drivers/main-stub.c: separate from main.c and register_upsdrv_callbacks() to allow dynamic linking [#2800]

Signed-off-by: Jim Klimov <jimklimov+nut@gmail.com>

Author: jimklimov

drivers/Makefile.am

@@ -51,7 +51,7 @@ if ENABLE_SHARED_PRIVATE_LIBS
   LDADD_COMMON = \
 	$(top_builddir)/common/libnutprivate-common-all.la
 
-  LDADD_DRIVERS = libnutprivate-drivers-common.la $(LDADD_COMMON)
+  LDADD_DRIVERS = libdummy_main_stub.la libnutprivate-drivers-common.la $(LDADD_COMMON)
 
   LDADD_DRIVERS_SERIAL = libnutprivate-drivers-serial.la $(LDADD_DRIVERS)
 
@@ -508,14 +508,24 @@ dist_noinst_HEADERS = \
 # because per-object CFLAGS can only be specified for libraries, not
 # for object files. This library is used during the build process,
 # and is not meant to be installed.
-EXTRA_LTLIBRARIES = libdummy_main.la libdummy_serial.la libdummy_upsdrvquery.la
-
+EXTRA_LTLIBRARIES = libdummy_main.la libdummy_main_stub.la libdummy_serial.la libdummy_upsdrvquery.la
+
+# NOTE: main.c includes tons of shared logic and global variables
+# used by all NUT drivers, so is a prime candidate for being a
+# shared library if we ENABLE_SHARED_PRIVATE_LIBS. But then we
+# have a chicken-and-egg problem, that actual NUT driver program
+# sources, which declare upsdrv_*() methods and info, have no way
+# to make the shared library variant of main code to see them.
+# This is where the main-stub.c comes in, with a small bit of code
+# compiled and linked into each driver binary to marry the two worlds.
 libdummy_main_la_SOURCES = main.c dstate.c
+libdummy_main_stub_la_SOURCES = main-stub.c
 libdummy_serial_la_SOURCES = serial.c
 libdummy_upsdrvquery_la_SOURCES = upsdrvquery.c
 
 if !ENABLE_SHARED_PRIVATE_LIBS
 libdummy_main_la_LDFLAGS = -no-undefined -static
+libdummy_main_stub_la_LDFLAGS = -no-undefined -static
 libdummy_serial_la_LDFLAGS = -no-undefined -static
 libdummy_upsdrvquery_la_LDFLAGS = -no-undefined -static
 endif !ENABLE_SHARED_PRIVATE_LIBS

drivers/main-stub.c

@@ -0,0 +1,50 @@
+/* main-stub.c - Network UPS Tools driver core stub
+
+   This code is linked into each binary to ensure that even if the
+   NUT drivers are built as dynamically-linked programs, the shared
+   library can find needed methods and data provided by each specific
+   driver program source file(s) and declared by main.h.
+
+   Copyright (C)
+   2026 -     	Jim Klimov <jimklimov+nut@gmail.com>
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+*/
+
+#include "common.h"
+#include "main.h"
+
+#if (defined ENABLE_SHARED_PRIVATE_LIBS) && ENABLE_SHARED_PRIVATE_LIBS
+/* Provided by main.c in shared-mode builds */
+int drv_main(int argc, char **argv);
+
+/* This source file is used in some unit tests to mock realistic driver
+ * behavior - using a production driver skeleton, but their own main().
+ */
+# ifndef DRIVERS_MAIN_WITHOUT_MAIN
+int main(int argc, char **argv)
+{
+	/* shared build, symbols should be visible to us right away,
+	 * but not to (library-stored copy of) main.c */
+	default_register_upsdrv_callbacks();
+	return drv_main(argc, argv);
+}
+# endif /* DRIVERS_MAIN_WITHOUT_MAIN */
+#else	/* !ENABLE_SHARED_PRIVATE_LIBS */
+
+/* Just avoid warning/error: ISO C forbids an empty translation unit [-Werror=pedantic] */
+int main (int argc, char ** argv);
+
+#endif	/* !ENABLE_SHARED_PRIVATE_LIBS */

drivers/main.c

@@ -1,10 +1,10 @@
-/* main.c - Network UPS Tools driver core
+/* main.c - Network UPS Tools driver core logic and global variables
 
    Copyright (C)
    1999			Russell Kroll <rkroll@exploits.org>
    2005 - 2017	Arnaud Quette <arnaud.quette@free.fr>
    2017 		Eaton (author: Emilien Kia <EmilienKia@Eaton.com>)
-   2017 - 2025	Jim Klimov <jimklimov+nut@gmail.com>
+   2017 - 2026	Jim Klimov <jimklimov+nut@gmail.com>
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -164,30 +164,39 @@ static int handle_reload_flag(void);
 /* Set in do_ups_confargs() for consumers like handle_reload_flag() */
 static int reload_requires_restart = -1;
 
