How can I create an OID from time - c

Question

In pymongo you can do something like this to create an OID from time:
dummy_id = ObjectId.from_datetime(time)
Is there something like that in mongoc?
I saw that there's a "bson_oid_get_time_t()" function, but is there a reverse function of this, and if not, How can it be implemented in C?

Answer 1

I don't believe there is a reverse function, but it should be easy for you to generate your own using the default constructor and "fixing" the time.
Here is an example where I create an object ID.
Then I create a timestamp for December 25, 2014 and modify the OID to that date.
#include <time.h>
#include <bson.h>
int
main (int argc,
char *argv[])
{
time_t oid_thinks_time; //what time does the OID think it is
bson_oid_t oid;
bson_oid_t *oid_pointer = &oid;
bson_oid_init (&oid, NULL); // get a standard ObjectId
oid_thinks_time = bson_oid_get_time_t (&oid); //It was just made
printf ("The OID was generated at %u\n", (unsigned) oid_thinks_time); //prove it
time_t ts = time(NULL); //make a new time
struct tm * timeinfo = localtime(&ts);
timeinfo->tm_year = 2014-1900; //-1900 because time.h
timeinfo->tm_mon = 12 - 1; // time.h off by one (starts at 0)
timeinfo->tm_mday = 25;
ts = mktime(timeinfo); // create the time
u_int32_t ts_uint = (uint32_t)ts;
ts_uint = BSON_UINT32_TO_BE (ts_uint); //BSON wants big endian time
memcpy (&oid_pointer->bytes[0], &ts_uint, sizeof (ts_uint)); //overwrite the first 4 bytes with user selected time
oid_thinks_time = bson_oid_get_time_t (&oid);
printf ("The OID was fixed to time %u\n", (unsigned) oid_thinks_time);//prove it
}
The output of this code is:
The OID was generated at 1491238015
The OID was fixed to time 1419526015