Interrupted system call linux

select interrupted system call

I am creating a timer which runs approximately every second and which is waiting for a key to be pressed (which i am not doing). While it is running it shows:

select : interrupted system call select : interrupted system call select : interrupted system call select : interrupted system call 

struct sigaction s1; static timer_t tid3; sigfillset(&s1.sa_mask); s1.sa_flags = SA_SIGINFO; s1.sa_sigaction = SignalHandler; if (sigaction(SIGU, &s1, NULL) == -1) < perror("s1 failed"); exit( EXIT_FAILURE ); >printf("\nTimer %d is setting up \n",TimerIdentity); tid3=SetTimer(SIGU, 1000, 1); // ---------- SET timer values ------------------- static struct sigevent sigev; static timer_t tid; static struct itimerspec itval; static struct itimerspec oitval; sigev.sigev_notify = SIGEV_SIGNAL; sigev.sigev_signo = signo; sigev.sigev_value.sival_ptr = &tid; if (timer_create(CLOCK_REALTIME, &sigev, &tid) == 0) < itval.it_value.tv_sec = sec/1000; itval.it_value.tv_nsec = (long)(sec % 1000) * (1000000L); //itval.it_value.tv_nsec = 0; if (mode == 1) < itval.it_interval.tv_sec = itval.it_value.tv_sec; itval.it_interval.tv_nsec = itval.it_value.tv_nsec; >if (timer_settime(tid, 0, &itval, NULL) == 0) < printf("Timer_settime \n"); >else < perror("time_settime error!"); >> //---------------- SIGNAL HANDLER ---------------- void SignalHandler(int signo, siginfo_t* info, void* context) < else if (signo == SIGU) // for keypad being pressed

//-----------------calltimer3function------------------------ unsigned char key5_debounce=0,key5_debounce_count=0; calltimer3function() < if(!key5_debounce) < if((GPIORead(INPUT_SW5)==0)) < key5_debounce=1; >> if(key5_debounce) < if((GPIORead(INPUT_SW5)==0)) < key5_debounce_count++; >else key5_debounce=0; if(key5_debounce_count>=KEY_DEBOUNCE) < printf("key5 pressed\n"); extr_count=1; printf("\nDisplay menu called"); display_menu(); key5_debounce=0; key5_debounce_count=0; >> > 

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interrupted system call error when writing to a pipe

Pretty straight forward. It's been working fine for quite a while now. But recently, the write call will fail with "interrupted system call" error after the programme went under some stress test. Strangely, the stuff actually went through the pipe no problem. Of course I'd still like to go to the bottom of the error message and get rid of it. Thanks,

2 Answers 2

Conforming to SVr4, 4.3BSD, POSIX.1-2001.

Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written.

I guess you were just lucky that it didn't occur so far.

If you google just for the "interrupted system call", you will find this thread which tells you to use siginterrupt() to auto-restart the write call.

@lang2: I assume the writes are enormeously large in the stress tests. Maybe you should try to chunk the data into buffers of a reasonable size and then write these buffers.

A signal can arrive and be handled while an I/O primitive such as open or read is waiting for an I/O device. If the signal handler returns, the system faces the question: what should happen next?

POSIX specifies one approach: make the primitive fail right away. The error code for this kind of failure is EINTR. This is flexible, but usually inconvenient. Typically, POSIX applications that use signal handlers must check for EINTR after each library function that can return it, in order to try the call again. Often programmers forget to check, which is a common source of error.

So you can handle the EINTR error, there is another choice by the way, You can use sigaction to establish a signal handler specifying how that handler should behave. Using the SA_RESTART flag, return from that handler will resume a primitive; otherwise, return from that handler will cause EINTR.

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Interrupted System Call with CIFS Mount on Kernel 5.3.0

The above kernel causes previously working CIFS mounts to fail with an “interrupted system call” error. A Linux system which had been using a NAS share for years acquired the new kernel on 17th March 2020 and the mount became unstable thereafter.

Update. Resolved 13th June 2020. The issue was not seen again after an update to kernel version 5.3.0-53.

Interrupted System Call

It was during regular system backups that the problem surfaced. While attempting to read files on the mount, the backup software failed with this message:

Cannot open timestamp file for backup pluto.190305_193221.full: Interrupted system call at /home/backups/scripts/Backup.pm line 106, line 2.

