feat(c-api): add cuOptSetLogCallback and cuOptSetLogLevel

cpp/include/cuopt/mathematical_optimization/cuopt_c.h

@@ -823,6 +823,41 @@ cuopt_int_t cuOptGetFloatParameter(cuOptSolverSettings settings,
                                    const char* parameter_name,
                                    cuopt_float_t* parameter_value);
 
+/**
+ * @brief Type of callback invoked once per log line emitted by the solver.
+ *
+ * @param level    Severity of the log line, increasing with value
+ *  (0=trace, 1=debug, 2=info, 3=warn, 4=error, 5=critical). Intended only for
+ *  display/filtering of output, not as a stable programmatic API.
+ * @param message  Null-terminated log line without trailing newline.
+ * @param user_data Opaque pointer passed to cuOptSetLogCallback.
+ *
+ * @note The callback is invoked from the solver thread. Do not call back into
+ *  cuOpt from inside the callback.
+ * @warning Log message formatting is not part of the stable API and may change
+ *  between releases. The callback is intended for display purposes (GUI integration,
+ *  log forwarding, stdout capture) — do not parse message content for programmatic
+ *  control flow.
+ */
+typedef void (*cuOptLogCallback)(int level, const char* message, void* user_data);
+
+/**
+ * @brief Register a callback to receive solver log messages.
+ *
+ * The callback is invoked once per log line. It is called in addition to any
+ * file or console sink already enabled via ``log_to_console`` / ``log_file``
+ * parameters. Pass NULL to remove a previously registered callback.
+ *
+ * @param[in] settings  The solver settings object.
+ * @param[in] callback  Callback function, or NULL to clear.
+ * @param[in] user_data Opaque pointer forwarded to the callback unchanged.
+ *
+ * @return A status code indicating success or failure.
+ */
+cuopt_int_t cuOptSetLogCallback(cuOptSolverSettings settings,
+                                cuOptLogCallback callback,
+                                void* user_data);
+
 /**
  * @brief Type of callback for receiving incumbent MIP solutions with user context.
  *

cpp/src/mip_heuristics/diversity/diversity_manager.cu

@@ -207,7 +207,7 @@ void diversity_manager_t<i_t, f_t>::add_user_given_solutions(
         *problem_ptr->original_problem_ptr, h_original, h_crushed);
       init_sol_assignment = cuopt::device_copy(h_crushed, sol.handle_ptr->get_stream());
 
-#if CUOPT_LOG_ACTIVE_LEVEL <= CUOPT_LOG_LEVEL_DEBUG
+#if CUOPT_LOG_ACTIVE_LEVEL <= RAPIDS_LOGGER_LOG_LEVEL_DEBUG
       const auto& reduced_problem       = *problem_ptr->original_problem_ptr;
       const std::vector<f_t> h_red_obj  = reduced_problem.get_objective_coefficients_host();
       const std::vector<f_t>& h_ori_obj = presolver_ptr->get_original_objective_coefficients();

cpp/src/mip_heuristics/feasibility_jump/feasibility_jump.cu

@@ -945,7 +945,7 @@ i_t fj_t<i_t, f_t>::host_loop(solution_t<i_t, f_t>& solution, i_t climber_idx)
       }
     }
   }
-#if CUOPT_LOG_ACTIVE_LEVEL == CUOPT_LOG_LEVEL_TRACE
+#if CUOPT_LOG_ACTIVE_LEVEL == RAPIDS_LOGGER_LOG_LEVEL_TRACE
   auto h_sol = cuopt::host_copy(solution.assignment, climber_stream);
   static std::set<std::vector<f_t>> solutions_set;
   bool same_sol = solutions_set.count(h_sol) > 0;

cpp/src/pdlp/cuopt_c.cpp

@@ -89,6 +89,9 @@ struct solver_settings_handle_t {
   ~solver_settings_handle_t() { delete settings; }
   solver_settings_t<cuopt_int_t, cuopt_float_t>* settings;
   std::vector<std::unique_ptr<cuopt::internals::base_solution_callback_t>> callbacks;
+  // Log callback registered via cuOptSetLogCallback
+  cuOptLogCallback log_callback{nullptr};
+  void* log_callback_user_data{nullptr};
 };
 
 solver_settings_handle_t* get_settings_handle(cuOptSolverSettings settings)
@@ -1069,6 +1072,17 @@ cuopt_int_t cuOptSetMIPSetSolutionCallback(cuOptSolverSettings settings,
   return CUOPT_SUCCESS;
 }
 
