Browse zlib specs (31 of 62)

Proofs / zlib / The stream interface

api_counters

The stream's pointers and counters move by exactly the bytes read and written.

spec
50 lines of C
with its headers
90 lines
status
Proved
31 of 62
On this page

The spec

The spec is the C program harness/api_counters.c, 50 lines, shown whole as it is in the repository at 37189f4. It includes harness/laws_common.h (40 lines). Its first comment states the property in full, with every bound.

C
// Law api_counters: For any windowBits that inflateInit2 accepts and any input (any length the allocator provides), fed
// to any number of inflate() calls in any pieces below 2^32 bytes, each with any flush and any output space (input and
// output each an object of exactly avail_in and avail_out bytes), with inflateSetDictionary of any bytes between calls
// or not, inflate's counters are exact: after every call, next_in and next_out have moved on by exactly the bytes it
// read and wrote (avail_in and avail_out never grow), and total_in and total_out are the sums of those over all calls
// (Z_MEM_ERROR ends the run).
#include "laws_common.h"

int main(void) {
	int window_bits = (int) nondet_u32();
	size_t input_len = upto(PTRDIFF_MAX);
	unsigned char *input = alloc_any_bytes(input_len);
	z_stream strm;
	new_stream(&strm);
	if (input == NULL || inflateInit2(&strm, window_bits) != Z_OK)
		return 0;
	size_t read = 0;
	size_t written = 0;
	do {
		size_t take = upto(clip(input_len - read));
		size_t room = nondet_u32();
		unsigned char *piece = copy_of(input + read, take);
		unsigned char *out = alloc_any_bytes(room);
		if (piece == NULL || out == NULL)
			return 0;
		set_buffer(&strm, piece, take, out, room);

		if (inflate(&strm, (int) nondet_u32()) == Z_MEM_ERROR)
			return 0;

		// zlib.h, z_stream: "next_in: next input byte", "avail_in: number of bytes available at next_in",
		// "total_in: total number of input bytes read so far", and the same for next_out, avail_out, total_out
		gp_assert(strm.avail_in <= take && strm.avail_out <= room);
		gp_assert(strm.next_in == piece + (take - strm.avail_in));
		gp_assert(strm.next_out == out + (room - strm.avail_out));
		read += take - strm.avail_in;
		written += room - strm.avail_out;
		gp_assert(strm.total_in == read && strm.total_out == written);
		free(piece);
		free(out);
		if (nondet_u8() == 0) {
			size_t dict_len = nondet_u32();
			unsigned char *dict = alloc_any_bytes(dict_len);
			if (dict == NULL || inflateSetDictionary(&strm, dict, (uInt) dict_len) == Z_MEM_ERROR)
				return 0;
			free(dict);
		}
	} while (nondet_u8() != 0);
	return 0;
}
harness/laws_common.h, 40 lines
C
// laws_common.h: trivial helpers shared by the zlib laws (each law includes it once). No models, no algorithms.
#include <limits.h>
#include <stdlib.h>
#include "quickproof.h"
#include "zlib.h"
static size_t upto(size_t max) {
	size_t value = nondet_size();
	return value < max ? value : max;
}
static uInt clip(size_t n) { return n < UINT_MAX ? (uInt) n : UINT_MAX; }
static unsigned char *alloc_any_bytes(size_t n) {
	unsigned char *bytes = malloc(n);
	if (bytes != NULL)
		nondet_bytes(bytes, n);
	return bytes;
}
static unsigned char *copy_of(const unsigned char *source, size_t n) {
	unsigned char *copy = malloc(n);
	for (size_t i = 0; copy != NULL && i < n; i++)
		copy[i] = source[i];
	return copy;
}
static int bytes_eq(const unsigned char *a, const unsigned char *b, size_t n) {
	for (size_t i = 0; i < n; i++)
		if (a[i] != b[i])
			return 0;
	return 1;
}
static void set_buffer(z_stream *strm, unsigned char *in, size_t in_len, unsigned char *out, size_t out_len) {
	strm->next_in = in;
	strm->avail_in = (uInt) in_len;
	strm->next_out = out;
	strm->avail_out = (uInt) out_len;
}
static void new_stream(z_stream *strm) {
	strm->zalloc = Z_NULL;
	strm->zfree = Z_NULL;
	strm->opaque = Z_NULL;
	set_buffer(strm, Z_NULL, 0, Z_NULL, 0);
}

The theorem

QuickProof generated this statement from the spec, in src/contracts/api_counters.v:

Rocq
Theorem api_counters : harness_safe whole_prog.

src/contracts/laws.v states it on the spec's own program, whole_prog (the spec linked with the code under test), with its proof:

Rocq
Theorem api_counters : harness_safe Zlib.contracts.api_counters.whole_prog.
Proof. exact (proj1 Zlib.contracts.suite.suite). Qed.

Read as a statement about runs, it says that no run of the program has undefined behaviour or a failing check, for every value the spec can draw. That reading rests on VST's adequacy theorem, which is not yet instantiated in Rocq (formerly Coq) for QuickProof's environment.

The C files (units) its program links
  • adler32.c
  • crc32.c
  • inflate.c
  • inffast.c
  • inftrees.c
  • zutil.c
  • quickproof/libc/libc.c

What it leaves out

  • What the first comment of the spec excludes, and its bounds.
  • Like every spec of this proof: termination (a run that never returns is not a counterexample), and binaries built by a compiler (the theorem is about the code as CompCert defines C for x86_64, LP64), one thread.
  • Where zlib reports Z_MEM_ERROR, most specs end the run and claim nothing about it; the first comment of the spec says so.
  • The edits to zlib's code, listed in the repository, are tested, not proved.

The summary page says what the proof as a whole covers.

How it was proved

AI agents wrote the proof, and Rocq checks it. qp verify passes on this repository: this statement is the one generated from the spec at the contract commit, and its proof uses only Rocq's standard axioms and VST's abstract malloc library.