For this example we'll be creating our own custom type of eina value. Eina value can already store struct timeval(man gettimeofday for more information) but it has no type to store struct timezone, so that's what this example will do.
- Note
- Struct timezone is actually obsolete, so using it in real world programs is probably not a good idea, but this is an example so, bear with us.
To create our own custom eina value type we need to define functions to do the following operations on it:
- Setup
- Flush
- Copy
- Compare
- Set
- Get
- Conversion
Most of this functions are very simple, so let's look at them, starting with setup which only clear the memory so that we can be certain we won't be using stale data:
Now the flush function, which is even simpler, it does nothing, that's because there is nothing we need to do, all the necessary steps are taken by eina value itself:
Our next function, copy, is a bit more interesting, but not much, it just casts our void pointers to struct timezone pointers and does the copy:
_tz_copy(
const Eina_Value_Type *type EINA_UNUSED,
const void *src,
void * dst)
{
struct timezone *tzsrc = (struct timezone *)src;
struct timezone *tzdst = dst;
*tzdst = *tzsrc;
}
- Note
- By now you might be wondering why our functions receive void pointers instead of pointers to struct timezone, and this is a good point. The reason for this is that eina value doesn't know anything about our type so it must use a generic void pointer, casting that pointer into a proper value is the job of the implementer of the new type.
Next we have the comparison function, which compares the tz_minuteswest
field of struct timezone, we don't compare tz_dsttime
because that field is not used in linux:
static int
_tz_compare(
const Eina_Value_Type *type EINA_UNUSED,
const void *a,
const void *b)
{
struct timezone tza = *(struct timezone*)a;
struct timezone tzb = *(struct timezone*)b;
if (tza.tz_minuteswest < tzb.tz_minuteswest)
return -1;
else if (tza.tz_minuteswest > tzb.tz_minuteswest)
return 1;
return 0;
}
Next we have setting, this however requires not one but rather two functions, the reason for this is because to be able to receive arguments of any type eina value uses variadic functions, so we need a function to get the argument from a va_list and another to actually to the setting.
Lets first look at the pset function which sets the received value to a pointer:
_tz_pset(
const Eina_Value_Type *type EINA_UNUSED,
void *mem,
const void *ptr)
{
*(struct timezone*)mem = *(struct timezone*)ptr;
}
Next we have the vset function which get the argument from the va_list and passes it to the pset function:
{
const struct timezone tz = va_arg(args, struct timezone);
return _tz_pset(type, mem, &tz);
}
And now the function to get the value, a very simple copying of the value to the given pointer:
And finally our conversion function, this is our longest and most interesting one. For numeric type we simply assign the value of tz_minuteswest
to the new type and call a set function using it:
{
struct timezone v = *(struct timezone*)type_mem;
{
unsigned char other_mem = v.tz_minuteswest;
}
{
unsigned short other_mem = v.tz_minuteswest;
}
{
unsigned int other_mem = v.tz_minuteswest;
}
{
unsigned long other_mem = v.tz_minuteswest;
}
{
uint64_t other_mem = v.tz_minuteswest;
}
{
char other_mem = v.tz_minuteswest;
}
{
short other_mem = v.tz_minuteswest;
}
{
int other_mem = v.tz_minuteswest;
}
{
long other_mem = v.tz_minuteswest;
}
{
int64_t other_mem = v.tz_minuteswest;
}
- Note
- It would be a good idea to add checks for over and underflow for these types and return EINA_FALSE in those cases, we omit this here for brevity.
For string types we use snprintf()
to format our tz_minuteswest
field and put it in a string(again tz_dsttime
is ignored because it's not used):
{
const char *other_mem;
char buf[64];
snprintf(buf, sizeof(buf), "%d", v.tz_minuteswest);
other_mem = buf;
}
Finally we handle any other types by returning an error in that case:
Now that we have all the functions, we can populate an Eina_Value_Type
to later use it with eina_value_setup()
:
sizeof(struct timezone),
"struct timezone",
_tz_setup,
_tz_flush,
_tz_copy,
_tz_compare,
_tz_convert_to,
NULL,
_tz_vset,
_tz_pset,
_tz_pget
};
We can now finally use our new TZ_TYPE with eina value, so lets conclude our example by practicing that by setting its value and printing it:
int main(int argc EINA_UNUSED, char **argv EINA_UNUSED)
{
struct timeval tv;
struct timezone tz;
char *s;
gettimeofday(&tv, &tz);
printf("time: %s\n", s);
free(s);
printf("timezone: %s\n", s);
free(s);
}
For the full source code see eina_value_03.c.