+static upsdrv_callback_t upsdrv_callbacks = {0};
+void register_upsdrv_callbacks(upsdrv_callback_t callbacks) {
+	/*FIXME: mempcy of arbitrarily ordered list of function pointers
+	 * does not feel safe, better use named methods to initialize?
+	 * upsdrv_callbacks = callbacks;
+	 */
+	memcpy(&upsdrv_callbacks, &callbacks, sizeof(callbacks));
+}
+
 /* print the driver banner */
 void upsdrv_banner (void)
 {
 	int i;
 
 	printf("Network UPS Tools %s - %s%s %s\n",
 		describe_NUT_VERSION_once(),
-		upsdrv_info.name,
-		strstr(upsdrv_info.name, "river") ? "" : " driver",
-		upsdrv_info.version);
+		upsdrv_callbacks.upsdrv_info.name,
+		strstr(upsdrv_callbacks.upsdrv_info.name, "river") ? "" : " driver",
+		upsdrv_callbacks.upsdrv_info.version);
 
 	/* process sub driver(s) information */
-	for (i = 0; upsdrv_info.subdrv_info[i]; i++) {
+	for (i = 0; upsdrv_callbacks.upsdrv_info.subdrv_info[i]; i++) {
 
-		if (!upsdrv_info.subdrv_info[i]->name) {
+		if (!upsdrv_callbacks.upsdrv_info.subdrv_info[i]->name) {
 			continue;
 		}
 
-		if (!upsdrv_info.subdrv_info[i]->version) {
+		if (!upsdrv_callbacks.upsdrv_info.subdrv_info[i]->version) {
 			continue;
 		}
 
-		printf("%s %s\n", upsdrv_info.subdrv_info[i]->name,
-			upsdrv_info.subdrv_info[i]->version);
+		printf("%s %s\n", upsdrv_callbacks.upsdrv_info.subdrv_info[i]->name,
+			upsdrv_callbacks.upsdrv_info.subdrv_info[i]->version);
 	}
 
 	fflush(stdout);
@@ -304,7 +313,7 @@ static void help_msg(void)
 		}
 	}
 
-	upsdrv_help();
+	upsdrv_callbacks.upsdrv_help();
 
 	printf("\n%s", suggest_doc_links(progname, "ups.conf"));
 }
@@ -846,7 +855,7 @@ int do_loop_shutdown_commands(const char *sdcmds, char **cmdused) {
 					sdcmds);
 				/* TOTHINK : These logs are handled in driver codes */
 				/* upslog_INSTCMD_POWERSTATE_CHANGE("shutdown.default", NULL); */
-				upsdrv_shutdown();
+				upsdrv_callbacks.upsdrv_shutdown();
 				cmdret = STAT_INSTCMD_HANDLED;
 				/* commented below */
 				if (cmdused && !(*cmdused))
@@ -865,7 +874,7 @@ int do_loop_shutdown_commands(const char *sdcmds, char **cmdused) {
 			/* TOTHINK : These logs are handled in driver codes */
 			/* We are trying (if at all implemented), so "maybe"... */
 			/* upslog_INSTCMD_POWERSTATE_MAYBE(s, NULL); */
-			upsdrv_shutdown();
+			upsdrv_callbacks.upsdrv_shutdown();
 			cmdret = STAT_INSTCMD_HANDLED;
 		} else {
 			/* TOTHINK : These logs are handled in driver codes */
@@ -1017,7 +1026,7 @@ int main_instcmd(const char *cmdname, const char *extra, conn_t *conn) {
 		 * called by default if none were passed (or this one
 		 * was passed explicitly).
 		 */
-		upsdrv_shutdown();
+		upsdrv_callbacks.upsdrv_shutdown();
 		return STAT_INSTCMD_HANDLED;
 	}
 
@@ -1896,7 +1905,7 @@ void vartab_free(void)
 static void exit_upsdrv_cleanup(void)
 {
 	dstate_setinfo("driver.state", "cleanup.upsdrv");
-	upsdrv_cleanup();
+	upsdrv_callbacks.upsdrv_cleanup();
 }
 
 static void exit_cleanup(void)
@@ -2082,9 +2091,15 @@ void setup_signals(void)
 
