pg_dump and data consistency in the backup

How pg_dump honor data consistency on its output?

ie:

I have production system with some transaction in progress, some commited,
some not yet, clients normally work with the database.
Let database have some tables like "data",
"archive","something","something_else".

when i start pg_dump for creating backup

(like $ pg_dump mydb > output_file )

on production and loaded system , IS IT teoretically POSIBLE that
somebody update (and commit) for example table "something", before
execution of pg_dump ended ?? For example :pg_dump just process table
"data", but other users can change table "archive" ?
or other example : pg_dump processed table "data", just process table
"archive" and while processing somebody can updata table "data" (witch is
currently dumped) and table "something" ?So it can lead to an unconsistent
backup ????

Or pg_dump lock entire database mydb and backup is perfectly consistent
and make (useful and consistent) backup from running server is complettly
safe ????

Thanx for answer and execuse my wrong english.

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