creates /tmp file which can be written to through the console.

exercises/15_RAM/src/main.cpp

@@ -21,6 +21,11 @@
 
 static U8G2_SH1106_128X64_NONAME_F_HW_I2C g_oled(U8G2_R0, U8X8_PIN_NONE);
 
+static const char *kTmpPath = "/tmp/volatile.txt";
+static const size_t kTmpFileCapacity = 4096;
+static char g_tmpFileBuffer[kTmpFileCapacity];
+static size_t g_tmpFileSize = 0;
+
 static void oledShowLines(const char *l1,
                           const char *l2 = nullptr,
                           const char *l3 = nullptr,
@@ -65,6 +70,112 @@ static void printRamStatus()
   oledShowLines(line1, line2, line3, line4, line5);
 }
 
+static void showHelp()
+{
+  Serial.println("RAM command list:");
+  Serial.println("  help           - show this menu");
+  Serial.println("  stat           - show /tmp file state");
+  Serial.println("  read           - read /tmp contents");
+  Serial.println("  clear          - clear /tmp contents");
+  Serial.println("  write <text>   - write text to /tmp");
+  Serial.println("  append <text>  - append text to /tmp");
+}
+
+static void printTmpFileStat()
+{
+  Serial.printf("Path: %s\r\n", kTmpPath);
+  Serial.printf("Size: %u bytes\r\n", (unsigned)g_tmpFileSize);
+  Serial.printf("Capacity: %u bytes\r\n", (unsigned)kTmpFileCapacity);
+}
+
+static void printTmpFileContents()
+{
+  if (g_tmpFileSize == 0) {
+    Serial.println("/tmp file is empty");
+    return;
+  }
+
+  Serial.print("/tmp contents: ");
+  Serial.write((const uint8_t *)g_tmpFileBuffer, g_tmpFileSize);
+  if (g_tmpFileBuffer[g_tmpFileSize - 1] != '\n')
+    Serial.println();
+}
+
+static void setTmpFileContent(const char *text)
+{
+  if (!text) {
+    g_tmpFileSize = 0;
+    return;
+  }
+  const size_t newLen = strlen(text);
+  if (newLen > kTmpFileCapacity - 1) {
+    Serial.printf("Error: content too large (%u/%u)\r\n", (unsigned)newLen, (unsigned)kTmpFileCapacity);
+    return;
+  }
+  memcpy(g_tmpFileBuffer, text, newLen);
+  g_tmpFileSize = newLen;
+}
+
+static void appendTmpFileContent(const char *text)
+{
+  if (!text || text[0] == '\0') return;
+  const size_t textLen = strlen(text);
+  if (g_tmpFileSize + textLen > kTmpFileCapacity - 1) {
+    Serial.printf("Error: append would exceed %u bytes\r\n", (unsigned)kTmpFileCapacity);
+    return;
+  }
+  memcpy(g_tmpFileBuffer + g_tmpFileSize, text, textLen);
+  g_tmpFileSize += textLen;
+}
+
+static void processSerialCommand(const char *line)
+{
+  if (!line || line[0] == '\0') return;
+
+  char tmp[384];
+  strncpy(tmp, line, sizeof(tmp) - 1);
+  tmp[sizeof(tmp) - 1] = '\0';
+
+  char *cmd = strtok(tmp, " \t\r\n");
+  if (!cmd) return;
+
+  if (strcasecmp(cmd, "help") == 0) {
+    showHelp();
+    return;
+  }
+
+  if (strcasecmp(cmd, "stat") == 0) {
+    printTmpFileStat();
+    return;
+  }
+
+  if (strcasecmp(cmd, "read") == 0) {
+    printTmpFileContents();
+    return;
+  }
+
+  if (strcasecmp(cmd, "clear") == 0) {
+    g_tmpFileSize = 0;
+    Serial.println("/tmp cleared");
+    return;
+  }
+
+  if (strcasecmp(cmd, "write") == 0 || strcasecmp(cmd, "append") == 0) {
+    const char *payload = line + strlen(cmd);
+    while (*payload == ' ' || *payload == '\t') payload++;
+    if (strcasecmp(cmd, "write") == 0)
+      setTmpFileContent(payload);
+    else
+      appendTmpFileContent(payload);
+
+    Serial.printf("%s: %u bytes\r\n", cmd,
+                  (unsigned)g_tmpFileSize);
+    return;
+  }
+
+  Serial.println("Unknown command (help for list)");
+}
+
 void setup()
 {
   Serial.begin(115200);
@@ -83,6 +194,25 @@ void loop()
 {
   static uint32_t lastMs = 0;
   const uint32_t now = millis();
+
+  // check serial commands at all times
+  static char rxLine[384];
+  static size_t rxLen = 0;
+
+  while (Serial.available()) {
+    int c = Serial.read();
+    if (c <= 0) continue;
+    if (c == '\r' || c == '\n') {
+      if (rxLen > 0) {
+        rxLine[rxLen] = '\0';
+        processSerialCommand(rxLine);
+        rxLen = 0;
+      }
+    } else if (rxLen + 1 < sizeof(rxLine)) {
+      rxLine[rxLen++] = (char)c;
+    }
+  }
+
   if (now - lastMs < 1000) {
     delay(10);
     return;