samples, tests, boards: Switch main return type from void to int

As both C and C++ standards require applications running under an OS to
return 'int', adapt that for Zephyr to align with those standard. This also
eliminates errors when building with clang when not using -ffreestanding,
and reduces the need for compiler flags to silence warnings for both clang
and gcc.

Most of these changes were automated using coccinelle with the following
script:

@@
@@
- void
+ int
main(...) {
	...
-	return;
+	return 0;
	...
}

Approximately 40 files had to be edited by hand as coccinelle was unable to
fix them.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

boards/arm/mps2_an521/empty_cpu0/src/main.c

@@ -8,11 +8,12 @@
 
 extern void wakeup_cpu1(void);
 
-void main(void)
+int main(void)
 {
 	/* Simply wake-up the remote core */
 	wakeup_cpu1();
 
 	while (1) {
 	}
+	return 0;
 }

kernel/init.c

@@ -332,11 +332,7 @@ static void bg_thread_main(void *unused1, void *unused2, void *unused3)
 	z_mem_manage_boot_finish();
 #endif /* CONFIG_MMU */
 
-#ifdef CONFIG_CPP_MAIN
 	extern int main(void);
-#else
-	extern void main(void);
-#endif
 
 	(void)main();
 

samples/application_development/code_relocation_nocopy/src/main.c

@@ -42,7 +42,7 @@ void disable_mpu_rasr_xn(void)
 extern void function_in_ext_flash(void);
 extern void function_in_sram(void);
 
-void main(void)
+int main(void)
 {
 #ifdef CONFIG_ARM_MPU
 	disable_mpu_rasr_xn();
@@ -54,4 +54,5 @@ void main(void)
 	function_in_sram();
 
 	printk("Hello World! %s\n", CONFIG_BOARD);
+	return 0;
 }

samples/application_development/external_lib/src/main.c

@@ -12,8 +12,9 @@
 
 #include <mylib.h>
 
-void main(void)
+int main(void)
 {
 	printf("Hello World!\n");
 	mylib_hello_world();
+	return 0;
 }

samples/application_development/out_of_tree_board/src/main.c

@@ -7,7 +7,8 @@
 #include <zephyr/kernel.h>
 #include <zephyr/sys/printk.h>
 
-void main(void)
+int main(void)
 {
 	printk("Hello World! %s\n", CONFIG_ARCH);
+	return 0;
 }

samples/application_development/out_of_tree_driver/src/main.c

@@ -15,7 +15,7 @@ static void user_entry(void *p1, void *p2, void *p3)
 	hello_world_print(dev);
 }
 
-void main(void)
+int main(void)
 {
 	printk("Hello World from the app!\n");
 
@@ -27,4 +27,5 @@ void main(void)
 
 	k_object_access_grant(dev, k_current_get());
 	k_thread_user_mode_enter(user_entry, NULL, NULL, NULL);
+	return 0;
 }

samples/application_development/sysbuild/with_mcuboot/src/main.c

@@ -7,8 +7,9 @@
 #include <zephyr/kernel.h>
 #include <zephyr/linker/linker-defs.h>
 
-void main(void)
+int main(void)
 {
 	printk("Address of sample %p\n", (void *)__rom_region_start);
 	printk("Hello sysbuild with mcuboot! %s\n", CONFIG_BOARD);
+	return 0;
 }

samples/arch/mpu/mpu_test/src/main.c

@@ -139,15 +139,16 @@ static int cmd_mtest(const struct shell *shell, size_t argc, char *argv[])
 	return 0;
 }
 
-void main(void)
+int main(void)
 {
 #if defined(CONFIG_SOC_FLASH_MCUX) || defined(CONFIG_SOC_FLASH_LPC) || \
 	defined(CONFIG_SOC_FLASH_STM32)
 	if (!device_is_ready(flash_dev)) {
 		printk("Flash device not ready\n");
-		return;
+		return 0;
 	}
 #endif
+	return 0;
 }
 
 SHELL_STATIC_SUBCMD_SET_CREATE(sub_mpu,

samples/arch/smp/pi/src/main.c

@@ -77,7 +77,7 @@ void test_thread(void *arg1, void *arg2, void *arg3)
 	atomic_dec(counter);
 }
 
-void main(void)
+int main(void)
 {
 	uint32_t start_time, stop_time, cycles_spent, nanoseconds_spent;
 	int i;
@@ -112,4 +112,5 @@ void main(void)
 
 	printk("All %d threads executed by %d cores in %d msec\n", THREADS_NUM,
 	       CORES_NUM, nanoseconds_spent / 1000 / 1000);
+	return 0;
 }

samples/arch/smp/pktqueue/src/main.c

@@ -133,7 +133,7 @@ void queue_thread(void *arg1, void *arg2, void *arg3)
 	k_mutex_unlock(&fetch_queue_mtx);
 }
 
-void main(void)
+int main(void)
 {
 	uint32_t start_time, stop_time, cycles_spent, nanoseconds_spent;
 
@@ -201,4 +201,5 @@ void main(void)
 	}
 
 	k_sleep(K_MSEC(10));
+	return 0;
 }