type text in c functions

i tried to write a simple typechecking function in c. but it does not work 
correct

-- CODE --
#include <postgres.h>
#include <string.h>
#include <fmgr.h>

PG_FUNCTION_INFO_V1(text_to_card);

Datum text_to_card(PG_FUNCTION_ARGS)
{ int i,l; int32 ret;
 text *t = PG_GETARG_TEXT_P(0);
 l = VARSIZE(t) - VARHDRSZ;
 for (i = 0; i < l; i++) { if ( (t->vl_dat[i] < '0') || (t->vl_dat[i] > 
'9')) break; }
 if (i==l) ret = 0; else ret = i;
 PG_RETURN_INT32(ret);
}

--END --

this function should check if it is possible to cast a TEXT in an unsigned 
INTEGER. if OK it returns 0
otherwise the position in the string where the error occured.
( so you can produce error messages like ERROR[234] text_to_card('paramete') 
failed at <3>)

i assumed - after reading the corresponding header files and docs and google 
- the type structure of a text as above.
i only get zeros and ones as result

the algorithm works fine in 'normal' c.

is there a possibilty to treat a TEXT parameter like a char array ?

thanks and regards sepp

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