Merge pull request #196 from jnpkrn/demystify-qblog.h

Low: explain mysterious lines in a public header (qblog.h)

configure.ac

@@ -549,10 +549,12 @@ AC_SUBST(HAVE_SYSLOG_TESTS)
 
 # --- callsite sections ---
 if test "x${GCC}" = xyes; then
-	AC_MSG_CHECKING([whether GCC supports __attribute__((section())])
+	AC_MSG_CHECKING([whether GCC supports __attribute__((section()) + ld supports orphan sections])
 	if test "x${ac_cv_link_attribute_section}" = x ; then
-		AC_TRY_LINK([],
+		AC_TRY_LINK([#include <assert.h>
+			     extern void * __start___verbose, * __stop___verbose;],
 			    [static int my_var __attribute__((section("__verbose"))) = 5;
+			     if (__start___verbose == __stop___verbose) assert(0);
 			     if (my_var == 5) return 0;
 			     else return -1;
 			     ],

include/qb/qbdefs.h

@@ -56,6 +56,15 @@ extern "C" {
 #define qb_bit_is_set(barray, bit) (barray & qb_bit_value(bit))
 #define qb_bit_is_clear(barray, bit) (!(barray & qb_bit_value(bit)))
 
+/*
+ * wrappers over preprocessor operators
+ */
+
+#define QB_PP_JOIN_(a, b)	a##b
+#define QB_PP_JOIN(a, b)	QB_PP_JOIN_(a, b)
+#define QB_PP_STRINGIFY_(arg)	#arg
+#define QB_PP_STRINGIFY(arg)	QB_PP_STRINGIFY_(arg)
+
 
 /*
  * handy time based converters.

include/qb/qblog.h

@@ -253,14 +253,26 @@ struct qb_log_callsite {
 
 typedef void (*qb_log_filter_fn)(struct qb_log_callsite * cs);
 
-/* will be assigned by ld linker magic */
+/* will be assigned by linker magic (assuming linker supports that):
+ * https://sourceware.org/binutils/docs/ld/Orphan-Sections.html
+ */
 #ifdef QB_HAVE_ATTRIBUTE_SECTION
-extern struct qb_log_callsite __start___verbose[];
-extern struct qb_log_callsite __stop___verbose[];
 
-#define QB_LOG_INIT_DATA(name)						\
-    void name(void);							\
-    void name(void) { if (__start___verbose != __stop___verbose) {assert(1);} }	\
+#define QB_ATTR_SECTION			__verbose  /* conforms to C ident. */
+#define QB_ATTR_SECTION_STR		QB_PP_STRINGIFY(QB_ATTR_SECTION)
+#define QB_ATTR_SECTION_START		QB_PP_JOIN(__start_, QB_ATTR_SECTION)
+#define QB_ATTR_SECTION_STOP		QB_PP_JOIN(__stop_, QB_ATTR_SECTION)
+#define QB_ATTR_SECTION_START_STR	QB_PP_STRINGIFY(QB_ATTR_SECTION_START)
+#define QB_ATTR_SECTION_STOP_STR	QB_PP_STRINGIFY(QB_ATTR_SECTION_STOP)
+
+extern struct qb_log_callsite QB_ATTR_SECTION_START[];
+extern struct qb_log_callsite QB_ATTR_SECTION_STOP[];
+
+/* mere linker sanity check, possible future extension for internal purposes */
+#define QB_LOG_INIT_DATA(name)							\
+    void name(void);								\
+    void name(void) 								\
+    { if (QB_ATTR_SECTION_START == QB_ATTR_SECTION_STOP) assert(0); }	\
     void __attribute__ ((constructor)) name(void);
 #else
 #define QB_LOG_INIT_DATA(name)
@@ -345,7 +357,7 @@ void qb_log_from_external_source_va(const char *function,
 #ifdef QB_HAVE_ATTRIBUTE_SECTION
 #define qb_logt(priority, tags, fmt, args...) do {			\
 	static struct qb_log_callsite descriptor			\
-	__attribute__((section("__verbose"), aligned(8))) =		\
+	__attribute__((section(QB_ATTR_SECTION_STR), aligned(8))) =	\
 	{ __func__, __FILE__, fmt, priority, __LINE__, 0, tags };	\
 	qb_log_real_(&descriptor, ##args);				\
     } while(0)
@@ -506,8 +518,8 @@ void qb_log_fini(void);
  * you will need to do the following to get the filters to work
  * in that module:
  * @code
- * 	_start = dlsym (dl_handle, "__start___verbose");
- *	_stop = dlsym (dl_handle, "__stop___verbose");
+ * 	_start = dlsym (dl_handle, QB_ATTR_SECTION_START_STR);
+ *	_stop = dlsym (dl_handle, QB_ATTR_SECTION_STOP_STR);
  *	qb_log_callsites_register(_start, _stop);
  * @endcode
  */

lib/log.c

@@ -800,13 +800,13 @@ _log_so_walk_dlnames(void)
 			goto done;
 		}
 
-		start = dlsym(handle, "__start___verbose");
+		start = dlsym(handle, QB_ATTR_SECTION_START_STR);
 		error = dlerror();
 		if (error) {
 			goto done;
 		}
 
-		stop = dlsym(handle, "__stop___verbose");
+		stop = dlsym(handle, QB_ATTR_SECTION_STOP_STR);
 		error = dlerror();
 		if (error) {
 			goto done;
@@ -868,7 +868,7 @@ qb_log_init(const char *name, int32_t facility, uint8_t priority)
 
 	qb_log_dcs_init();
 #ifdef QB_HAVE_ATTRIBUTE_SECTION
-	qb_log_callsites_register(__start___verbose, __stop___verbose);
+	qb_log_callsites_register(QB_ATTR_SECTION_START, QB_ATTR_SECTION_STOP);
 	dl_iterate_phdr(_log_so_walk_callback, NULL);
 	_log_so_walk_dlnames();
 #endif /* QB_HAVE_ATTRIBUTE_SECTION */