EINTR

[Bruno Haible]

Ralf Wildenhues wrote:
> In the first example using 'read', don't you have to take care of EINTR
> so that './mycat9 < file' doesn't drop forget half the file after being
> suspended and continued again?

Excellent point. I thought that in programs that don't install signal handlers,
EINTR was last seen in SunOS 4, 15 years ago. But no, it also occurs in
MacOS X! Test program:

==========================================================================
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int
main ()
{
  for (;;)
    {
      char buf[4096];
      ssize_t count = read (0, buf, sizeof (buf));
      if (count == 0)
        break;
      if (count < 0)
        {
          perror ("read");
          break;
        }
      fwrite (buf, 1, count, stdout);
    }
  return 0;
}
==========================================================================

Run it, type Hello<Return>, then Ctrl-Z and the job control command 'fg'.
Result on all systems except MacOS X:

$ ./a.out 
Hello
Hello
^Z
[1]+  Stopped                 ./a.out
$ fg
./a.out
World
World

Result on MacOS X:

$ ./a.out 
Hello
Hello
^Z
[1]+  Stopped                 ./a.out
$ fg
./a.out
read: Interrupted system call

I have updated the blog / tutorial lesson, and I'm adding this to the gnulib
documentation:


2011-07-03  Bruno Haible  <address@hidden>

        Comments about EINTR.
        * lib/safe-read.h: Explain the purpose of this module.
        * lib/safe-write.h: Likewise.
        * doc/posix-functions/read.texi: Mention EINTR and the 'safe-read'
        module.
        * doc/posix-functions/write.texi: Mention EINTR and the 'safe-write'
        module.
        Reported by Ralf Wildenhues <address@hidden>.

--- doc/posix-functions/read.texi.orig  Sun Jul  3 13:33:13 2011
+++ doc/posix-functions/read.texi       Sun Jul  3 12:29:44 2011
@@ -17,4 +17,11 @@
 
 Portability problems not fixed by Gnulib:
 @itemize
address@hidden
+This function may fail with error @code{EINTR}, even in programs that don't
+install any signal handlers, on some platforms:
+MacOS X 10.5.
 @end itemize
+
+For handling @code{EINTR}, Gnulib provides a module @samp{safe-read} with a
+function @code{safe_read}.
--- doc/posix-functions/write.texi.orig Sun Jul  3 13:33:13 2011
+++ doc/posix-functions/write.texi      Sun Jul  3 12:29:40 2011
@@ -32,4 +32,11 @@
 
 Portability problems not fixed by Gnulib:
 @itemize
address@hidden
+This function may fail with error @code{EINTR}, even in programs that don't
+install any signal handlers, on some platforms:
+MacOS X 10.5.
 @end itemize
+
+For handling @code{EINTR}, Gnulib provides a module @samp{safe-write} with a
+function @code{safe_write}.
--- lib/safe-read.h.orig        Sun Jul  3 13:33:13 2011
+++ lib/safe-read.h     Sun Jul  3 12:57:35 2011
@@ -14,6 +14,19 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+/* Some system calls may be interrupted and fail with errno = EINTR in the
+   following situations:
+     - The process is stopped and restarted (signal SIGSTOP and SIGCONT, user
+       types Ctrl-Z) on some platforms: MacOS X.
+     - The process receives a signal for which a signal handler was installed
+       with sigaction() with an sa_flags field that does not contain
+       SA_RESTART.
+     - The process receives a signal for which a signal handler was installed
+       with signal() and for which no call to siginterrupt(sig,0) was done,
+       on some platforms: AIX, HP-UX, IRIX, OSF/1, Solaris.
+
+   This module provides a wrapper around read() that handles EINTR.  */
+
 #include <stddef.h>
 
 #ifdef __cplusplus
--- lib/safe-write.h.orig       Sun Jul  3 13:33:13 2011
+++ lib/safe-write.h    Sun Jul  3 12:57:36 2011
@@ -14,6 +14,19 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+/* Some system calls may be interrupted and fail with errno = EINTR in the
+   following situations:
+     - The process is stopped and restarted (signal SIGSTOP and SIGCONT, user
+       types Ctrl-Z) on some platforms: MacOS X.
+     - The process receives a signal for which a signal handler was installed
+       with sigaction() with an sa_flags field that does not contain
+       SA_RESTART.
+     - The process receives a signal for which a signal handler was installed
+       with signal() and for which no call to siginterrupt(sig,0) was done,
+       on some platforms: AIX, HP-UX, IRIX, OSF/1, Solaris.
+
+   This module provides a wrapper around write() that handles EINTR.  */
+
 #include <stddef.h>
 
 #define SAFE_WRITE_ERROR ((size_t) -1)

-- 
In memoriam Yuri Shchekochikhin 
<http://en.wikipedia.org/wiki/Yuri_Shchekochikhin>