Accept Interrupted System Call Error
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Interrupted System Call Errno
million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up TCP sockets, server fails to respond to client, accept: Interrupted system call up vote 0 down vote favorite I am interrupted system call @ rb_sysopen trying to implement a TCP server and client in C, running on Solaris. I am new to sockets and am using Beej's Guide as an example. For starters, what I would like is for the client to send a message to the server in the form of word1 word2. Upon receipt, I want the server to extract word2 from the message and send that back to the client. The initial client --> server interrupted system call linux message sending works fine. But the server --> client response is not working. There are several failure symptoms: The server does not appear to even try to send() anything to the client. After receiving the client's message, the server prints: accept: Interrupted system call, then returns to the top of the while(1) loop and remains there until I Ctrl-C out of it. The client's call to recv() returns 0 bytes. I found an old thread here, where the last post says this: accept is being interrupted by the child process sending a signal back to the parent when it terminates (SIGCHLD, if I remember write). You can either ignore SIGCHLD, or you can code accept() to handle the interrupt better (errno is set to EINTR) However, I'm not understanding this. Why is the child process terminating before even attempting the send() portion? What does "handle the interrupt better" mean? After searching some more, I found this question on Stack Overflow: How to handle EINTR (interrupted System Call). I tried adding the code in the accepted answer, replacing write() with send(), but I still see the same behavior. Server code: #include
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Unix - Decision Making Unix - Shell Loops Unix - Loop Control Unix - Shell Substitutions Unix - Quoting Mechanisms Unix - IO Redirections Unix - Shell http://stackoverflow.com/questions/20067657/tcp-sockets-server-fails-to-respond-to-client-accept-interrupted-system-call Functions Unix - Manpage Help Advanced Unix Unix - Regular Expressions Unix - File System Basics Unix - User Administration Unix - System Performance Unix - System Logging Unix - Signals and Traps Unix Useful References Unix - Useful Commands Unix - Quick Guide Unix - Builtin Functions Unix - System Calls Unix - Commands http://www.tutorialspoint.com/unix_system_calls/accept.htm List Unix Useful Resources Unix Useful Resources Selected Reading Computer Glossary Who is Who Copyright © 2014 by tutorialspoint Home References Discussion Forums About TP accept() - Unix, Linux System Call Advertisements NAME accept - accept a connection on a socket SYNOPSIS #include
point about the behavior of the Unix (socket API) connect() system call when it is interrupted by a signal. It points out how obscure the Single Unix Specification is, and notes http://www.madore.org/~david/computers/connect-intr.html that many existing Unix implementations seem to have f*cked this up somehow. http://poincare.matf.bg.ac.rs/~ivana/courses/tos/sistemi_knjige/pomocno/apue/APUE/0201433079/ch10lev1sec5.html The question is this: if a blocking connect() (on a blocking stream socket, that is) is interrupted by a signal, returning EINTR, in what state is the socket left, and is it permissible to restart the system call? What happens if a second connect() with the same arguments is attempted interrupted system immediately after one failed with EINTR? The reference for connect() (hereafter, “the Spec”) is part of the Open Group's Single Unix Specification, version3 (note: you may need to register to read this; see also here). Here is the relevant part of it: If the initiating socket is connection-mode, then connect() shall attempt to establish a connection to the address specified by the interrupted system call address argument. If the connection cannot be established immediately and O_NONBLOCK is not set for the file descriptor for the socket, connect() shall block for up to an unspecified timeout interval until the connection is established. If the timeout interval expires before the connection is established, connect() shall fail and the connection attempt shall be aborted. If connect() is interrupted by a signal that is caught while blocked waiting to establish a connection, connect() shall fail and set connect() to [EINTR], but the connection request shall not be aborted, and the connection shall be established asynchronously. If the connection cannot be established immediately and O_NONBLOCK is set for the file descriptor for the socket, connect() shall fail and set errno to [EINPROGRESS], but the connection request shall not be aborted, and the connection shall be established asynchronously. Subsequent calls to connect() for the same socket, before the connection is established, shall fail and set errno to [EALREADY]. When the connection has been established asynchronously, select() and poll() shall indicate that the file descriptor for the socket is ready for writing. Later on, when listing
errno was set to EINTR. This was done under the assumption that since a signal occurred and the process caught it, there is a good chance that something has happened that should wake up the blocked system call. Here, we have to differentiate between a system call and a function. It is a system call within the kernel that is interrupted when a signal is caught. To support this feature, the system calls are divided into two categories: the "slow" system calls and all the others. The slow system calls are those that can block forever. Included in this category are Reads that can block the caller forever if data isn't present with certain file types (pipes, terminal devices, and network devices)Writes that can block the caller forever if the data can't be accepted immediately by these same file typesOpens that block until some condition occurs on certain file types (such as an open of a terminal device that waits until an attached modem answers the phone)The pause function (which by definition puts the calling process to sleep until a signal is caught) and the wait functionCertain ioctl operationsSome of the interprocess communication functions (Chapter 15) The notable exception to these slow system calls is anything related to disk I/O. Although a read or a write of a disk file can block the caller temporarily (while the disk driver queues the request and then the request is executed), unless a hardware error occurs, the I/O operation always returns and unblocks the caller quickly. One condition that is handled by interrupted system calls, for example, is when a process initiates a read from a terminal device and the user at the terminal walks away from the terminal for an extended period. In this example, the process could be blocked for hours or days and would remain so unless the system was taken down. POSIX.1 semantics for interrupted reads and writes changed with the 2001 version of the standard. Earlier versions gave implementations a choice for how to deal with reads and writes that have processed partial amounts of data. If read has received and transferred data to an application's buffer, but has not yet received all that the application requested and is then interrupted, the operating system could either fail the system call with errno set to EINTR or allow the system call to succeed, returning the partial amount of data received. Similarly, if write is interrupted after transferring some of the data in an application's buff