why msvcrt.printf show the first char only?

[installman at 189.cn]

from ctypes import *
msvcrt = cdll.msvcrt
message_string = "Hello world!\n"
print(msvcrt.printf("Testing: %s", message_string))

when running in eclipse, the result is:
1
T

when running in IDLE, then result is:
1

why is that?