Re: EINTR

[Bastien ROUCARIES]

On Sun, Jul 3, 2011 at 5:36 PM, Bruno Haible <address@hidden> wrote:
> Hi all,
>
> I wrote:
>> ... in programs that don't install signal handlers, EINTR ... also occurs in
>> MacOS X!
>
> It is worse than that:
>
> 1) Even on Linux, even when the signal handlers have the SA_RESTART flag set,
>   some system call (like msgrcv(), but not read()) can fail with EINTR.
>   See [1].

Report to vger al viro and alan cox have fixed problem like this
sometime ago. It is considered backported to stable

> 2) On all systems, when a signal handler has the SA_RESTART flag cleared,
>   not only the read() system call will fail with EINTR, but also an fread()
>   call will fail and set the stream's error bit.
>
>   POSIX specifies it like this: [2]
>
>   Test case: [3]. Run it, type Hello<Return> then Ctrl-C. Result is:
>
>   $ ./a.out
>   Hello
>   ^CSIGINT!
>   fread: Interrupted system call
>   Hello
>
>   Tested on Linux, MacOS X, FreeBSD, OpenBSD, AIX, HP-UX, IRIX, OSF/1,
>   Solaris, Cygwin.
>
> 3) On MacOS X, SIGSTOP and SIGCONT make not only read() fail with EINTR, but
>   fread() as well.
>
>   Test case: [3]. Run it, type Hello<Return> then Ctrl-Z then fg. Result is:
>
>   $ ./a.out
>   Hello
>   ^Z
>   [1]+  Stopped                 ./a.out
>   $ fg
>   ./a.out
>   fread: Interrupted system call
>   Hello
>
>   $ ./a.out --unbuffered
>   Hello
>   Hello
>   ^Z
>   [1]+  Stopped                 ./a.out --unbuffered
>   $ fg
>   ./a.out --unbuffered
>   fread: Interrupted system call
>
>   This was also reported in [4].
>
>   I would expect that EINTR also occurs in write() and fwrite() when writing
>   to a pipe that is temporarily full, but I can't easily reproduce this.
>
> I think we need safe_fread() and safe_fgetc() functions. What do you think?
>
> Bruno
>
>
> [1] 
> http://us.generation-nt.com/sigstop-interrupted-system-call-help-183657841.html
> [2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/fread.html
> [3]
> =============================================================================
> #include <errno.h>
> #include <signal.h>
> #include <stdio.h>
> #include <string.h>
> #include <unistd.h>
>
> static void handler (int sig) { write (1, "SIGINT!\n", 8); }
>
> int
> main (int argc, char *argv[])
> {
>  int unbuffered = (argc > 1 && strcmp (argv[1], "--unbuffered") == 0);
>  signal (SIGINT, handler);
>  siginterrupt (SIGINT, 1);
>  for (;;)
>    {
>      char buf[4096];
>      size_t bufsize = (unbuffered ? 1 : sizeof (buf));
>      size_t count;
>      errno = 0;
>      count = fread (buf, 1, bufsize, stdin);
>      if (count < bufsize && errno != 0)
>        perror ("fread");
>      if (count > 0)
>        fwrite (buf, 1, count, stdout);
>      if (count < bufsize)
>        break;
>    }
>  return 0;
> }
> =============================================================================
> [4] 
> http://factor-language.blogspot.com/2010/09/two-things-every-unix-developer-should.html
> --
> In memoriam Yuri Shchekochikhin 
> <http://en.wikipedia.org/wiki/Yuri_Shchekochikhin>
>
>