sizeof reporting wrong value - c

Question

I'm trying to get a "column" elements count from my own structure using sizeof(_test.header.columns)/sizeof(struct _column). Unfortunately I'm always getting it as 0, because the sizeof(_test.header.columns) is always 4. Here is the code:
struct _column{
char title[40];
int length;
};
struct test_struct{
struct{
struct _column* columns;
}header;
struct{
struct _column* columns;
}details;
struct{
struct _column* columns;
}end;
};
struct test_struct _test = {
.header = {
.columns = {
{
"a",
1,
},
{
"ab",
2,
},
},
},
.details = {
.columns = {
{
"b",
2,
},
{
"bc",
3,
},
},
},
.end = {
.columns = {
{
"c",
3,
},
{
"cd",
4,
},
},
},
};
void testme(){
char buff[20];
itoa(sizeof(_test.header.columns)/sizeof(struct _column),buff,sizeof(buff));
MessageBoxA(NULL,buff,NULL,NULL);
}
please help me to resolve the problem, thanks. Any help would be appreciated.

Answer 1

You might try to do an approach like the following in which you do the same kind of initialization however you include as part of it the column count itself.
struct _column{
char title[40];
int length;
};
struct test_struct{
struct{
struct _column* columns;
int nColumns;
}header;
struct{
struct _column* columns;
int nColumns;
}details;
struct{
struct _column* columns;
int nColumns;
}end;
};
struct _column headerColumns [] = {
{
"a",
1
},
{
"ab",
2
}
};
struct _column detailColumns[] = {
{
"b",
2
},
{
"bc",
3
},
};
struct _column endColumns [] = {
{
"c",
3
},
{
"cd",
4
}
};
struct test_struct _test = {
{ headerColumns, sizeof(headerColumns)/sizeof(headerColumns[0])},
{ detailColumns, sizeof(detailColumns)/sizeof(detailColumns[0])},
{ endColumns, sizeof(endColumns)/sizeof(endColumns[0])}
};

Answer 2

The reason it fails is that you're checking the sizeof on a pointer, which returns the size of the pointer, NOT of the actual Array the memory address points to.
If you know the maximum length of the array, you can declare it as such:
_column[ integerSize ]
But then if you knew the size, you wouldn't be querying it using sizeof, I'd think ;-) You could extend the struct by adding another property of an int type that describes the size of the columns array ?