C/C++ API to decode cron-style timings - c++

Question

Does anyone know of a library which will assist in decoding cron style timings, i.e.
30 7 * * 1-5
Which is 7:30am every Monday, Tuesday, Wednesday, Thursday, Friday.
M.

Answer 1

There is a library for PHP, Perl but I never saw one for C++.
The good thing is that Cron's source is freely available and you can reuse its code to parse entries in the cron format.
The entry data structure is defined in "cron.h" file:
typedef struct _entry {
struct _entry *next;
uid_t uid;
gid_t gid;
char **envp;
char *cmd;
bitstr_t bit_decl(minute, MINUTE_COUNT);
bitstr_t bit_decl(hour, HOUR_COUNT);
bitstr_t bit_decl(dom, DOM_COUNT);
bitstr_t bit_decl(month, MONTH_COUNT);
bitstr_t bit_decl(dow, DOW_COUNT);
int flags;
#define DOM_STAR 0x01
#define DOW_STAR 0x02
#define WHEN_REBOOT 0x04
#define MIN_STAR 0x08
#define HR_STAR 0x10
} entry;
And there are two functions you need from "entry.c" file (too large to post code here):
void free_entry (e);
entry *load_entry (file, error_func, pw, envp);
You can compile those files into a shared library or object files and use directly in your project.
This is an example of getting cron source code in Debian (Ubuntu):
apt-get source cron
You can also download it from http://cron.sourcearchive.com/

Answer 2

For those that wish to achieve the same goal as #ScaryAardvark
Dependency:
http://cron.sourcearchive.com/downloads/3.0pl1/cron_3.0pl1.orig.tar.gz
Build:
gcc -o main main.c cron-3.0pl1.orig/entry.c cron-3.0pl1.orig/env.c
cron-3.0pl1.orig/misc.c -I cron-3.0pl1.orig
Source:
#include <pwd.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <uuid/uuid.h>
#define MAIN_PROGRAM 1
#include "cron-3.0pl1.orig/cron.h"
void error_handler( char* message )
{
fprintf( stderr, "Error: %s\n", message );
}
void print_entry( const entry* e )
{
fprintf( stdout, "uid: %i\n", e->uid );
fprintf( stdout, "gid: %i\n", e->gid );
fprintf( stdout, "command: %s\n", e->cmd);
//etc...
}
int main( int argc, char** argv, char** envp )
{
const char* filename = "crontab";
const char* username = "bcrowhurst";
//Retreive Crontab File
FILE *file = fopen( filename, "r" );
if ( file == NULL )
{
error_handler( strerror( errno ) );
return EXIT_FAILURE;
}
//Retreive Password Entry
struct passwd *pw = getpwnam( username );
if ( pw == NULL )
{
error_handler( strerror( errno ) );
return EXIT_FAILURE;
}
//Read Entry
entry *e = load_entry( file, &error_handler, pw, envp );
if ( e == NULL )
{
error_handler( "No entry found!" );
return EXIT_FAILURE;
}
print_entry( e );
//Clean-up
fclose( file );
free_entry( e );
return EXIT_SUCCESS;
}
Example Crontab
#yearly /home/bcrowhurst/annual-process
*/10 * * * * /home/bcrowhurst/fschk