ∧ h.gzheaders g = some gh) →
    ∃ rc after, C.run (Zlib.Inflate.inflateGetHeader (some r) head) h = .ok (rc, after) ∧
      (if (s.wrap &&& 2) = 0 then rc = Z_STREAM_ERROR ∧ after = h
       else ∃ g gh, head = some (.tab g) ∧ h.gzheaders g = some gh ∧ rc = Z_OK ∧
         after = { putInflate h i { s with head := head } with
           gzheaders := upd h.gzheaders g (some { gh with done := 0 }) })

Related definitions and source ↗