Custom data type in C with one fixed and one variable attribute

Hi,

I am trying to create a custom data type in C that has a fixed size
and a variable size attribute - is that actually possible? The
documentation mentions only one or the other but a struct in the
pg_trgm extension (TRGM) seems to have that.

The data type I have is

typedef struct {
int4 length;
uint32 foo;
char bar[1];
} oefp;

The external representation of that data type would be (1,
'hexadecimal string here'), for example.

This is my _in function to parse the external cstring.

PG_FUNCTION_INFO_V1(mydatatype_in);
Datum mydatatype_in(PG_FUNCTION_ARGS)
{
char *rawcstring = PG_GETARG_CSTRING(0);

uint32 foo;
char *buffer = (char *) palloc(strlen(rawcstring));

if (sscanf(rawcstring, "(%u,%[^)])", &foo, buffer) != 2)
{
ereport(ERROR,
(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
errmsg("Invalid input syntax: \"%s\"", rawcstring)));
}

mydatatype *dt = (mydatatype*) palloc(VARHDRSZ + sizeof(uint32) +
strlen(buffer));

SET_VARSIZE(dt, VARHDRSZ + sizeof(uint32) + strlen(buffer));
memcpy(dt->bar, buffer, strlen(buffer));
dt->foo = foo;

PG_RETURN_POINTER(dt);
}

The problem is however that dt->bar contains not only the input string
but random characters or other garbage as well, so something must go
wrong at the end of the function. Any thoughts what it could be?

Cheers,

Adrian