128-bit integers can range only up to (2 ^ 63 -1)

include/pg_config.h
14: #define ALIGNOF_PG_INT128_TYPE 16
355: #define MAXIMUM_ALIGNOF 8
374: #define PG_INT128_TYPE __int128

/include/c.h
507: /*
508:  * 128-bit signed and unsigned integers
509:  * There currently is only limited support for such types.
510:  * E.g. 128bit literals and snprintf are not supported; but math is.
511:  * Also, because we exclude such types when choosing MAXIMUM_ALIGNOF,
512:  * it must be possible to coerce the compiler to allocate them on no
513:  * more than MAXALIGN boundaries.
514:  */
515: #if defined(PG_INT128_TYPE)
516: #if defined(pg_attribute_aligned) || ALIGNOF_PG_INT128_TYPE <= MAXIMUM_ALIGNOF
517: #define HAVE_INT128 1
518:
519: typedef PG_INT128_TYPE int128
520: #if defined(pg_attribute_aligned)
521: pg_attribute_aligned(MAXIMUM_ALIGNOF)
522: #endif
523:   ;
524:
525: typedef unsigned PG_INT128_TYPE uint128
526: #if defined(pg_attribute_aligned)
527: pg_attribute_aligned(MAXIMUM_ALIGNOF)
528: #endif
529:   ;
530:
531: #endif
532: #endif
533:

Hi.

I am slightly confused by the int128 type. I thought the 128 bit integer means range type will be upto 2 ^ 127 - 1.

Now just copy the above code and test the int128 range.

int128 can only up to  9223372036854775807 (2 ^ 63 -1).

also

File: /home/jian/helloc/pg/pg_interval/include/pg_config_ext.h
6: /* Define to the name of a signed 64-bit integer type. */
7: #define PG_INT64_TYPE long int

I also thought  that 64-bit means range up to 2 ^ 63 -1. Obviously I was wrong.

So when we say "128 bit" what does it actually mean?