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>

samples/basic/blinky/src/main.c

@@ -19,24 +19,25 @@
  */
 static const struct gpio_dt_spec led = GPIO_DT_SPEC_GET(LED0_NODE, gpios);
 
-void main(void)
+int main(void)
 {
 	int ret;
 
 	if (!gpio_is_ready_dt(&led)) {
-		return;
+		return 0;
 	}
 
 	ret = gpio_pin_configure_dt(&led, GPIO_OUTPUT_ACTIVE);
 	if (ret < 0) {
-		return;
+		return 0;
 	}
 
 	while (1) {
 		ret = gpio_pin_toggle_dt(&led);
 		if (ret < 0) {
-			return;
+			return 0;
 		}
 		k_msleep(SLEEP_TIME_MS);
 	}
+	return 0;
 }

samples/basic/blinky_pwm/src/main.c

@@ -19,7 +19,7 @@ static const struct pwm_dt_spec pwm_led0 = PWM_DT_SPEC_GET(DT_ALIAS(pwm_led0));
 #define MIN_PERIOD PWM_SEC(1U) / 128U
 #define MAX_PERIOD PWM_SEC(1U)
 
-void main(void)
+int main(void)
 {
 	uint32_t max_period;
 	uint32_t period;
@@ -31,7 +31,7 @@ void main(void)
 	if (!device_is_ready(pwm_led0.dev)) {
 		printk("Error: PWM device %s is not ready\n",
 		       pwm_led0.dev->name);
-		return;
+		return 0;
 	}
 
 	/*
@@ -49,7 +49,7 @@ void main(void)
 			printk("Error: PWM device "
 			       "does not support a period at least %lu\n",
 			       4U * MIN_PERIOD);
-			return;
+			return 0;
 		}
 	}
 
@@ -61,7 +61,7 @@ void main(void)
 		ret = pwm_set_dt(&pwm_led0, period, period / 2U);
 		if (ret) {
 			printk("Error %d: failed to set pulse width\n", ret);
-			return;
+			return 0;
 		}
 
 		period = dir ? (period * 2U) : (period / 2U);
@@ -75,4 +75,5 @@ void main(void)
 
 		k_sleep(K_SECONDS(4U));
 	}
+	return 0;
 }

samples/basic/button/src/main.c

@@ -38,21 +38,21 @@ void button_pressed(const struct device *dev, struct gpio_callback *cb,
 	printk("Button pressed at %" PRIu32 "\n", k_cycle_get_32());
 }
 
-void main(void)
+int main(void)
 {
 	int ret;
 
 	if (!gpio_is_ready_dt(&button)) {
 		printk("Error: button device %s is not ready\n",
 		       button.port->name);
-		return;
+		return 0;
 	}
 
 	ret = gpio_pin_configure_dt(&button, GPIO_INPUT);
 	if (ret != 0) {
 		printk("Error %d: failed to configure %s pin %d\n",
 		       ret, button.port->name, button.pin);
-		return;
+		return 0;
 	}
 
 	ret = gpio_pin_interrupt_configure_dt(&button,
@@ -60,7 +60,7 @@ void main(void)
 	if (ret != 0) {
 		printk("Error %d: failed to configure interrupt on %s pin %d\n",
 			ret, button.port->name, button.pin);
-		return;
+		return 0;
 	}
 
 	gpio_init_callback(&button_cb_data, button_pressed, BIT(button.pin));
@@ -95,4 +95,5 @@ void main(void)
 			k_msleep(SLEEP_TIME_MS);
 		}
 	}
+	return 0;
 }

samples/basic/custom_dts_binding/src/main.c

@@ -17,13 +17,13 @@
 static const struct gpio_dt_spec load_switch =
 	GPIO_DT_SPEC_GET_OR(DT_NODELABEL(load_switch), gpios, {0});
 
-void main(void)
+int main(void)
 {
 	int err;
 
 	if (!gpio_is_ready_dt(&load_switch)) {
 		printf("The load switch pin GPIO port is not ready.\n");
-		return;
+		return 0;
 	}
 
 	printf("Initializing pin with inactive level.\n");
@@ -31,7 +31,7 @@ void main(void)
 	err = gpio_pin_configure_dt(&load_switch, GPIO_OUTPUT_INACTIVE);
 	if (err != 0) {
 		printf("Configuring GPIO pin failed: %d\n", err);
-		return;
+		return 0;
 	}
 
 	printf("Waiting one second.\n");
@@ -44,4 +44,5 @@ void main(void)
 	if (err != 0) {
 		printf("Setting GPIO pin level failed: %d\n", err);
 	}
+	return 0;
 }

samples/basic/fade_led/src/main.c

@@ -19,7 +19,7 @@ static const struct pwm_dt_spec pwm_led0 = PWM_DT_SPEC_GET(DT_ALIAS(pwm_led0));
 #define NUM_STEPS	50U
 #define SLEEP_MSEC	25U
 
