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

Question 1

I need to create a converter from epoch time stored in atime_t variable, to astruct tm variable, in order to check / create a certain task every hour or a day. This function should get also a pasttime_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 timetm 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

Question 2

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

Question 3

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

Here you are overwriting the parameterconvTime. There is no point inpassing 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 toiot.convEpoch()doesn't change that (the parameter is passed by value, not byreference).

  Serial.print(newtime->tm_year);

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

The simplest solution I see is to havemyIOT32::convEpoch() return thepointer it got fromgmtime(). 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); }

Question 4

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... ) ?

Question 5

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.

Edgar Bonet 45.2k4 gold badges42 silver badges81 bronze badges · Accepted Answer · 2020-06-14 10:07:38Z

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

Here you are overwriting the parameterconvTime. There is no point inpassing 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 toiot.convEpoch()doesn't change that (the parameter is passed by value, not byreference).

  Serial.print(newtime->tm_year);

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

The simplest solution I see is to havemyIOT32::convEpoch() return thepointer it got fromgmtime(). 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); }

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... ) ?
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.

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

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