The file in question (“timestamp” in the example) varied with each occurrence of the error. An attempt to “cat” the file immediately afterwards would sometimes return the file contents, and sometimes hang the shell.

Kernel Crash

Kernel reactions to the error varied in severity. Sometimes there were no additional messages. Sometimes this appeared in /var/log/kern.log:

CIFS VFS: No task to wake, unknown frame received! NumMids 1

and on at least one occasion, the kernel jumped on a stool as if it had seen a mouse, dumping into kern.log nearly half a million of these:

Apr 6 09:55:57 cx61 kernel: [ 3951.546518] 00000000: 23000000 424d53ff 000008a2 c80180c0 …#.SMB……..
Apr 6 09:55:57 cx61 kernel: [ 3951.546519] 00000010: 00000000 00000000 00000000 39460001 …………..F9
Apr 6 09:55:57 cx61 kernel: [ 3951.546520] 00000020: 65 00 9d 0e 00 e….

NAS Server

The NAS of interest is a rather ancient Linkstation. However the share has been working flawlessly for many years and continues to work with 5 other Linux clients, also Debian based. The difference seems to be that they are running Kernel 4 and the problematic box, running Linux Mint 19.3, is on 5.3.0-45.

For the record, these corresponding errors were left in the NAS’s log. An “oplock” referring to the same file. More likely a symptom than the cause. The same message was seen with earlier kernels.

smbd[26984]: Oplock break failed for file pluto.190305_193221.full/timestamp

Kernel Bug

It looks very similar to Gentoo bug 694780, although putting “ver=1.0” in fstab is not a workaround here. Options appear to be (a) go back to an earlier kernel eg. 4.19, or wait for kernel 5 to be fixed.

Updates

The NAS unit in question, a rather ancient Buffalo Linkstation HS-DHGL, was updated to the latest firmware (v. 2.21-0.83, posted 9th Jan 2018). After updating, the issue is was there.

The client Linux Mint kernel was updated from 5.3.0-45 to 5.3.0-51. After updating, the issue was still there.

The client Linux Mint kernel was updated from 5.3.0.51 to 5.3.0.53. The seems to have resolved matters and the error has not recurred.

Notes

The client encountering the error was a MSI CX61 laptop, about 5 years old.

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What is interrupted system call?

A characteristic of earlier UNIX systems was 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.

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.

So before automatic restarting was introduced, I had to handle interrupted system call on my own. I need write code like:

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:

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

But nowadays I don't have to write this kind of code, beacause of the automatic restarting mechanism.

So if I my understanding are all correct, what/why should I care about interrupted system call now. It seems the system/OS handles it automatically.

related to, but not completely to this link, I removed my duplicate suggestion: EINTR : is there a rationale behind it?

2 Answers 2

Interruption of a system call by a signal handler occurs only in the case of various blocking system calls, and happens when the system call is interrupted by a signal handler that was explicitly established by the programmer.

Furthermore, in the case where a blocking system call is interrupted by a signal handler, automatic system call restarting is an optional feature. You elect to automatically restart system calls by specifying the SA_RESTART flag when establishing the signal handler. As stated in (for example) the Linux signal(7) manual page:

 If a signal handler is invoked while a system call or library function call is blocked, then either: * the call is automatically restarted after the signal handler returns; or * the call fails with the error EINTR. Which of these two behaviors occurs depends on the interface and whether or not the signal handler was established using the SA_RESTART flag (see sigaction(2)). 

As hinted by the last sentence quoted above, even when you elect to use this feature, it does not work for all system calls, and the set of system calls for which it does work varies across UNIX implementations. The Linux signal(7) manual page notes a number of system calls that are automatically restarted when using the SA_RESTART flag, but also goes on to note various system calls that are never restarted, even if you specify that flag when establishing a handler, including:

 * "Input" socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the socket using setsockopt(2): accept(2), recv(2), recvfrom(2), recvmmsg(2) (also with a non-NULL timeout argu‐ ment), and recvmsg(2). * "Output" socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the socket using setsockopt(2): connect(2), send(2), sendto(2), and sendmsg(2). * File descriptor multiplexing interfaces: epoll_wait(2), epoll_pwait(2), poll(2), ppoll(2), select(2), and pselect(2). * System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and semtimedop(2). 

For these system calls, manual restarting using a loop of the form described in APUE is essential, something like:

while ((ret = some_syscall(. )) == -1 && errno == EINTR) continue; if (ret == -1) /* Handle error */ ; 

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