cgo: add FreeRTOS compatibility headers

This is especially useful if we ever want to support the ESP-IDF.

Currently implemented:

  - xSemaphoreCreateRecursiveMutex
  - xSemaphoreDelete
  - xSemaphoreTakeRecursive
  - xSemaphoreGiveRecursive

compileopts/config.go

@@ -201,6 +201,9 @@ func (c *Config) RP2040BootPatch() bool {
 // preprocessing.
 func (c *Config) CFlags() []string {
 	var cflags []string
+	// Compatibility CFlags.
+	cflags = append(cflags, "-I"+filepath.Join(goenv.Get("TINYGOROOT"), "src/compat/freertos/include"))
+	// CFlags for the target.
 	for _, flag := range c.Target.CFlags {
 		cflags = append(cflags, strings.ReplaceAll(flag, "{root}", goenv.Get("TINYGOROOT")))
 	}

loader/goroot.go

@@ -220,6 +220,7 @@ func needsSyscallPackage(buildTags []string) bool {
 func pathsToOverride(needsSyscallPackage bool) map[string]bool {
 	paths := map[string]bool{
 		"/":                     true,
+		"compat/":               false,
 		"crypto/":               true,
 		"crypto/rand/":          false,
 		"device/":               false,

src/compat/freertos/include/freertos/FreeRTOS.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <stdint.h>
+
+typedef uint32_t     TickType_t;
+typedef int          BaseType_t;
+typedef unsigned int UBaseType_t;
+
+#define portMAX_DELAY (TickType_t)0xffffffffUL

src/compat/freertos/include/freertos/queue.h

@@ -0,0 +1,3 @@
+#pragma once
+
+typedef struct QueueDefinition * QueueHandle_t;

src/compat/freertos/include/freertos/semphr.h

@@ -0,0 +1,12 @@
+#pragma once
+
+// Note: in FreeRTOS, SemaphoreHandle_t is an alias for QueueHandle_t.
+typedef struct SemaphoreDefinition * SemaphoreHandle_t;
+
+SemaphoreHandle_t xSemaphoreCreateRecursiveMutex(void);
+
+void vSemaphoreDelete(SemaphoreHandle_t xSemaphore);
+
+// Note: these two functions are macros in FreeRTOS.
+BaseType_t xSemaphoreTakeRecursive(SemaphoreHandle_t xMutex, TickType_t xTicksToWait);
+BaseType_t xSemaphoreGiveRecursive(SemaphoreHandle_t xMutex);