What is the difference between CStringGetTextDatum() and CStringGetDatum() in Postgresql?-CodePudding

In a previous question about using CStrings to create table data in an SQL extension, it was a problem to use Datums made from CStringGetDatum() for table columns that expect VARCHAR. The solution was to use CStringGetTextDatum(). Now i am curious why.

Here are the function definitions, but i am not sure, in which situation to use CStringGetDatum() over the second, if you can't use the first with CStrings:

 #define CStringGetDatum(X) PointerGetDatum(X)
 #define CStringGetTextDatum(s) PointerGetDatum(cstring_to_text(s))

 #define PointerGetDatum(X) ((Datum) (X))

 text *
 cstring_to_text(const char *s)
 {
     return cstring_to_text_with_len(s, strlen(s));
 }
 
 text *
 cstring_to_text_with_len(const char *s, int len)
 {
     text       *result = (text *) palloc(len   VARHDRSZ);
 
     SET_VARSIZE(result, len   VARHDRSZ);
     memcpy(VARDATA(result), s, len);
 
     return result;
 }

CodePudding user response：

The data type text or varchar is not stored as a zero-terminated array of characters. It is a varlena, which has the length (and other stuff) stored in the beginning.

The data type cstring is for a C string and is an internal data type in PostgreSQL. You can never use it in SQL.

Use CStringGetDatum whenever you need to pass a Datum that is a C string and use CStringGetTextDatum to convert a C string to a text or varchar that you need to pass as a Datum.