I have a pipe which I opened with FILE *telnet = popen("telnet server", "w". If telnet exits after a while because server is not found, the pipe is closed from the other extreme.
Then I would expect some error, either in fprintf(telnet, ...) or fflush(telnet) calls, but instead, my program suddenly dies at fflush(telnet) without reporting the error. Is this normal behaviour? Why is it?
Converting (expanded) comments into an answer.
If you write to a pipe when there's no process at the other end of the pipe to read the data, you get a SIGPIPE signal to let you know, and the default behaviour for SIGPIPE is to exit (no core dump, but exit with prejudice).
If you examine the exit status in the shell, you should see $? is 141 (128 + SIGPIPE, which is normally 13).
If you don't mind that the process exits, you need do nothing. Alternatively, you can set the signal handler for SIGPIPE to SIG_IGN, in which case your writing operation should fail with an error, rather than terminating the process. Or you can set up more elaborate signal handling.
Note that one of the reasons you need to be careful to close unused file descriptors from pipes is that if the current process is writing to a pipe but also has the read end of the pipe open, it won't get SIGPIPE — but it might get blocked because it can't write more information to the pipe until some process reads from the pipe, but the only process that can read from the pipe is the one that's trying to write to it.