Croncpp:

Software:

C++ source.

"Croncpp" is a light library (iot-friendly) for processing CRON expressions.

It implements two basic operations: find the next date or find the last date of execution of a periodic processing defined in cron format.

This allows to find the current status of the iot device after rebooting and plan its next state ...

cron()	initialze a cron object
cron( std::string( `a cron format definition (see below)` )	initialize and set a cron job object
std::string cron.expression()	return the command of the job
cron& cron.assign( std::string )	assign a new cron definition
cron = std:string()	assign a new cron definition
cron& cron.clear()	clear the cron object
bool cron.error() (return true on error)	allows to verify the cron expression
const time_t cron.nextDate( std::tn* )	Find the next rawtime of the cron job (-1 on error)
const time_t cron.nextDate( const time_t* )	Find the next rawtime of the cron job
const time_t cron.nextDate( const time_t& )	Find the next rawtime of the cron job
const time_t cron.previousDate( std::tn )	give the last rawtime of the previous cron job exec
const time_t cron.previousDate( const time_t* )	give the last rawtime of the previous cron job exec
const time_t cron.previousDate( const time_t& )	give the last rawtime of the previous cron job exec

Format of a cron string : "S M H d m w [Y] cmd" - (Year is optional ; default limit values of the year: +/- 8 years relative to the current year).

Use example :

    # define out /*
    g++ -o main $0 cron.cpp; exit $?
    # define out */
    # undef out

    #include <string>
    #include <iostream>
    #include "cron.h"

    int main() {
    time_t rawtime(time(NULL)), result;
    char buffer [80];
    std::string cronexpr( "*/10 55 * * apr * 2021-2025 myCommand" );
    // Or:
    // localtime( &rawtime );
    // cron myCron = "*/10 55 * * apr * 2021-2025 myCommand";
    // rawtime = myCron.nextDate( &rawtime );
    // std::cout << localtime( &rawtime ) << std::endl;
    // ...

    strftime ( buffer, 80, "%Y/%m/%d %H:%M:%S", localtime(&rawtime) );
    std::cout << "Now() is " << rawtime << " (" << buffer << ")" << std::endl << "Cron expression is: " << cronexpr << std::endl;

    // Next date:
    result = cron( cronexpr ).nextDate( rawtime );
    strftime ( buffer, 80, "%Y/%m/%d %H:%M:%S", localtime(&result) );
    std::cout << "The next \"" << cron( cronexpr ).expression() << "\" task will be performed at: " << result << " (" << buffer << ")." << std::endl;

    // Previous date:
    result = cron( cronexpr ).previousDate( rawtime );
    strftime ( buffer, 80, "%Y/%m/%d %H:%M:%S", localtime(&result) );
    std::cout << "The last execution of the \"" << cron( cronexpr ).expression() << "\" task was at: " << result << " (" << buffer << ")." << std::endl;

    return 0;
    }

Display:

Now() is 1656593363 (2022/06/30 02:49:23)
Cron expression is: */10 55 * * apr * 2021,2025 myCommand
The next "myCommand" task will be performed at: 1743504900 (2025/04/01 00:55:00).
The last execution of the "myCommand" task was at: 1617274500 (2021/04/30 23:55:50)

Next job of a crontab :

# define out /*
g++ -o main $0 cron.cpp && ./main; exit $?
# define out */
# undef out

#include <string>
#include <vector>
#include <iostream>
#include "cron.h"

int main() {
std::vector<std::string> crontab = {
      "5-55 55 * * apr * myCmd1",
      "* 25-30 * * * * myCmd2",
      "*/10 55 * * apr * 2021,2025 myCmd3",
      "* 05-30 * * * 2-3 * myCmd4",
      "* * * 31 * * myCmd5"
};

char buffer [80];
time_t t, Now(time(NULL));
time_t rawtime( cron( crontab[0] ).nextDate( Now ) );
std::string job;
cron c;

strftime ( buffer, 80, "%Y/%m/%d %H:%M:%S", localtime(&Now) );
std::cout << "Now() is " << Now << " (" << buffer << "):" << std::endl << std::endl;

for( auto& x : crontab ){
   c.assign(x);
   t = c.nextDate( Now );

   strftime ( buffer, 80, "%Y/%m/%d %H:%M:%S", localtime( &t ) );
   std::cout << "The job \"" << c.expression() << "\" will be lanched at: " << t << " (" << buffer << ") - \"" << x << "\"" << std::endl;

   if ( t != -1 && t < rawtime ){
      rawtime = t; job = c.expression();
    }    }

strftime ( buffer, 80, "%Y/%m/%d %H:%M:%S", localtime(&rawtime) );
std::cout << std::endl << "The next job will be \"" << job << "\" at: " << rawtime << " (" << buffer << ")." << std::endl;

rawtime=-1; for( auto& x : crontab ){
   c.assign(x);
   t = c.previousDate( Now );

   strftime ( buffer, 80, "%Y/%m/%d %H:%M:%S", localtime( &t ) );
   std::cout << "The job \"" << c.expression() << "\" has been lanched at: " << t << " (" << buffer << ") - \"" << x << "\"" << std::endl;

   if ( t != -1 && t > rawtime ){
      rawtime = t; job = c.expression();
    }    }

strftime ( buffer, 80, "%Y/%m/%d %H:%M:%S", localtime(&rawtime) );
std::cout << std::endl << "The last job was \"" << job << "\" at: " << rawtime << " (" << buffer << ")." << std::endl << std::endl;

return 0;
}

Display:

    Now() is 1657317135 (2022/07/08 11:52:15):

    The job "myCmd1" will be lanched at: 1680346505 (2023/04/01 00:55:05) - "5-55 55 * * apr * myCmd1"
    The job "myCmd2" will be lanched at: 1657319100 (2022/07/08 12:25:00) - "* 25-30 * * * * myCmd2"
    The job "myCmd3" will be lanched at: 1743504900 (2025/04/01 00:55:00) - "*/10 55 * * apr * 2021,2025 myCmd3"
    The job "myCmd4" will be lanched at: 1657620300 (2022/07/12 00:05:00) - "* 05-30 * * * 2-3 * myCmd4"
    The job "myCmd5" will be lanched at: 1659261600 (2022/07/31 00:00:00) - "* * * 31 * * myCmd5"

    The next job will be "myCmd2" at: 1657319100 (2022/07/08 12:25:00).

    The job "myCmd1" has been lanched at: 1651398955 (2022/04/30 23:55:55) - "5-55 55 * * apr * myCmd1"
    The job "myCmd2" has been lanched at: 1657315800 (2022/07/08 11:30:00) - "* 25-30 * * * * myCmd2"
    The job "myCmd3" has been lanched at: 1619862950 (2021/04/30 23:55:50) - "*/10 55 * * apr * 2021,2025 myCmd3"
    The job "myCmd4" has been lanched at: 1657186200 (2022/07/06 23:30:00) - "* 05-30 * * * 2-3 * myCmd4"
    The job "myCmd5" has been lanched at: 1653991200 (2022/05/31 00:00:00) - "* * * 31 * * myCmd5"

    The last job was "myCmd2" at: 1657315800 (2022/07/08 11:30:00).