keith%tira@gargoyle.uchicago.edu (Keith Waclena) (08/05/89)
Hardware: Sun 4/110, Sun 3/50
OS: SunOS 4.0, SunOS 4.0.1
The following shell script gives different results when invoked with
standard input redirected from a file, compared to when the same file is
piped into it via cat.
$ cat testread.sh
#!/bin/sh
read FIRSTLINE
read SECONDLINE
(echo "$FIRSTLINE"; echo "$SECONDLINE"; cat)
$ cat FOO
AAA
BBB
CCC
$ cat FOO | /bin/sh testread.sh
AAA
BBB
CCC
$ /bin/sh testread.sh <FOO
AAA
BBB
AAA
BBB
CCC
$
(This problem showed up in a realistic context; testread.sh is the
smallest script I can write that illustrates the problem.)
The script works (i.e., produces the same output regardless of whether the
input comes from redirection or from a pipe) under the following standard
Bourne shells: on a Sun 3/50 running SunOS 3.4, on an Elxsi running their
version of Unix, and on an AT&T 3B15 running System V r2.
Is this a known bug? Does it exist in the latest version of SunOS?
(We're upgrading soon.)
Thanks for the info,
Keith
--
Keith WACLENA keith@curry.uchicago.edu
GLS / TIRA / U of Chicago keith%curry@uchimvs1.bitnet
1100 E.57th.St Chi IL 60637 USA ...!uunet!curry.uchicago.edu!keithgorpong@uunet.uu.net (Gordon C. Galligher) (08/29/89)
In article <763@brazos.Rice.edu> keith%tira@gargoyle.uchicago.edu (Keith Waclena) writes: >X-Sun-Spots-Digest: Volume 8, Issue 93, message 13 of 16 > >Hardware: Sun 4/110, Sun 3/50 >OS: SunOS 4.0, SunOS 4.0.1 > >The following shell script gives different results when invoked with >standard input redirected from a file, compared to when the same file is >piped into it via cat. [text of script deleted] > >Is this a known bug? Does it exist in the latest version of SunOS? >(We're upgrading soon.) We currently have SunOS 4.0.3 on a Sun 4/280 and it still exists in that version. Are you listening SUN? -- Gordon. Gordon C. Galligher <|> ...!uunet!telxon!gorpong <|> gorpong@telxon.uucp.uu.net Telxon Corporation <|> "What are ya standin' around for? Don't ya know Akron, Ohio, 44313 <|> a Jail Break when ya see one?" - Scotty (216) 867-3700 (3512)<|> Star Trek V: The Final Frontier
essick@prisma.com (Ray Essick) (09/25/89)
Keith Waclena shows a simple shell script that surprisingly changes behavior depending on whether its input is a pipe or a file. The alleged culprit is the Bourne shell under SunOS 4.x We recreated the same situation here at Prisma and found that the problem is NOT in the Bourne shell. Instead, the bad program is /bin/cat. SunOS 4 /bin/cat knows about mmap() and tries to use a mmap/write cycle instead of a read/write cycle. This saves a buffer copy from the kernel to user space; so it does make sense. The bad news is that the code that says "can we use mmap()" within /bin/cat assumes that the entire file is to be copied. It starts the mmap() at offset 0. In Keith's script, the shell hands cat a perfectly positioned file descriptor and, in essence, cat blindly does an lseek(,0,0) when it uses mmap(). Our fix was to modify /bin/cat's test so that it checked the file position and only uses mmap() if the file was positioned on a page boundary. A more complete solution could have done the appropriate mmap() and offset into the page, but we felt that it wasn't worth the effort. Our fix reverts to the read/write loop when things aren't perfectly aligned. -- Ray Essick, essick@prisma.com