In the following code, what happens to the buffer from sysread() when the system call is restarted due to EINTR? I am reading from a pipe and not a file and cant afford to lose a single bit. However, my program complains like it is losing data. Are the contents of $buffer in an undefined state or is it guaranteed that the read will be consistent? That is if I have the following data in the pipe:

"1 2 3 4"

and call sysread but the call is interruped and restarted after reading "1 2" can it be guaranteed that $buffer will have "1 2 3 4" or will it have "3 4" and lose "1 2"? Something else? How can I guarantee that I will read all the data even if the call is interrupted by SIGCHLD?

Furthermore, I must get the return code from the INPIPE process and the only way that seems to work ok is to install a blank signal handler and let close() wait on the child so letting perl reap the process by itself is unacceptable.

I am using AIX 5.3 perl 5.8.8

Apologies if the code is rough, I pulled the relevant bits out of a larger program. I'm not good at low level systems programming/semantics so feel free to challenge any of my assumptions or suggest alternate methods.

my $sigset = POSIX::SigSet->new(SIGCHLD);  
my $sigaction = POSIX::SigAction->new( sub { }, $sigset, &POSIX::SA_RE
+START );
my $old_sigset = POSIX::SigSet->new;        
sigprocmask(SIG_SETMASK, $sigset, $old_sigset)
    or die "Could not set signal mask\n";

$reader_pid = open(INPIPE,"cat /tmp/file |")

my $buffer;
#Pipe buffer on AIX seems to be 32K
while($rbytes = sysread(INPIPE,$buffer,32768)){
    $wbytes = syswrite(OUTPIPE,$buffer);
    #print OUTPIPE $buffer;
    if($wbytes != $rbytes){
        die("Error writing to outpipe.");
    }
}
if(!defined($rbytes)){
    bomb("INPIPE had error: $!\n",2);
}
close(INPIPE)
    or die $! ? "Error closing inpipe: $!"
                   : "Exit status $? from inpipe";

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In reply to EINTR and sysread() by bot403

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