-void main(void)
+int main(void)
 {
 	uint32_t pulse_width = 0U;
 	uint32_t step = pwm_led0.period / NUM_STEPS;
@@ -31,14 +31,14 @@ void main(void)
 	if (!device_is_ready(pwm_led0.dev)) {
 		printk("Error: PWM device %s is not ready\n",
 		       pwm_led0.dev->name);
-		return;
+		return 0;
 	}
 
 	while (1) {
 		ret = pwm_set_pulse_dt(&pwm_led0, pulse_width);
 		if (ret) {
 			printk("Error %d: failed to set pulse width\n", ret);
-			return;
+			return 0;
 		}
 
 		if (dir) {
@@ -58,4 +58,5 @@ void main(void)
 
 		k_sleep(K_MSEC(SLEEP_MSEC));
 	}
+	return 0;
 }

samples/basic/hash_map/src/main.c

@@ -26,7 +26,7 @@ struct _stats {
 
 static void print_stats(const struct _stats *stats);
 
-void main(void)
+int main(void)
 {
 	size_t i;
 	int ires;
@@ -89,9 +89,7 @@ out:
 
 	LOG_INF("success");
 
-	if (IS_ENABLED(CONFIG_ARCH_POSIX)) {
-		exit(0);
-	}
+	return 0;
 }
 
 static void print_stats(const struct _stats *stats)

samples/basic/minimal/src/main.c

@@ -6,6 +6,7 @@
 
 #include <zephyr/kernel.h>
 
-void main(void)
+int main(void)
 {
+	return 0;
 }

samples/basic/rgb_led/src/main.c

@@ -22,7 +22,7 @@ static const struct pwm_dt_spec blue_pwm_led =
 
 #define STEP_SIZE PWM_USEC(2000)
 
-void main(void)
+int main(void)
 {
 	uint32_t pulse_red, pulse_green, pulse_blue; /* pulse widths */
 	int ret;
@@ -33,7 +33,7 @@ void main(void)
 	    !device_is_ready(green_pwm_led.dev) ||
 	    !device_is_ready(blue_pwm_led.dev)) {
 		printk("Error: one or more PWM devices not ready\n");
-		return;
+		return 0;
 	}
 
 	while (1) {
@@ -42,7 +42,7 @@ void main(void)
 			ret = pwm_set_pulse_dt(&red_pwm_led, pulse_red);
 			if (ret != 0) {
 				printk("Error %d: red write failed\n", ret);
-				return;
+				return 0;
 			}
 
 			for (pulse_green = 0U;
@@ -53,7 +53,7 @@ void main(void)
 				if (ret != 0) {
 					printk("Error %d: green write failed\n",
 					       ret);
-					return;
+					return 0;
 				}
 
 				for (pulse_blue = 0U;
@@ -65,11 +65,12 @@ void main(void)
 						printk("Error %d: "
 						       "blue write failed\n",
 						       ret);
-						return;
+						return 0;
 					}
 					k_sleep(K_SECONDS(1));
 				}
 			}
 		}
 	}
+	return 0;
 }

samples/basic/servo_motor/src/main.c

@@ -24,7 +24,7 @@ enum direction {
 	UP,
 };
 
-void main(void)
+int main(void)
 {
 	uint32_t pulse_width = min_pulse;
 	enum direction dir = UP;
@@ -34,14 +34,14 @@ void main(void)
 
 	if (!device_is_ready(servo.dev)) {
 		printk("Error: PWM device %s is not ready\n", servo.dev->name);
-		return;
+		return 0;
 	}
 
 	while (1) {
 		ret = pwm_set_pulse_dt(&servo, pulse_width);
 		if (ret < 0) {
 			printk("Error %d: failed to set pulse width\n", ret);
-			return;
+			return 0;
 		}
 
 		if (dir == DOWN) {
@@ -62,4 +62,5 @@ void main(void)
 
 		k_sleep(K_SECONDS(1));
 	}
+	return 0;
 }

samples/basic/sys_heap/src/main.c

@@ -14,7 +14,7 @@ static struct sys_heap heap;
 
 void print_sys_memory_stats(void);
 
-void main(void)
+int main(void)
 {
 	void *p;
 
@@ -31,6 +31,7 @@ void main(void)
 
 	sys_heap_free(&heap, p);
 	print_sys_memory_stats();
+	return 0;
 }
 
 void print_sys_memory_stats(void)