fatal error: cannot open file – does awk flush output when interrupted by SIGINT?

Direct answer

No. GNU Awk does not flush remaining output buffers when it receives SIGINT. The default signal disposition terminates the process immediately, so any data still in user-space stdio buffers—whether bound for stdout or a redirected file—is typically lost. The error string fatal error: cannot open file is unrelated to this behavior; it indicates a file-open failure, not a signal path.

Established behavior vs. uncertainty

Well-established: POSIX does not specify awk signal semantics, and gawk has no documented configuration or extension that installs a graceful SIGINT handler. The C runtime flushes stdio streams during normal exit(), but not when a signal kills the process with the default action.

Uncertain: whether a particular gawk release or non-GNU variant ever flushed on SIGINT, and whether the operating system performs any post-mortem cleanup. These details are platform-dependent. Because the underlying research flags current verification as required, treat version-specific claims as provisional until you test on your target system.

Reliable mitigation

To ensure records survive interruption, flush explicitly after critical writes:

gawk '{ print > "out.txt"; fflush("out.txt") }'

If you cannot modify the awk script, write to a temporary file and rename it from a shell trap. This only works when the shell—not the awk process directly—receives the signal:

tmp=out.tmp
trap 'mv -f "$tmp" out.txt' INT TERM
gawk '{ print > "'"$tmp"'" }'
mv -f "$tmp" out.txt

Scoped verification

Confirm the behavior on your system with a minimal test. Start a gawk process that writes without flushing, send SIGINT, then inspect the file:

printf 'a\nb\nc\n' | gawk '{ print > "out.txt"; system("sleep 5") }' &
pid=$!
sleep 1
kill -INT "$pid"
wait "$pid"; echo "exit=$?"

If out.txt is empty or incomplete, buffers were not flushed. Repeat with fflush() after each print to verify the fix. Record gawk --version and your OS, because other awk implementations (mawk, nawk, BusyBox awk) may differ.