diff --git a/src/client/client_send.c b/src/client/client_send.c
index 535cb5d..c777d52 100644
--- a/src/client/client_send.c
+++ b/src/client/client_send.c
@@ -1967,7 +1967,7 @@ static int incremental_check(Client* client, File* file, const Config* config,
       send_status(client->file_descriptor, STATUS_ERROR);
       return -1;
     }
-    log_debug_message(LOG_DEBUG_RECV, "recv: delta signature for %s (%d blocks)",
+    log_debug_message(LOG_DEBUG_RECV, "recv: delta signature for %s (%u blocks)",
                       file_wire_path(file), sig->block_count);
     *out_sig = sig;
     return 2;
diff --git a/src/client/usage.c b/src/client/usage.c
index b4e0584..cd5e35c 100644
--- a/src/client/usage.c
+++ b/src/client/usage.c
@@ -165,7 +165,7 @@ void print_usage(void) {
   printf("                    --no-delta, or --no-incremental)\n");
   printf("  --no-fuzzy          Disable --fuzzy\n");
   printf("  -B <n>, --block-size <n>, --delta-block <n>\n");
-  printf("                    Delta block size in bytes (default: %d)\n", DELTA_BLOCK_SIZE_DEFAULT);
+  printf("                    Delta block size in bytes (default: %u)\n", DELTA_BLOCK_SIZE_DEFAULT);
   printf("  --delta-max <n>     Max file size for delta transfer (default: %llu)\n",
          DELTA_MAX_FILE_SIZE);
   printf("  -j, --threads[=N]   Enable the multithreaded scanner/loader/sender\n");
diff --git a/src/shared/log.h b/src/shared/log.h
index b3a258c..d64c467 100644
--- a/src/shared/log.h
+++ b/src/shared/log.h
@@ -5,6 +5,15 @@
 #include <stdbool.h>
 #include <stdint.h>
 
+/* Ask the compiler to type-check the printf-style arguments of the variadic
+ * logging helpers.  Only enabled for GNU-compatible compilers (gcc/clang). */
+#if defined(__GNUC__)
+#define LOG_PRINTF_ATTR(fmt_idx, first_vararg_idx)                                                 \
+  __attribute__((format(printf, fmt_idx, first_vararg_idx)))
+#else
+#define LOG_PRINTF_ATTR(fmt_idx, first_vararg_idx)
+#endif
+
 typedef enum { LOG_LEVEL_DEBUG, LOG_LEVEL_INFO, LOG_LEVEL_WARNING, LOG_LEVEL_ERROR } LogLevel;
 typedef enum { LOG_STDERR_ERRORS, LOG_STDERR_ALL } LogStderrMode;
 
@@ -59,7 +68,7 @@ typedef enum {
                  LOG_INFO_PROGRESS | LOG_INFO_MOUNT,
 } LogInfoFlag;
 
-void log_message(LogLevel log_level, const char* message, ...);
+void log_message(LogLevel log_level, const char* message, ...) LOG_PRINTF_ATTR(2, 3);
 void log_perror(const char* context);
 void set_log_level(LogLevel level);
 void set_log_debug_flags(uint32_t flags);
@@ -68,10 +77,10 @@ uint32_t get_log_debug_flags(void);
  * the debug log level is enabled AND the flag is selected.  Hot paths use this
  * to skip expensive message formatting/escaping when the line is filtered. */
 bool log_debug_enabled(LogDebugFlag flag);
-void log_debug_message(LogDebugFlag flag, const char* message, ...);
+void log_debug_message(LogDebugFlag flag, const char* message, ...) LOG_PRINTF_ATTR(2, 3);
 void set_log_info_flags(uint32_t flags);
 uint32_t get_log_info_flags(void);
-void log_info_message(LogInfoFlag flag, const char* message, ...);
+void log_info_message(LogInfoFlag flag, const char* message, ...) LOG_PRINTF_ATTR(2, 3);
 void log_set_file(FILE* fp);
 void log_set_8_bit_output(bool enabled);
 bool log_get_8_bit_output(void);
diff --git a/src/shared/protocol.c b/src/shared/protocol.c
index 581ebb8..fa1fff2 100644
--- a/src/shared/protocol.c
+++ b/src/shared/protocol.c
@@ -390,7 +390,7 @@ bool protocol_send_n_data(ProtocolSession* session, const void* data, size_t dat
     if (session->ssl)
       wait_events = POLLOUT;
   }
-  log_debug_message(LOG_DEBUG_IO, "    Send n Data: %zu", total_bytes_send);
+  log_debug_message(LOG_DEBUG_IO, "    Send n Data: %zd", total_bytes_send);
   atomic_fetch_add(&io_bytes_written, (unsigned long long)total_bytes_send);
   return true;
 }