Getting SIGPIPE with non-blocking sockets -- is this normal?

Question

I'm writing an epoll-based network server in C. When I create my socket to listen for incoming connections, I make it non-blocking using fcntl. Similarly when incoming connections arrive from clients, I make their sockets non-blocking before doing anything with them, and likewise for outgoing connections' sockets.

Sometimes my server gets a SIGPIPE -- I think this is when I try to write to a client connection that has been closed by the client. This seems strange to me; I thought that with non-blocking sockets instead of a SIGPIPE I should get an -1 back from the call to write and ECONNRESET in errno.

Is there something I'm missing? Or is it just normal to get both a SIGPIPE and an error code even with non-blocking sockets (meaning that I should explicitly ignore the signal with signal(SIGPIPE, SIG_IGN) in my setup)?

Answer 1

Yes, this is normal. If you write to a socket (non-blocking or not) where the other end has closed the connection, you will get a SIGPIPE or (if you are blocking the SIGPIPE signal) an error return (-1) with errno set to EPIPE.

From the man page for write:

EPIPE: fd is connected to a pipe or socket whose reading end is closed. When this happens the writing process will also receive a SIGPIPE signal. (Thus, the write return value is seen only if the program catches, blocks or ignores this signal.)

The POSIX standard is here: http://pubs.opengroup.org/onlinepubs/009695399/functions/write.html and says:

[EPIPE] An attempt is made to write to a pipe or FIFO that is not open for reading by any process, or that only has one end open. A SIGPIPE signal shall also be sent to the thread.

Answer 2

The SIGPIPE is normal. Another option beside setting signal handler solely for this purpose is to use flag MSG_NOSIGNAL whenever you send.