 /* This source file is used in some unit tests to mock realistic driver
  * behavior - using a production driver skeleton, but their own main().
+ * It is called from main-stub.c in shared-mode builds.
  */
 #ifndef DRIVERS_MAIN_WITHOUT_MAIN
+# if (defined ENABLE_SHARED_PRIVATE_LIBS) && ENABLE_SHARED_PRIVATE_LIBS
+int drv_main(int argc, char **argv)
+# else
+/* Used right away */
 int main(int argc, char **argv)
+# endif
 {
 	struct	passwd	*new_uid = NULL;
 	int	i, do_forceshutdown = 0;
@@ -2110,6 +2125,9 @@ int main(int argc, char **argv)
 
 #if (defined ENABLE_SHARED_PRIVATE_LIBS) && ENABLE_SHARED_PRIVATE_LIBS
 	callback_upsconf_args = do_upsconf_args;
+#else
+	/* static build, symbols should be visible to main.c right away */
+	default_register_upsdrv_callbacks();
 #endif
 
 	/* init verbosity from default in common.c (0 probably) */
@@ -2206,21 +2224,21 @@ int main(int argc, char **argv)
 	}
 #endif	/* WIN32 */
 
-	upsdrv_tweak_prognames();
+	upsdrv_callbacks.upsdrv_tweak_prognames();
 
 	open_syslog(progname);
 
 	if (!banner_is_disabled()) {
 		upsdrv_banner();
 	}
 
-	if (upsdrv_info.status == DRV_EXPERIMENTAL) {
+	if (upsdrv_callbacks.upsdrv_info.status == DRV_EXPERIMENTAL) {
 		printf("Warning: This is an experimental driver.\n");
 		printf("Some features may not function correctly.\n\n");
 	}
 
 	/* build the driver's extra (-x) variable table */
-	upsdrv_makevartable();
+	upsdrv_callbacks.upsdrv_makevartable();
 
 	while ((i = getopt(argc, argv, optstring)) != -1) {
 		switch (i) {
@@ -2903,20 +2921,20 @@ int main(int argc, char **argv)
 	dstate_setinfo("device.type", "ups");
 
 	dstate_setinfo("driver.state", "init.device");
-	upsdrv_initups();
+	upsdrv_callbacks.upsdrv_initups();
 	dstate_setinfo("driver.state", "init.quiet");
 
 	/* UPS is detected now, cleanup upon exit */
 	atexit(exit_upsdrv_cleanup);
 
 	/* now see if things are very wrong out there */
-	if (upsdrv_info.status == DRV_BROKEN) {
+	if (upsdrv_callbacks.upsdrv_info.status == DRV_BROKEN) {
 		fatalx(EXIT_FAILURE, "Fatal error: broken driver. It probably needs to be converted.\n");
 	}
 
 	/* publish the top-level data: version numbers, driver name */
 	dstate_setinfo("driver.version", "%s", UPS_VERSION);
-	dstate_setinfo("driver.version.internal", "%s", upsdrv_info.version);
+	dstate_setinfo("driver.version.internal", "%s", upsdrv_callbacks.upsdrv_info.version);
 	dstate_setinfo("driver.name", "%s", progname);
 
 	/*
@@ -2929,7 +2947,7 @@ int main(int argc, char **argv)
 
 	/* get the base data established before allowing connections */
 	dstate_setinfo("driver.state", "init.info");
-	upsdrv_initinfo();
+	upsdrv_callbacks.upsdrv_initinfo();
 
 	/* Register a way to call upsdrv_shutdown() among `sdcommands` */
 	dstate_addcmd("shutdown.default");
@@ -2942,7 +2960,7 @@ int main(int argc, char **argv)
 	/* Note: a few drivers also call their upsdrv_updateinfo() during
 	 * their upsdrv_initinfo(), possibly to impact the initialization */
 	dstate_setinfo("driver.state", "init.updateinfo");
-	upsdrv_updateinfo();
+	upsdrv_callbacks.upsdrv_updateinfo();
 	dstate_setinfo("driver.state", "init.quiet");
 
 	if (dstate_getinfo("driver.flag.ignorelb")) {
@@ -3183,7 +3201,7 @@ int main(int argc, char **argv)
 		}
 
 		dstate_setinfo("driver.state", "updateinfo");
-		upsdrv_updateinfo();
+		upsdrv_callbacks.upsdrv_updateinfo();
 		dstate_setinfo("driver.state", "quiet");
 
 		/* Dump the data tree (in upsc-like format) to stdout and exit */

drivers/main.h

@@ -28,18 +28,24 @@ extern const char	*prognames[MAX_PROGNAMES];
 extern char	prognames_should_free[MAX_PROGNAMES];
 #define	progname	(prognames[0])
 
