Error passing a `time_t` and `struct tm`, ESP32

Question

I need to create a converter from epoch time stored in a time_t variable, to a struct tm variable, in order to check / create a certain task every hour or a day. This function should get also a past time_t for other uses.

Converting function, inside a library, works OK ( last two lines are for verification purposes, meaning conversion was done as expected ):

void myIOT32::convEpoch(time_t in_time, struct tm *convTime)
{
 convTime = gmtime(&in_time);
 char time_char[40];
 sprintf(time_char, "%04d-%02d-%02d %02d:%02d:%02d", convTime->tm_year + 1900, convTime->tm_mon+1, convTime->tm_mday,
 convTime->tm_hour, convTime->tm_min, convTime->tm_sec);
 Serial.print(" from funct: ");
 Serial.println(time_char);

Now, when calling it from my code and try to use time tm var, it causes the code to crash. In code below, I just try to print it to console:

void sendnewNotif()
{
 struct tm *newtime;
 time_t t=182423032;
 iot.convEpoch(t, newtime); // <--- Using the conversion
 char timeStamp[50];
 // Serial.print(newtime->tm_year); 
}

What may be the problem ?

Guy

• Juraj: struct tm newtime; to alocate on stack. struct tm *newtime is a pointer to nowhere

Answer 1

void myIOT32::convEpoch(time_t in_time, struct tm *convTime)
{
 convTime = gmtime(&in_time);

Here you are overwriting the parameter convTime. There is no point in passing such a parameter if you are not going to use the value provided.

void sendnewNotif()
{
 struct tm *newtime;
 time_t t=182423032;
 iot.convEpoch(t, newtime); // <--- Using the conversion

newtime is initially uninitialized. The call to iot.convEpoch() doesn't change that (the parameter is passed by value, not by reference).

 Serial.print(newtime->tm_year);

This is dereferencing an uninitialized pointer, which is undefined behavior and can crash the program.

The simplest solution I see is to have myIOT32::convEpoch() return the pointer it got from gmtime(). Note that this points to static storage, which makes the method not reentrant.

Example (not tested):

struct tm *myIOT32::convEpoch(time_t in_time)
{
 struct tm *convTime = gmtime(&in_time);
 // ...
 return convTime;
}
void sendnewNotif()
{
 time_t t = 182423032;
 struct tm *newtime = iot.convEpoch(t);
 Serial.print(newtime->tm_year); 
}

• guyd: thank you for your kind and comprehensive reply :), but your answer rised some more: a) why does it matter if it is initialized ? since it get its value later on ( though as a pointer ). b) are there guide lines when to use as a return value ( as you just suggested ) or as I did, passing its value/ reference ( in a case it works as expected... ) ?
• Edgar Bonet: a) It does not get it's value later on. b) You could in principle pass the pointer by reference, but mixing pointers and references can quickly become highly confusing.