On 12/13/25 1:51 AM, Grisha Levit wrote:
On Fri, Dec 12, 2025, 19:20 Robert Elz <[email protected] <mailto:[email protected]>> wrote:> I'm guessing that the difference between using awk, and most other > processes that might be used (echo, sed, even cat probably) inside the > cmdsub, is that awk (gawk) might be doing > > freopen(filename, "r", stdin); > > or something similar, which it is perfectly entitled to do (ie: doing that > is certainly not, of itself, a bug). What gawk had was essentially a call like: fcntl(2, F_SETFL, fcntl(2, F_GETFL) | O_APPEND);
And coreutils cat changes the test it uses to determine whether or not an
output file is an input file depending on whether O_APPEND is set in the
output file's flags.
So let's posit that fd 1 and fd 2 are the same at startup (dup2 (1, 2) or
equivalent somewhere). Then gawk changing fd 2 to add O_APPEND to the
file flags will change fd 1's flags. Further posit that fd 0 is the same
device but opened by a different call to open(2), so it will test as having
the same device and inode, but with a different entry in the file table and
a different flag set.
This appears to change how cat tests whether fd 0 and fd 1 are the same
file (this is from coreutils-git):
if (! (S_ISFIFO (stat_buf.st_mode) || S_ISSOCK (stat_buf.st_mode)
|| S_TYPEISSHM (&stat_buf) || S_TYPEISTMO (&stat_buf))
&& have_out_dev
&& SAME_INODE (stat_buf, out_id))
{
off_t in_pos = lseek (input_desc, 0, SEEK_CUR);
if (0 <= in_pos)
{
if (out_flags < -1)
out_flags = fcntl (STDOUT_FILENO, F_GETFL);
int whence = (0 <= out_flags && out_flags & O_APPEND
? SEEK_END : SEEK_CUR);
if (in_pos < lseek (STDOUT_FILENO, 0, whence))
{
error (0, 0, _("%s: input file is output file"),
quotef (infile));
ok = false;
goto contin;
}
}
}
and the difference between fd 0/SEEK_CUR and fd 1/SEEK_END is enough.
I don't have a better explanation and I've already spent way too much
time on this.
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU [email protected] http://tiswww.cwru.edu/~chet/
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