-/* functions & variables required in each driver */
+int drv_main(int argc, char **argv);
+
+/* functions & variables required in each driver
+ * See also: register_upsdrv_callbacks()
+ */
 void upsdrv_tweak_prognames(void);	/* optionally add aliases and/or set preferred name into [0] (for pipe name etc.); called just after populating prognames[0] and prognames_should_free[] entries */
 void upsdrv_initups(void);	/* open connection to UPS, fail if not found */
 void upsdrv_initinfo(void);	/* prep data, settings for UPS monitoring */
 void upsdrv_updateinfo(void);	/* update state data if possible */
 void upsdrv_shutdown(void);	/* make the UPS power off the load */
 void upsdrv_help(void);		/* tack on anything useful for the -h text */
-void upsdrv_banner(void);	/* print your version information */
 void upsdrv_cleanup(void);	/* free any resources before shutdown */
+void upsdrv_makevartable(void);	/* main calls this driver function - it needs to call addvar */
 
 void set_exit_flag(int sig);
 
+void upsdrv_banner(void);	/* print your version information - shared in main.c */
+
 /* --- details for the variable/value sharing --- */
 
 /* Try each instant command in the comma-separated list of
@@ -79,9 +85,6 @@ int main_instcmd(const char *cmdname, const char *extra, conn_t *conn);
  */
 int main_setvar(const char *varname, const char *val, conn_t *conn);
 
-/* main calls this driver function - it needs to call addvar */
-void upsdrv_makevartable(void);
-
 /* retrieve the value of variable <var> if possible */
 char *getval(const char *var);
 
@@ -155,7 +158,9 @@ typedef struct upsdrv_info_s {
 					 * pass */
 /* FIXME: complete with mfr support, and other interesting info */
 
-/* public driver information from the driver file */
+/* public driver information from the driver file
+ * See also: register_upsdrv_callbacks()
+ */
 extern upsdrv_info_t	upsdrv_info;
 
 /* functions and data possibly used via libdummy_mockdrv.la for unit-tests */
@@ -197,4 +202,39 @@ void setup_signals(void);
 # define SIGCMD_RELOAD_OR_ERROR         "driver.reload-or-error"	/* NUT_WIN32_INCOMPLETE */
 #endif	/* WIN32 */
 
+/* allow main.c code in both static and shared driver builds
+ * to see implementations defined by specific driver sources,
+ * called by main-stub.c where used:
+ */
+typedef struct upsdrv_callback_s {
+	void (*upsdrv_tweak_prognames)(void);	/* optionally add aliases and/or set preferred name into [0] (for pipe name etc.); called just after populating prognames[0] and prognames_should_free[] entries */
+	void (*upsdrv_initups)(void);	/* open connection to UPS, fail if not found */
+	void (*upsdrv_initinfo)(void);	/* prep data, settings for UPS monitoring */
+	void (*upsdrv_updateinfo)(void);	/* update state data if possible */
+	void (*upsdrv_shutdown)(void);	/* make the UPS power off the load */
+	void (*upsdrv_help)(void);		/* tack on anything useful for the -h text */
+	void (*upsdrv_cleanup)(void);	/* free any resources before shutdown */
+	void (*upsdrv_makevartable)(void);	/* main calls this driver function - it needs to call addvar */
+	upsdrv_info_t	upsdrv_info;	/* public driver information from the driver file */
+} upsdrv_callback_t;
+void register_upsdrv_callbacks(upsdrv_callback_t callbacks);
+
+/* simple call to register implementations named as dictated
+ * by this header, which (being a macro) can be called easily
+ * from both static and shared builds: */
+#define default_register_upsdrv_callbacks() do {			\
+	upsdrv_callback_t callbacksTmp;					\
+	memset(&callbacksTmp, 0, sizeof(callbacksTmp));			\
+	callbacksTmp.upsdrv_cleanup		= upsdrv_cleanup;	\
+	callbacksTmp.upsdrv_help		= upsdrv_help;		\
+	callbacksTmp.upsdrv_initups		= upsdrv_initups;	\
+	callbacksTmp.upsdrv_initinfo		= upsdrv_initinfo;	\
+	callbacksTmp.upsdrv_makevartable	= upsdrv_makevartable;	\
+	callbacksTmp.upsdrv_shutdown		= upsdrv_shutdown;	\
+	callbacksTmp.upsdrv_tweak_prognames	= upsdrv_tweak_prognames;	\
+	callbacksTmp.upsdrv_updateinfo		= upsdrv_updateinfo;	\
+	memcpy(&callbacksTmp.upsdrv_info, &upsdrv_info, sizeof(upsdrv_info));	\
+	register_upsdrv_callbacks(callbacksTmp);	\
+	} while (0)
+
 #endif /* NUT_MAIN_H_SEEN */