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):

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:

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:

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

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).

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!

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