Shell Programming and Scripting

  #!/bin/ksh
  typeset TEXT=""
  
  trap 'echo "got a signal!"' USR1
  
  echo "put some text here.... my pid is $$"
  read TEXT
  echo "I'm done"
  
  exit 0

trap 'echo "got a signal!"' USR1 0

trap 'echo "got a signal!"' USR1
trap 'echo ""' 0

[Linux]> cat a.ksh
#!/bin/ksh
typeset TEXT=""

trap 'echo "got a signal!"' USR1
trap 'echo ""' 0

# send USR1 signal after 5 seconds
( sleep 5; kill -USR1 ${$} ) &

echo "put some text here.... my pid is $$"
date
read TEXT
date
echo "I'm done $TEXT"

exit 0
[Linux]>

[Linux]> ksh -x a.ksh
+ TEXT=''
+ typeset TEXT
+ trap 'echo "got a signal!"' USR1
+ trap 'echo ""' 0
+ echo 'put some text here.... my pid is 17403'
put some text here.... my pid is 17403
+ date
+ sleep 5
Wed Nov 30 15:54:50 EST 2011
+ read TEXT
+ kill -USR1 17403
+ echo 'got a signal!'
got a signal!
waiting for a while and pressing enter...
+ date
Wed Nov 30 15:55:20 EST 2011
+ echo $'I\'m done waiting for a while and pressing enter...'
I'm done waiting for a while and pressing enter...
+ exit 0
+ echo ''

[Linux]>

[Linux]> cat a.ksh
#!/bin/ksh
typeset TEXT=""

trap 'echo "got a signal!"' USR1
trap 'echo ""' 0

# send USR1 signal after 5 seconds
( sleep 5; kill -USR1 ${$} ) &

echo "put some text here.... my pid is $$"
date
sleep 20
date
echo "I'm done $TEXT"

exit 0


# Now run it

[Linux]> ksh -x ./a.ksh
+ TEXT=''
+ typeset TEXT
+ trap 'echo "got a signal!"' USR1
+ trap 'echo ""' 0
+ echo 'put some text here.... my pid is 16012'
put some text here.... my pid is 16012
+ date
+ sleep 5
Wed Nov 30 15:51:26 EST 2011
+ sleep 20
+ kill -USR1 16012
+ echo 'got a signal!'
got a signal!
+ date
Wed Nov 30 15:51:31 EST 2011
+ echo $'I\'m done '
I'm done
+ exit 0
+ echo ''

[Linux]>

#include <stdio.h>
#include <signal.h>
#include <unistd.h>

int handler(int sig)
{
        fprintf(stderr, "!!!\n");
}

int main(void)
{
        char buf[512];
        ssize_t s;
        signal(SIGALRM, handler);
        alarm(1);
        s=read(STDIN_FILENO, buf, 512);
        if(s <= 0) perror("Error");
        return(0);
}

$ ./a.out
!!!
^C
$

   Interruption of System Calls and Library Functions by Signal Handlers
       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)).  The details vary across Unix systems; below,
       the details for Linux.

       If a blocked call to one of the following interfaces is interrupted  by
       a  signal  handler, then the call will be automatically restarted after
       the signal handler returns if the SA_RESTART flag was  used;  otherwise
       the call will fail with the error EINTR:

           * read(2),  readv(2),  write(2),  writev(2),  and ioctl(2) calls on
             "slow" devices.  A "slow" device is one where the  I/O  call  may
             block  for  an indefinite time, for example, a terminal, pipe, or
             socket.  (A disk is not a slow device according to  this  defini-
             tion.)   If  an I/O call on a slow device has already transferred
             some data by the time it is interrupted by a signal handler, then
             the  call  will  return a success status (normally, the number of
             bytes transferred).

...

#include <stdio.h>
#include <signal.h>
#include <unistd.h>

int handler(int sig)
{
        fprintf(stderr, "!!!\n");
}

int main(void)
{
        struct sigaction act, old;
        char buf[512];
        ssize_t s;

        act.sa_handler=handler;
        sigemptyset(&act.sa_mask);
        act.sa_flags=SA_RESETHAND;
        sigaction(SIGALRM, &act, &old);

        alarm(1);
        s=read(STDIN_FILENO, buf, 512);
        if(s <= 0) perror("Error");
        return(0);
}

$ ./a.out
!!!
Error: Interrupted system call

$