c-cpp-integration.md

C/C++ Integration

liblogjet is a shared library for C and C++ callers that want to emit OTLP logs into ljd without embedding Rust directly in the appliance application.

The ABI is intentionally small:

• create a logger for OTLP/HTTP or OTLP/gRPC
• send one log record at a time
• provide:
  • message body as a string
  • severity
  • timestamp in Unix nanoseconds
  • zero or more string key/value attributes

Those key/value pairs become OTLP LogRecord.attributes, which is the standard OpenTelemetry metadata field for logs.

Build

From the project root:

cargo build -p liblogjet

Header:

liblogjet/include/liblogjet.h

Shared object:

target/debug/libliblogjet.so

Minimal C++ Example

This is the smallest useful flow:

1. load the .so
2. resolve the needed symbols
3. create a logger
4. send one warning-level log

#include "liblogjet.h"
#include <dlfcn.h>

using new_grpc_fn = lj_logger *(*)(const char *, const char *, uint64_t);
using log_fn = bool (*)(lj_logger *, const lj_log_record *);
using free_fn = void (*)(lj_logger *);
using err_fn = const char *(*)();

int main() {
    void *so = dlopen("./liblogjet.so", RTLD_NOW | RTLD_LOCAL);
    auto lj_logger_new_grpc = reinterpret_cast<new_grpc_fn>(dlsym(so, "lj_logger_new_grpc"));
    auto lj_logger_log = reinterpret_cast<log_fn>(dlsym(so, "lj_logger_log"));
    auto lj_logger_free = reinterpret_cast<free_fn>(dlsym(so, "lj_logger_free"));
    auto lj_error_message = reinterpret_cast<err_fn>(dlsym(so, "lj_error_message"));

    lj_logger *logger = lj_logger_new_grpc("127.0.0.1:4317", "hello-cpp", 2000);
    if (logger == nullptr) return 1;

    const lj_attribute attrs[] = {
        {"tag", "hello-world"},
        {"version", "2.04"},
    };
    const lj_log_record record{
        1700000000000000000ULL,           // timestamp in unix ns
        LJ_SEVERITY_WARN,                 // severity number
        "WARN",                           // severity text
        "Biohazard: C++ is in use!",      // :-)
        attrs,                            // attributes
        sizeof(attrs) / sizeof(attrs[0]), // attributes (len)
    };

    if (!lj_logger_log(logger, &record)) {
        const char *err = lj_error_message();
        (void)err;
    }

    lj_logger_free(logger);
    return 0;
}

Notes

• use lj_logger_new_http(...) for OTLP/HTTP
• use lj_logger_new_grpc(...) for OTLP/gRPC
• strings must be valid UTF-8
• attribute keys and values are currently string-only by design
• richer C++ usage lives in the demo:
  • demo/cpp-shared-lib