ECONNRESET in Send Linux C
According to Unix Network Programming when a socket writes twice to a closed socket (after a FIN packet), then in the first time it succeeded to send, but receives an RST packet from the other host. Since the host receives an RST, the socket is destroyed. Thus in the second time it writes, the SIGPIPE signal is received, and an EPIPE error is returned.
However, in send man pages ECONNRESET can be returned, which means that an RST packet is received. When it returns ECONNRESET -there no signal is returned.
What are the cases ECONNRESET can be returned? and why does there is no SIGPIPE signal in this case?
Note: I have checked I similar question here. However, when I run in my linux computer, send returned the EPIPE error, and not ECONNRESET.
Answer
If the peer closed the connection while there were still unhandled data in the socket buffer it will send a RST packet back. This will cause a flag to be set on the socket and the next send will return ECONNRESET as the result . EPIPE instead is returned (or SIGPIPE triggered) on send if the connection was closed by the peer with no outstanding data. In both cases the local socket is still open (i.e. the file descriptor is valid), but the underlying connection is closed.
Example: Imagine a server which reads a single byte and then closes the connection:
- EPIPE: The client sends first one byte. After the server read the byte and closed the connection the client will send some more data and then again some data. The latest send call will trigger EPIPE/SIGPIPE.
- ECONNRESET: The client sends first more than one byte. The server will read a single byte and close the connection with more bytes in the sockets receive buffer. This will trigger a connection RST packet from the server and on the next send the client will receive ECONNRESET.