Why does awk do full buffering when reading from a pipe

I know it is an old question, but a one-liner may help those who come here searching:

cat /dev/ttyPSC9 | awk '{ print $0; system("")}'

system("") does the trick, and is POSIX compliant. Non-posix systems: beware.

There exists a more specific function fflush() that does the same, but is not available in older versions of awk.

An important piece of information from the docs regarding the use of system(""):

gawk treats this use of the system() function as a special case and is smart enough not to run a shell (or other command interpreter) with the empty command. Therefore, with gawk, this idiom is not only useful, it is also efficient.

It is likely to be buffering in awk, not cat. In the first case, awk believes it is interactive because its input and output are TTYs (even though they're different TTYs - I'm guessing that awk is not checking that). In the second, the input is a pipe so it runs non-interactively.

You will need to explicitly flush in your awk program. This is not portable, though.

For more background and details on how to flush output, read: http://www.gnu.org/software/gawk/manual/html_node/I_002fO-Functions.html