croncpp – a C++ library for CRON expressions

A CRON expression is a string composed of six fields (in some implementation seven), separated by a whites space, representing a time schedule. CRON expressions are used in various job schedulers (such as the Linux job scheduler, the Quartz scheduler, Azure scheduler for functions, etc.). Recently, I have written a C++ library for parsing such expressions and determining the next occurrence of the scheduled time. The library is called croncpp. It is written in C++17, is header-only, open-source and cross-platform, and can be found on GitHub.

CRON expressions

The general form of a CRON expression is the following (with the years being optional):

<seconds> <minutes> <hours> <days of month> <months> <days of week> <years>

The following values are allowed for these fields:

Field	Required	Allowed value *	Allowed value (alternative 1) **	Allowed value (alternative 2) ***	Allowed special characters
seconds	yes	0-59	0-59	0-59	* , -
minutes	yes	0-59	0-59	0-59	* , -
hours	yes	0-23	0-23	0-23	* , -
days of month	1-31	1-31	1-31	1-31	* , - ? L W
months	yes	1-12	0-11	1-12	* , -
days of week	yes	0-6	1-7	1-7	* , - ? L #
years	no	1970-2099	1970-2099	1970-2099	* , -

* – As described on Wikipedia Cron

** – As described on Oracle Role Manager Integration Guide – A Cron Expressions

*** – As described for the Quartz scheduler CronTrigger Tutorial

The special characters have the following meaning:

Special character	Meaning	Description
*	all values	selects all values within a field
?	no specific value	specify one field and leave the other unspecified
-	range	specify ranges
,	comma	specify additional values
/	slash	speficy increments
L	last	last day of the month or last day of the week
W	weekday	the weekday nearest to the given day
#	nth	specify the Nth day of the month

Examples:

CRON	Description
* * * * * *	Every second
*/5 * * * * ?	Every 5 seconds
0 */5 */2 * * ?	Every 5 minutes, every 2 hours
0 */2 */2 ? */2 */2	Every 2 minutes, every 2 hours, every 2 days of the week, every 2 months
0 15 10 * * ? *	10:15 AM every day
0 0/5 14 * * ?	Every 5 minutes starting at 2 PM and ending at 2:55 PM, every day
0 10,44 14 ? 3 WED	2:10 PM and at 2:44 PM every Wednesday of March
0 15 10 ? * MON-FRI	10:15 AM every Monday, Tuesday, Wednesday, Thursday and Friday
0 15 10 L * ?	10:15 AM on the last day of every month
0 0 12 1/5 * ?	12 PM every 5 days every month, starting on the first day of the month
0 11 11 11 11 ?	Every November 11th at 11:11 AM

croncpp library

There are two main functionalities provided by the library: parsing a CRON expression and computing the next occurence of the scheduled time.

To parse a CRON expression use make_cron() as follows:

try
{
   auto cron = cron::make_cron("* 0/5 * * * ?");
}
catch (cron::bad_cronexpr const & ex)
{
   std::cerr << ex.what() << '\n';
}

make_cron() returns an object of the type cronexpr. The actual content of this object is not of real interest and, in fact, all its details are private. You can consider this as an implementation detail object that contains the necessary information for a CRON expression, in order to compute the next occurence of the time schedule, which is the actual important operation we are interested in.

To get the next occurence of the time schedule use the cron_next() function as follows:

try
{
   auto cron = cron::make_cron("* 0/5 * * * ?");
   
   std::time_t now = std::time(0);
   std::time_t next = cron::cron_next(cron, now);   
}
catch (cron::bad_cronexpr const & ex)
{
   std::cerr << ex.what() << '\n';
}

Alternatively, you can use std::tm instead of std::time_t:

try
{
   auto cron = cron::make_cron("* 0/5 * * * ?");
   
   std::tm time = cron::utils::to_tm("2018-08-08 20:30:45");
   std::tm next = cron::cron_next(cron, time);
}
catch (cron::bad_cronexpr const & ex)
{
   std::cerr << ex.what() << '\n';
}

When you use these functions as shown above you implicitly use the standard supported values for the fields, as described in the first section. However, you can use any other settings. The ones provided with the library are called cron_standard_traits, cron_oracle_traits and cron_quartz_traits (coresponding to the aforementioned settings).

try
{
   auto cron = cron::make_cron<cron_quartz_traits>("* 0/5 * * * ?");
   
   std::time_t now = std::time(0);
   std::time_t next = cron::cron_next<cron_quartz_traits>(cron, now);   
}
catch (cron::bad_cronexpr const & ex)
{
   std::cerr << ex.what() << '\n';
}

Performance

I tried to see how the library performs, both on Windows and Mac, although the machines I used for testing were pretty different. I used the following:

VC++ 15.7.4 running on:

• Windows 10 Enterprise build 17134
• Intel Core i7, 2.67 GHz, 1 CPU / 4 cores / 8 logical, 6 RAM

GCC 8.1.0 / Clang LLVM 9.1.0 running on:

• macOS 10.13.5
• Intel Core i7, 1.7 GHz, 1 CPU / 2 cores, 8 GB RAM

The following results are the average (in microseconds) for running the benchmark program ten times on these machines and compilers (all with release settings):

VC++ 32-bit	VC++ 64-bit	GCC 32-bit	GCC 64-bit	Clang 64-bit
11.52	8.30	8.95	7.03	4.48

I am looking forward to hearing your feedback!