• Check and report write(2) errors.

• Issue a single writev(2) call instead of multiple write(2)s. This change improves the inefficiencies introduced when echo went on an stdio diet.

The following figures are for echoing 1000 arguments.
original stdio-based echo:

0.01 real         0.01 user         0.00 sys

before:

0.05 real         0.00 user         0.04 sys

after:

0.01 real         0.00 user         0.00 sys