Saturday, October 8, 2011

interrupted system call linux

Interrupted System Calls

A characteristic of earlier UNIX systems is that if a process caught a signal while the process was blocked in a "slow" system call, the system call was interrupted. The system call returned an error and 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 types 
  • Opens 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 function 
  • Certain ioctl operations 
  • Some of the interprocess communication functions.
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 buffer, the operation 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 written. Historically, implementations derived from System V fail the system call, whereas BSD-derived implementations return partial success. With the 2001 version of the POSIX.1 standard, the BSD-style semantics are required.

The problem with interrupted system calls is that we now have to handle the error return explicitly. The typical code sequence (assuming a read operation and assuming that we want to restart the read even if it's interrupted) would be

   if ((n = read(fd, buf, BUFFSIZE)) < 0) {
      if (errno == EINTR)
         goto again; //just an interrupted system cal
       * handle other errors 

To prevent applications from having to handle interrupted system calls, 4.2BSD introduced the automatic restarting of certain interrupted system calls. The system calls that were automatically restarted are ioctl, read, readv, write, writev, wait, and waitpid. As we've mentioned, the first five of these functions are interrupted by a signal only if they are operating on a slow device; wait and waitpid are always interrupted when a signal is caught. Since this caused a problem for some applications that didn't want the operation restarted if it was interrupted, 4.3BSD allowed the process to disable this feature on a per signal basis.

POSIX.1 allows an implementation to restart system calls, but it is not required. The Single UNIX Specification defines the SA_RESTART flag as an XSI extension to sigaction to allow applications to request that interrupted system calls be restarted.

System V has never restarted system calls by default. BSD, on the other hand, restarts them if interrupted by signals. By default, FreeBSD 5.2.1, Linux 2.4.22, and Mac OS X 10.3 restart system calls interrupted by signals. The default on Solaris 9, however, is to return an error (EINTR) instead.

One of the reasons 4.2BSD introduced the automatic restart feature is that sometimes we don't know that the input or output device is a slow device. If the program we write can be used interactively, then it might be reading or writing a slow device, since terminals fall into this category. If we catch signals in this program, and if the system doesn't provide the restart capability, then we have to test every read or write for the interrupted error return and reissue the read or write.

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