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Dan Nicholson wrote:
> On Sat, Aug 04, 2007 at 04:04:17PM -0400, Bryan Kadzban wrote:
>> (if normal "read(0, ...);" works, why doesn't "test -r <that
>> device>"?)
>
> Why do you think read(0,...) works?
I assumed that the normal C-program method for reading standard input
(that is, fgets(..., stdin);) was equivalent to read(0, ...);, with the
addition of possible line buffering. Perhaps it isn't? Let me do some
testing...
OK, testing shows that calling read(0, ...); works, even when /dev/stdin
"test"s not readable. The program I used is attached. I su'ed to root,
then to another nonprivileged user, then ran "[ -r /dev/stdin ] ; echo
$?" and got 1. Then I ran this program, and it echo'ed my input, so its
read succeeded.
> I don't see anything in the tests that tries to read from stdin.
Right, it doesn't actually try to perform the read (if it did, then that
would be a better test of what I think it's trying to look for). It
just looks to see whether /dev/stdin's permissions mask (which it gets
from stat) allows reading. IMO that check is a poor one -- it should
just try to do the read, because there are cases where the test will
fail but reading is still possible.
> An alternative to using /dev/null is to use /dev/tty. This is what's
> done in the perl tests (not by us), but I guess we can't guarantee
> that'll be readable either.
Well, /dev/tty is "the process's controlling TTY", and its permissions
(AFAIK) should always be 0666. So that sounds like it might work better
too.
On your patch: Looks fine to me, unless we want to use /dev/tty.
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#include <stdio.h>
#include <unistd.h>
int main()
{
ssize_t bytes;
char buf[1024];
bytes = read(0, buf, sizeof(buf));
if(bytes < 0) {
perror("read");
return 1;
}
if(bytes >= 1024) bytes = 1023;
buf[bytes] = 0;
printf("Success: %s", buf);
return 0;
}
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