[sqlite] attempt at output with thousands separator via extension

[Bruce Hohl]

(1) SQLite will not return output with a thousands separator as follows:

sqlite> select printf("%15.2f",123456789.12789);
   123456789.13

sqlite> select printf("%'15.2f",123456789.12789);
<no output>

--------------------------------------------------------------------------------
(2) C language printf("%'15.2f",x) honors ' in printf() for thousands
separator:

vi comma1.c
#include <stdio.h>
#include <locale.h>
int main(void)
{
   printf("%'15.2f\n", 123456789.1234);
   setlocale(LC_ALL, "");
   printf("%'15.2f\n", 123456789.1234);
   return 0;
}

$ gcc comma1.c -o comma1

$ ./comma1
   123456789.12
 123,456,789.12

--------------------------------------------------------------------------------
(3) So I thought maybe a C extension to SQLite might honor the thousands
separator:

Using the half.c extension example from
https://www.sqlite.org/cvstrac/wiki?p=LoadableExtensions with
printf("%'12.2f",x) added:

#include <stdio.h>
#include <locale.h>
#include <sqlite3ext.h>
SQLITE_EXTENSION_INIT1

// The half() SQL function returns half of its input value.
static void halfFunc(
  sqlite3_context *context,
  int argc,
  sqlite3_value **argv
){
  sqlite3_result_double(context,
printf("%'12.2f",0.5*sqlite3_value_double(argv[0])));
}

// SQLite invokes this routine once when it loads the extension.
int sqlite3_extension_init(
  sqlite3 *db,
  char **pzErrMsg,
  const sqlite3_api_routines *pApi
){
  SQLITE_EXTENSION_INIT2(pApi)
  sqlite3_create_function(db, "half", 1, SQLITE_ANY, 0, halfFunc, 0, 0);
  return 0;
}


$ gcc -shared -fPIC -I. -o half.so half.c
$ sqlite3
sqlite> select load_extension('./half.so');

sqlite> select half(750000);
   375000.0012.0
sqlite> select half(7500000);
  3750000.0012.0
sqlite> select half(75000000);
 37500000.0012.0
sqlite> select half(750000000);
375000000.0012.0
sqlite> select half(7500000000);
3750000000.0013.0
sqlite> select half(75000000000);
37500000000.0014.0

Result:
(1) Output includes unwanted '12.0', '13.0', '14.0'.
    The 12, 13, 14 appear to be the number of characters printed.
    Not sure of the reason for the '.0'

(2) No thousands separator as wanted by including ' in printf():
    printf("%'12.2f",0.5*sqlite3_value_double(argv[0]))

I mostly use sqlite from the command line so it would really be nice to
have a thousands separator for more readable output.  Comments or
suggestions please.