+cuopt_int_t cuOptSetLogCallback(cuOptSolverSettings settings,
+                                cuOptLogCallback callback,
+                                void* user_data)
+{
+  if (settings == nullptr) { return CUOPT_INVALID_ARGUMENT; }
+  solver_settings_handle_t* handle = get_settings_handle(settings);
+  handle->log_callback             = callback;
+  handle->log_callback_user_data   = user_data;
+  return CUOPT_SUCCESS;
+}
+
 cuopt_int_t cuOptSetInitialPrimalSolution(cuOptSolverSettings settings,
                                           const cuopt_float_t* primal_solution,
                                           cuopt_int_t num_variables)
@@ -1145,6 +1159,23 @@ cuopt_int_t cuOptSolve(cuOptOptimizationProblem problem,
   if (settings == nullptr) { return CUOPT_INVALID_ARGUMENT; }
   if (solution_ptr == nullptr) { return CUOPT_INVALID_ARGUMENT; }
 
+  // Install user log callback so init_logger_t inside the solver picks it up.
+  // The RAII guard clears it on scope exit (whether by return or exception).
+  solver_settings_handle_t* handle = get_settings_handle(settings);
+  struct log_scope_guard_t {
+    bool has_callback;
+    ~log_scope_guard_t()
+    {
+      if (has_callback) { cuopt::clear_pending_log_callback(); }
+    }
+  } log_scope{false};
+
+  if (handle->log_callback) {
+    // cuOptLogCallback and log_callback_with_data_t share the same signature.
+    cuopt::set_pending_log_callback(handle->log_callback, handle->log_callback_user_data);
+    log_scope.has_callback = true;
+  }
+
   problem_and_stream_view_t* problem_and_stream_view =
     static_cast<problem_and_stream_view_t*>(problem);
 

cpp/src/utilities/logger.cpp

@@ -1,13 +1,18 @@
 /* clang-format off */
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2025, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+ * SPDX-FileCopyrightText: Copyright (c) 2025-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 /* clang-format on */
 
 #include <utilities/logger.hpp>
 #include <utilities/version_info.hpp>
 
+#include <algorithm>
+#include <cctype>
+#include <cstdlib>
+#include <optional>
+
 namespace cuopt {
 
 struct buffered_entry {
@@ -82,13 +87,44 @@ rapids_logger::sink_ptr default_sink()
  */
 inline std::string default_pattern() { return "[%Y-%m-%d %H:%M:%S:%f] [%n] [%-6l] %v"; }
 
+/**
+ * @brief Runtime log-level override from the `CUOPT_LOG_LEVEL` environment variable.
+ *
+ * Accepts a level name (case-insensitive): TRACE, DEBUG, INFO, WARN, ERROR, CRITICAL, OFF.
+ * Returns std::nullopt if the variable is unset or holds an unrecognised value.
+ *
+ * @note Statements below the compile-time `CUOPT_LOG_ACTIVE_LEVEL` (default INFO) are
+ *  removed at build time, so raising verbosity above the build level has no effect;
+ *  lowering it (e.g. WARN/ERROR/OFF to suppress output) always works.
+ */
+inline std::optional<rapids_logger::level_enum> env_log_level()
+{
+  const char* env = std::getenv("CUOPT_LOG_LEVEL");
+  if (env == nullptr) { return std::nullopt; }
+  std::string level{env};
+  std::transform(level.begin(), level.end(), level.begin(), [](unsigned char c) {
+    return static_cast<char>(std::toupper(c));
+  });
+  if (level == "TRACE") { return rapids_logger::level_enum::trace; }
+  if (level == "DEBUG") { return rapids_logger::level_enum::debug; }
+  if (level == "INFO") { return rapids_logger::level_enum::info; }
+  if (level == "WARN") { return rapids_logger::level_enum::warn; }
+  if (level == "ERROR") { return rapids_logger::level_enum::error; }
+  if (level == "CRITICAL") { return rapids_logger::level_enum::critical; }
+  if (level == "OFF") { return rapids_logger::level_enum::off; }
+  return std::nullopt;  // unrecognised value: keep the compiled default
+}
+
 /**
  * @brief Returns the default log level for the global logger.
  *
+ * The `CUOPT_LOG_LEVEL` environment variable, when set, overrides the compile-time default.
+ *
  * @return rapids_logger::level_enum The default log level.
  */
 inline rapids_logger::level_enum default_level()
 {
+  if (auto lvl = env_log_level()) { return *lvl; }
 #if CUOPT_LOG_ACTIVE_LEVEL == RAPIDS_LOGGER_LOG_LEVEL_TRACE
   return rapids_logger::level_enum::trace;
 #elif CUOPT_LOG_ACTIVE_LEVEL == RAPIDS_LOGGER_LOG_LEVEL_DEBUG
@@ -137,14 +173,62 @@ void reset_default_logger()
   default_logger().flush_on(rapids_logger::level_enum::debug);
 }
 
