REFERENCE / ZLIB × LEAN

deflatePrime

Prefix raw compression output with a caller-supplied bit fragment.

Source snapshot · 39ab1ace4bbe0b0e · 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 deflatePrime(z_streamp strm, int bits, int value);

Prefix raw compression output with a caller-supplied bit fragment.

Domain Restricted domain. Effect: append bits to pending output.

Pre live raw stream before its first deflate after init or deflateReset; bits is 0..16 and value is any int. The caller accounts for these prefix bits when interpreting the later compressed stream.

Post Z_OK appends the low bits bits of value, least significant first, to the logical pending bit sequence. Complete bytes join pending output and fewer than 8 bits remain buffered. No bytes are delivered to next_out; no allocation occurs. Z_BUF_ERROR inserts no bits when the pending buffer has insufficient space. Existing pending bytes may be compacted within their owned buffer before that refusal.

Outside focused scope: counts outside 0..16 give Z_BUF_ERROR before compaction; a failed stream check gives Z_STREAM_ERROR. The implementation does not check wrapper or first-call timing. Its public goal covers exact pending-bit effects there, without promising ordinary stream validity for arbitrary inserted fragments.

Shared conditions and scope

API manual · Shared stream rules · Stream decompression

Use a z_stream compressor when input arrives in pieces, output space is limited, or you need a chosen wrapper, dictionary or flush boundary. Initialize once, call deflate with successive buffers, and end the stream even if compression is abandoned. For a single buffer, see compress and compress2.

Task	Entries
Start or reuse	Init, configured init, reset, reset retaining history
Supply metadata or history	Dictionary, read history, gzip header
Compress and size output	Deflate, bound, pending output, last flush bits
Adjust or branch	Parameters, tuning, priming, copy
Release storage	End

These focused contracts follow the pinned deflate.c, applied patches and zlib.h in the default LP64 Linux build. Source readings are to confirm by proof. They leave the broader registered public goals unchanged. See contract conventions for domain/effect labels and progress for proof status and remaining work.

Shared conventions

Each stated Pre requires termination without runtime faults and the stated Post. A call that retains a live stream preserves its representation invariant.

Live stream: a writable z_stream with a compressor state established by successful init or copy, preserved by legal calls, and not ended. The caller does not alter its internal state or allocator fields. Calls on one stream are sequential.
Buffers: input covers avail_in readable bytes; output covers avail_out writable bytes. They do not overlap each other, the stream or its owned state. A non-NULL scalar result points to a writable cell separate from other results and stream memory. These separation conditions are explicit focused assumptions; see safety.md.
Allocator: the default allocator/free pair is used, or both fields are NULL before init. Allocation may fail on any attempt. Custom allocator callbacks are outside this page's scope.
Frame and ownership: calls change only their stream, its owned state and stated output regions. Caller input is read-only. Caller buffers and header records remain caller-owned. Only init and copy allocate; only their failure cleanup and deflateEnd free memory. Other calls preserve the stream's allocation set.
Header lifetime: a retained gzip header and its optional fields remain readable and unchanged while used. Non-NULL name and comment have a terminating zero; extra covers its stated length. Focused format contracts use extra_len <= 65535.
Checked refusals: except for init and bounds, a failed stream check returns Z_STREAM_ERROR before normal operation, without writes. deflateCopy additionally clears a non-NULL destination state. Query output cells are untouched on this refusal.
Restricted domain means these explicit memory, lifetime and call-order conditions, plus the entry's Pre. It does not mean every representable pointer or integer is accepted. Error summaries cover readable checked states, not arbitrary invalid addresses.

Formal contract

ZlibF.Spec.Public.Goal.deflatePrime

Proof status: Not completed · Evidence and remaining work

/-- `int deflatePrime(z_streamp strm, int bits, int value)`.
Append the requested low bits in transmission order. Compaction can occur before a buffer
error; invalid bit counts are refused before compaction. No allocation occurs. -/
def Goal.deflatePrime : Prop :=
  ∀ h r i z s count value old,
    Zlib.Deflate.Spec.DGood h r i z s → PendingBits h s old →
    ∃ rc after s', C.run (Zlib.Deflate.deflatePrime (some r) count value) h = .ok (rc, after) ∧
      Zlib.Deflate.Spec.DGood after r i z s' ∧
      (count.toInt < 0 ∨ 16 < count.toInt → rc = Z_BUF_ERROR ∧ after = h) ∧
      (0 ≤ count.toInt → count.toInt ≤ 16 →
        s'.pending_out = s'.pending_buf ∧
        if s.pending_buf_size.toNat - s.pending.toNat < (s.bi_valid.toInt.toNat + count.toInt.toNat) / 8 then
          rc = Z_BUF_ERROR ∧ PendingBits after s' old
        else rc = Z_OK ∧ PendingBits after s'
          (old ++ (List.range count.toInt.toNat).map (fun j => decide (value.toUInt32.toNat / 2 ^ j % 2 = 1)))) ∧
      Frame h after (DeflateOwned i s) ∧ LiveChange h after (fun _ => False) (fun _ => False) ∧
      after.nalloc = h.nalloc

Related definitions and source ↗