HP 3000 Manuals

HP C/iX Library Reference Manual

Mixed I/O from the C System and Other Systems 

With one exception, concurrent use of the HP C I/O system and another I/O
system to output data to the same disk file is not supported.  The one
exception is interleaved output using another I/O system and the HP C I/O
system through the standard C streams stdout and stderr.  In this case,
if you want the C output to appear in the file after a call to an output
routine is made, you must call the fflush function immediately after the
call or change the buffering scheme for the stdout and stderr streams to
completely unbuffered or line buffered by calling the C library function
setvbuf.  Refer to the description of the setvbuf function in chapter 5
for details.

MPE/iX 5.0 Documentation