Provide separate dlsym() wrappers for functions and data

jmarshall · daviesrob · commit 00c61d2c8d83 · 2020-07-07T14:24:26.000+01:00
Avoid -pedantic warnings about data pointer <-> function pointer conversions (which are required to work on POSIX platforms so that dlsym() works, but are nonetheless warned about by compilers). As we're providing wrappers around dlsym() anyway, we can provide separate function and data wrappers; change load_plugin()'s return type as it (practically) always returns a function. This uses the workaround recommended in POSIX Issue 6 dlsym() (since removed in Issue 7 by [1]; at least for GCC, type-punning through a union might be preferable). Hat tip Rob Davies. [1] https://www.austingroupbugs.net/view.php?id=74
diff --git a/hts_internal.h b/hts_internal.h
@@ -96,8 +96,10 @@ void hts_path_itr_setup(struct hts_path_itr *itr, const char *path,
 
 const char *hts_path_itr_next(struct hts_path_itr *itr);
 
-void *load_plugin(void **pluginp, const char *filename, const char *symbol);
+typedef void plugin_void_func(void);
+plugin_void_func *load_plugin(void **pluginp, const char *filename, const char *symbol);
 void *plugin_sym(void *plugin, const char *name, const char **errmsg);
+plugin_void_func *plugin_func(void *plugin, const char *name, const char **errmsg);
 void close_plugin(void *plugin);
 
 /*
diff --git a/plugin.c b/plugin.c
@@ -132,12 +132,13 @@ const char *hts_path_itr_next(struct hts_path_itr *itr)
 #define RTLD_NOLOAD 0
 #endif
 
-void *load_plugin(void **pluginp, const char *filename, const char *symbol)
+plugin_void_func *load_plugin(void **pluginp, const char *filename, const char *symbol)
 {
     void *lib = dlopen(filename, RTLD_NOW | RTLD_LOCAL);
     if (lib == NULL) goto error;
 
-    void *sym = dlsym(lib, symbol);
+    plugin_void_func *sym;
+    *(void **) &sym = dlsym(lib, symbol);
     if (sym == NULL) {
         // Reopen the plugin with RTLD_GLOBAL and check for uniquified symbol
         void *libg = dlopen(filename, RTLD_NOLOAD | RTLD_NOW | RTLD_GLOBAL);
@@ -152,7 +153,7 @@ void *load_plugin(void **pluginp, const char *filename, const char *symbol)
         const char *basename = slash? slash+1 : filename;
         kputsn(basename, strcspn(basename, ".-+"), &symbolg);
 
-        sym = dlsym(lib, symbolg.s);
+        *(void **) &sym = dlsym(lib, symbol);
         free(symbolg.s);
         if (sym == NULL) goto error;
     }
@@ -175,6 +176,13 @@ void *plugin_sym(void *plugin, const char *name, const char **errmsg)
     return sym;
 }
 
+plugin_void_func *plugin_func(void *plugin, const char *name, const char **errmsg)
+{
+    plugin_void_func *sym;
+    *(void **) &sym = plugin_sym(plugin, name, errmsg);
+    return sym;
+}
+
 void close_plugin(void *plugin)
 {
     if (dlclose(plugin) != 0) {
diff --git a/test/plugins-dlhts.c b/test/plugins-dlhts.c
@@ -55,6 +55,13 @@ void *sym(void *htslib, const char *name)
     return ptr;
 }
 
+typedef void void_func(void);
+void_func *func(void *htslib, const char *name) {
+    void_func *fptr;
+    *(void **) &fptr = sym(htslib, name);
+    return fptr;
+}
+
 int errors = 0;
 int verbose = 0;
 
@@ -112,11 +119,11 @@ int main(int argc, char **argv)
         *hts_verbosep += verbose;
 
         typedef const char *cstr_func(void);
-        printf("Loaded HTSlib %s\n", ((cstr_func *) sym(htslib, "hts_version"))());
+        printf("Loaded HTSlib %s\n", ((cstr_func *) func(htslib, "hts_version"))());
     }
 
-    hopen_p = (hopen_func *) sym(htslib, "hopen");
-    hclose_abruptly_p = (hclose_abruptly_func *) sym(htslib, "hclose_abruptly");
+    hopen_p = (hopen_func *) func(htslib, "hopen");
+    hclose_abruptly_p = (hclose_abruptly_func *) func(htslib, "hclose_abruptly");
 
     test_hopen("bad-scheme:unsupported", 0);
 #ifdef HAVE_LIBCURL
@@ -129,8 +136,7 @@ int main(int argc, char **argv)
     test_hopen("s3:invalid", 1);
 #endif
 
-    typedef void void_func(void);
-    ((void_func *) sym(htslib, "hts_lib_shutdown"))();
+    (func(htslib, "hts_lib_shutdown"))();
 
     if (dlclose(htslib) < 0) {
         fprintf(stderr, "Can't dlclose \"%s\": %s\n", argv[optind], dlerror());