-// Guard object whose destructor resets the logger
+// Forward declarations needed by logger_config_guard destructor.
+static std::mutex g_guard_mutex;
+static const struct captured_log_callback_t* g_active_log_callback;
+
+// Captured (immutable) callback state owned by the active logger guard.
+struct captured_log_callback_t {
+  log_callback_with_data_t callback;
+  void* user_data;
+};
+
+// Guard object whose destructor resets the logger.
+// Owns the captured callback state to guarantee its lifetime.
 struct logger_config_guard {
-  ~logger_config_guard() { cuopt::reset_default_logger(); }
+  std::unique_ptr<captured_log_callback_t> callback_state;
+  ~logger_config_guard()
+  {
+    cuopt::reset_default_logger();  // removes the sink; blocks until in-flight log calls finish
+    std::lock_guard<std::mutex> lock(g_guard_mutex);
+    g_active_log_callback = nullptr;  // safe: the sink (and the bridge) are already gone
+  }
 };
 
 // Weak reference to detect if any init_logger_t instance is still alive
 static std::weak_ptr<logger_config_guard> g_active_guard;
-static std::mutex g_guard_mutex;
+
+// g_active_log_callback: written only under g_guard_mutex (at guard create/destroy time).
+// Read lock-free by user_log_bridge — safe because the bridge is only reachable
+// while the sink is alive, and the sink is removed (in reset_default_logger) before
+// this pointer is cleared.
+
+// Pending user log callback set by the C API before cuOptSolve.
+// Consumed once (under g_guard_mutex) by init_logger_t to build the guard state.
+static log_callback_with_data_t g_pending_callback = nullptr;
+static void* g_pending_callback_data               = nullptr;
+
+static void user_log_bridge(int lvl, const char* msg)
+{
+  // g_active_log_callback is stable for the duration of any bridge call:
+  // it points into the guard's callback_state, which outlives the sink.
+  const captured_log_callback_t* state = g_active_log_callback;
+  if (state) { state->callback(lvl, msg, state->user_data); }
+}
+
+void set_pending_log_callback(log_callback_with_data_t cb, void* user_data)
+{
+  std::lock_guard<std::mutex> lock(g_guard_mutex);
+  g_pending_callback      = cb;
+  g_pending_callback_data = user_data;
+}
+
+void clear_pending_log_callback()
+{
+  std::lock_guard<std::mutex> lock(g_guard_mutex);
+  g_pending_callback      = nullptr;
+  g_pending_callback_data = nullptr;
+}
 
 init_logger_t::init_logger_t(std::string log_file, bool log_to_console)
 {
@@ -169,6 +253,15 @@ init_logger_t::init_logger_t(std::string log_file, bool log_to_console)
       std::make_shared<rapids_logger::basic_file_sink_mt>(log_file, true));
     cuopt::default_logger().flush_on(rapids_logger::level_enum::debug);
   }
+  // Capture pending callback into the guard so the bridge reads stable (immutable) state.
+  auto guard = std::make_shared<logger_config_guard>();
+  if (g_pending_callback) {
+    guard->callback_state = std::make_unique<captured_log_callback_t>(
+      captured_log_callback_t{g_pending_callback, g_pending_callback_data});
+    g_active_log_callback = guard->callback_state.get();
+    cuopt::default_logger().sinks().push_back(
+      std::make_shared<rapids_logger::callback_sink_mt>(user_log_bridge));
+  }
 
 #if CUOPT_LOG_ACTIVE_LEVEL >= RAPIDS_LOGGER_LOG_LEVEL_INFO
   cuopt::default_logger().set_pattern("%v");
@@ -182,8 +275,6 @@ init_logger_t::init_logger_t(std::string log_file, bool log_to_console)
     cuopt::default_logger().log(entry.level, entry.msg.c_str());
   }
 
-  // Create guard and store weak reference for future instances to find
-  auto guard     = std::make_shared<logger_config_guard>();
   g_active_guard = guard;
   guard_         = guard;
 }

cpp/src/utilities/logger.hpp

@@ -36,6 +36,20 @@ rapids_logger::logger& default_logger();
  */
 void reset_default_logger();
 
+// C-compatible log callback type: void callback(int level, const char* msg, void* user_data)
+// Matches cuOptLogCallback in cuopt_c.h — layout-compatible, no dependency on that header.
+using log_callback_with_data_t = void (*)(int level, const char* message, void* user_data);
+
+/**
+ * @brief Install a user log callback to be picked up by the next init_logger_t.
+ *
+ * Must be called before the init_logger_t that starts the targeted solve.
+ * Protected by the same mutex as init_logger_t so it is safe to call from
+ * any thread, but do not call from inside the callback itself.
+ */
+void set_pending_log_callback(log_callback_with_data_t cb, void* user_data);
+void clear_pending_log_callback();
+
 // Ref-counted logger initializer
 class init_logger_t {
   // Using shared_ptr for ref-counting