mysterious Serial.print

Projects Programming
1

Hi,
my sketch does not print within setup().
This code is part of a longer code for some tests. The function makeTimeStr() shall be used to print elapsed times, e.g.

...
tm=millis();
...
Serial.print(makeTimeStr(millis()-tm)):
...

#include <LCD5110_Graph.h>

#define DISP_BR 84
#define DISP_HO 48
#define BATT_BR 15
#define BATT_HO 30
#define A_BATT  6

LCD5110 myGLCD(9,10,11,12,13);

extern unsigned char SmallFont[];
extern unsigned char TinyFont[];

int pwm=100, dpwm=5, pw_1=0, pw_2=0, pw_off=185;
int V_batt, V_d, V_g, V_d0, V_d2, V_bold;
int V_charge, V_off1, V_off2, V_disch1, V_disch2, vw;
const word aR2V=45300/1023; // V++ = 4,75 V
//const word scale=10000;
unsigned long tm, t0, t_load=5000, t_off=1000, t_discharge=800;
int dt;
byte state=0;
int ncloops=0, ncl; // zaehlt die Anzahl Regelschleifenlaeufe
word V_soll=100;
char TStr[12];
bool SM=true;

// makeTimeStr erzeugt aus dt einen String der Form hh:mm:ss
char* makeTimeStr(unsigned long dt) {
  char hmsTime[12], hstr[6];
  word tt;

  dt /= 1000;    // make seconds
  tt = dt/3600; 
  if (SM) {
    Serial.print(tt); Serial.print(" ");
  }
  sprintf(hmsTime,"%02d:",tt);
  dt = dt- tt*3600;
  tt = dt/60; 
  if (SM) {
    Serial.print(tt); Serial.print(" ");
  }
  sprintf(hstr,"%02d:", tt);
  strcat(hmsTime, hstr);
  tt = dt- tt*60; 
  if (SM) {
    Serial.println(tt);
  }
  sprintf(hstr,"%02d ", tt);
  strcat(hmsTime, hstr);
  if (SM) Serial.println(hmsTime);
}


void setup() {
  // put your setup code here, to run once:
  myGLCD.InitLCD();
  myGLCD.setFont(SmallFont);  // SmallFont=6x8

  Serial.begin(38400);

  pinMode(3,OUTPUT);
  analogWrite(3,pwm);
  ncl=ncloops;
  delay(800);
  
  V_batt=analogRead(A6);
  V_bold=V_batt;
  
  V_d=V_batt;
  Serial.println("in setup  ");
if (SM) Serial.println(makeTimeStr(7000000));
  tm=millis();
  t0=tm;
  state=0;
  pw_off=185;
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.println("in loop");
  delay(1000);
}

When running this code the output is:

in setup
1 56 40         <--- print within makeTimeStr()
01:56:40       <--- print within makeTimeStr()
                    <--- missing print within setup()
in loop
in loop
...

I am in doubt if it is generally allowed to return a pointer to a string within makeTimeStr() because
the pointer is pointing to somewhere on the stack.

On the other hand the returned pointer is immediately used as a parameter of print.

What is the reason why there is no print output?

2

I am in doubt if it is generally allowed to return a pointer to a string within makeTimeStr() because
the pointer is pointing to somewhere on the stack.

Correct, and the stack can be overwritten at any time after the function returns.

In any case, the function makeTimeStr() currently does not return anything. You might as well have declared it "void".

The simplest approach is to declare "char hmsTime[12]" as global.

Finally, rather than have a global symbol (SM) control printing in makeTimStr(), it would be better programming practice to have a separate function print the string.