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Eina value custom type example

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:

//Compile with:
//gcc eina_value_03.c -o eina_value_03 `pkg-config --cflags --libs eina`
#include <Eina.h>
#include <sys/time.h>
static Eina_Bool
_tz_setup(const Eina_Value_Type *type, void *mem)
{
memset(mem, 0, type->value_size);
return EINA_TRUE;
}

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:

static Eina_Bool
_tz_flush(const Eina_Value_Type *type EINA_UNUSED, void *mem EINA_UNUSED)
{
return EINA_TRUE;
}

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:

static Eina_Bool
_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;
return EINA_TRUE;
}

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 implementor 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:

static Eina_Bool
_tz_pset(const Eina_Value_Type *type EINA_UNUSED, void *mem, const void *ptr)
{
*(struct timezone*)mem = *(struct timezone*)ptr;
return EINA_TRUE;
}

Next we have the vset function which get the argument from the va_list and passes it to the pset function:

static Eina_Bool
_tz_vset(const Eina_Value_Type *type, void *mem, va_list args)
{
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:

static Eina_Bool
_tz_pget(const Eina_Value_Type *type, const void *mem, void *ptr)
{
memcpy(ptr, mem, type->value_size);
return EINA_TRUE;
}

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:

static Eina_Bool
_tz_convert_to(const Eina_Value_Type *type EINA_UNUSED, const Eina_Value_Type *convert, const void *type_mem, void *convert_mem)
{
struct timezone v = *(struct timezone*)type_mem;
if (convert == EINA_VALUE_TYPE_UCHAR)
{
unsigned char other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_USHORT)
{
unsigned short other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_UINT)
{
unsigned int other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if ((convert == EINA_VALUE_TYPE_ULONG) || (convert == EINA_VALUE_TYPE_TIMESTAMP))
{
unsigned long other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_UINT64)
{
uint64_t other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_CHAR)
{
char other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_SHORT)
{
short other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_INT)
{
int other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_LONG)
{
long other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_INT64)
{
int64_t other_mem = v.tz_minuteswest;
return eina_value_type_pset(convert, convert_mem, &other_mem);
}
else if (convert == EINA_VALUE_TYPE_FLOAT)
return eina_value_type_pset(convert, convert_mem, &v.tz_minuteswest);
else if (convert == EINA_VALUE_TYPE_DOUBLE)
return eina_value_type_pset(convert, convert_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 thoses 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):

else if (convert == EINA_VALUE_TYPE_STRINGSHARE ||
convert == EINA_VALUE_TYPE_STRING)
{
const char *other_mem;
char buf[64];
snprintf(buf, sizeof(buf), "%d", v.tz_minuteswest);
other_mem = buf; /* required due &buf == buf */
return eina_value_type_pset(convert, convert_mem, &other_mem);
}

Finally we handle any other types by returning an error in that case:

return EINA_FALSE;
}

Now that we have all the functions, we can populate an `Eina_Value_Type` to later use it with `eina_value_setup()`:

static Eina_Value_Type TZ_TYPE = {
EINA_VALUE_TYPE_VERSION,
sizeof(struct timezone),
"struct timezone",
_tz_setup,
_tz_flush,
_tz_copy,
_tz_compare,
_tz_convert_to,
NULL, //No convert from
_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)
{
Eina_Value vtv, vtz;
struct timeval tv;
struct timezone tz;
char *s;
eina_init();
eina_value_setup(&vtv, EINA_VALUE_TYPE_TIMEVAL);
eina_value_setup(&vtz, &TZ_TYPE);
gettimeofday(&tv, &tz);
eina_value_set(&vtv, tv);
eina_value_set(&vtz, tz);
s = eina_value_to_string(&vtv);
printf("time: %s\n", s);
free(s);
s = eina_value_to_string(&vtz);
printf("timezone: %s\n", s);
free(s);
eina_value_flush(&vtz);
eina_value_flush(&vtv);
}

For the full source code see eina_value_03.c.