strlen and PROGMEM

Hello,

I have a function where one of the argument is a pointer to a text string in PROGMEM. The function need to find the length of the test string. I search and found that some have used strlen_P. But this seem to give me an error while compiling the sketch:

int readValue(const int line, const __FlashStringHelper *text, const int digits)
{
  int   val = 0;
  
  lcd1.writeLine(line, text);
  val = readNumber(digits, 1 + strlen_P(text));

  return val;
}

void someFunc()
{
  int val = readValue(1, F("Enter number:"), 2);
}

The error I get is:

cannot convert 'const __FlashStringHelper*' to 'const char*' for argument '1' to 'size_t strlen_P(const char*)'

What do I do wrong?

strlen_P() takes a 'const char *' argument, not a 'const __FlashStringHelper *'

strlen_P(reinterpret_cast<const char *>(text))

Reinterpret_cast is dangerous, and can only be used in a very limited number of cases: reinterpret_cast conversion - cppreference.com
In this case, it’s valid, because the underlying type of a progmem string actually is const char *. The F(…) macro reinterpret_casts it to a pointer to an incomplete helper type __FlashStringHelper. It does this to get the correct overload resolution for functions like Serial.print. The link I mentioned earlier explains that it’s safe to cast it back to the original type in that case (and that’s pretty much the only thing you can do with it).

F macro definition

Pieter

Thanks. I did not thought about using reinterpret_cast.