Comment by ajross
Interestingly it's actually a sort of degenerate use of ed. All it does is append one line to an empty buffer and write it to "hello.c". It's literally the equivalent of
echo 'int main(void) { printf("hello!\n"); }' > hello.c
It's not, because the shell redirection operators didn't exist yet at this point in time. Maybe (or maybe not?) it would work to cat to the file from stdin and send a Ctrl-D down the line to close the descriptor. But even that might have been present yet. Unix didn't really "look like Unix" until v7, which introduced the Bourne shell and most of the shell environment we know today.