lazyC

A small lazy-evaluation helper for C.

lazyC lets you wrap a computation and run it only once. Later evaluations reuse the cached state until you reset it.

Example

#include "lazy.h"

LazyStatus compute_answer(void *ctx, void *out) {
  (void)ctx;
  *(int *)out = 42;
  return LAZY_OK;
}

int main(void) {
  int result = 0;
  Lazy *lazy = NULL;

  if (lazy_create(&lazy, compute_answer, NULL, &result, sizeof(result)) != LAZY_OK) {
    return 1;
  }

  lazy_eval(lazy); // computes result
  lazy_eval(lazy); // cache hit; compute_answer is not called again

  lazy_destroy(lazy);
  return 0;
}

Ownership model

The caller owns the result storage.

Pass a pointer to output storage into lazy_create():

int result = 0;
lazy_create(&lazy, compute, ctx, &result, sizeof(result));

The library does not allocate, copy, clear, or free the result value. The output storage must remain valid until lazy_destroy() returns.

ctx may be NULL.

Reset behavior

lazy_reset() only marks the lazy value as not computed. It does not clear the caller-owned output storage. After reset, the output may contain stale data until the next successful lazy_eval().

Thread safety

lazy_eval(), lazy_reset(), and lazy_is_computed() are synchronized for use from multiple threads on the same Lazy object.

lazy_destroy() must not run concurrently with any other operation on the same Lazy object. The caller is responsible for ensuring no other thread is using the object before destroying it.

Build and test

make
make test
make sanitize

make sanitize builds and runs the test harness with AddressSanitizer and UBSan.

Public API

typedef struct Lazy Lazy;
typedef LazyStatus (*LazyComputeFn)(void *ctx, void *out);

LazyStatus lazy_create(Lazy **lazy, LazyComputeFn compute, void *ctx, void *out, size_t out_size);
LazyStatus lazy_eval(Lazy *lazy);
LazyStatus lazy_reset(Lazy *lazy);
LazyStatus lazy_destroy(Lazy *lazy);
LazyStatus lazy_is_computed(Lazy *lazy, bool *computed);

Current limitations

• Requires pthreads.
• lazy_destroy() is not safe to call concurrently with other operations.
• The library stores one caller-owned output pointer per lazy object.
• No packaged install target yet.