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Emitting events

Emitting an event is two steps: build a MessagePack payload, then hand it and an event type to the kernel. This guide shows both; event.h and msgpack.h are the full references. Emitting requires SeAuditPrivilege.

Build the payload

Use the MessagePack writer to encode a single top-level value — typically a map of fields:

peios_mp_writer *w = peios_mp_writer_new();

peios_mp_write_map(w, 2);                              /* {"user":…, "ok":…} */
peios_mp_write_str(w, "user", 4);  peios_mp_write_str(w, "alice", 5);
peios_mp_write_str(w, "ok", 2);    peios_mp_write_bool(w, true);

const void *payload;
ssize_t plen = peios_mp_writer_bytes(w, &payload);     /* validates as it borrows */
if (plen < 0) { /* EINVAL: malformed/under-filled — check peios_mp_writer_error */ }

peios_mp_writer_bytes validates that what you built is exactly one well-formed value, so a non-negative return means the payload is emit-ready. (Remember a map of n needs 2*n values — one per key and value.)

Emit it 

int rc = peios_event_emit("my.app.login", 12, payload, (uint32_t)plen);
peios_mp_writer_free(w);

if (rc != 0) {
    switch (errno) {
    case EPERM:  /* no SeAuditPrivilege */          break;
    case EINVAL: /* zero-length type or bad payload */ break;
    case ENOSPC: /* payload too large */            break;
    case EAGAIN: /* rate-limited — back off */      break;
    }
}

The event type is length-counted UTF-8 and must be non-zero length ("my.app.login" is 12 bytes — not NUL-terminated on the wire). On success the kernel stamps the trusted metadata (timestamp, sequence, identity GUIDs) and sets origin_class = userspace; you don't provide any of that.

Validating untrusted payloads first

If a payload's shape comes from dynamic or untrusted input, validate it in userspace before emitting so you handle the failure on your terms rather than as an EINVAL from the kernel:

if (peios_mp_validate(payload, plen, KMES_CONFIG_MAX_NESTING_DEPTH_DEFAULT) != 0) {
    /* reject it yourself */
}

The validator's acceptance matches the kernel's emit-time check at that depth bound.

Emitting in batches

A high-rate producer should batch. peios_event_emit_batch emits many events in one call, so a single timestamp capture, identity capture, and consumer wake cover the whole set:

struct peios_event_entry entries[3] = {
    { "my.app.a", 8, pa, pa_len },
    { "my.app.b", 8, pb, pb_len },
    { "my.app.c", 8, pc, pc_len },
};

uint32_t emitted = 0;
int rc = peios_event_emit_batch(entries, 3, &emitted);
if (rc != 0) {
    /* errno is from entries[emitted] — the first that failed.
       entries[0..emitted) were emitted; resume from `emitted`. */
}

count must be in [1, KMES_BATCH_MAX_ENTRIES]. On failure, errno is the reason the first failing entry failed and emitted tells you how many succeeded before it, so you know exactly where to resume. One caveat: rate-limiting is all-or-nothing for a batch — an EAGAIN emits none of it, so on EAGAIN back off and retry the whole batch.

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See also