Re: ncurses 6.6 regression: setupterm() rejects valid PTY, breaks tmux attach
Nicholas Marriott <[email protected]> Wed, 27 May 2026 11:21:58 +0100
| Newsgroups | gmane.comp.lib.ncurses.bugs |
|---|---|
| Message-ID | <CAEdLfcHqocqN7Cn7kszDTn0_cAE4=PGfJf+GFtzLDsqMtESNuw@mail.gmail.com> |
--0000000000000041d60652c9fc3b Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Normally we could bump the protocol version but this wasn't really noticed until after it was too late, and I'm not certain it would have worked anyway. I wouldn't mind if we did refuse to attach to older versions but people do hate it and it is a but harsh - it would mean for example that if you did the upgrade inside tmux you couldn't reattach to see the result even if there is in fact no incompatible changes (and that is more common than not)= . On Wed, 27 May 2026, 10:22 Liam Huang, <[email protected]> wrote: > Hi Nicholas, > > Thank you for chiming in =E2=80=94 I certainly didn't expect to draw the > attention of tmux's author on the ncurses mailing list! > > You're right, and I apologize for the noise. I should have read the > 3.6 release notes more carefully before going down this rabbit hole. > The note in https://github.com/tmux/tmux/issues/4699 describes exactly > the symptom I was seeing. > > For what it's worth, the investigation did eventually converge on the > same conclusion =E2=80=94 the imsg library protocol change between 3.5a a= nd > 3.6b makes old servers unable to receive fds from new clients. I just > took the scenic route getting there, and incorrectly blamed ncurses > along the way. > > One thought: would it be feasible for the tmux client to detect a > server version or protocol mismatch and print a more specific error > message (e.g., "server was started by an older tmux version, please > run 'tmux kill-server' and retry")? The current "open terminal failed: > not a terminal" doesn't hint at the actual cause, which is probably why > both I and the earlier 2026-02 reporter ended up looking in the wrong > places. > > In any case, thank you for the clarification, and sorry again for the > misdirected report. > > Best, > Liam > > > 2026=E5=B9=B45=E6=9C=8827=E6=97=A5 17:15=EF=BC=8CNicholas Marriott <nicho= [email protected]> =E5=86=99=E9=81=93=EF=BC=9A > > This was already mentioned in the tmux 3.6 release notes, see the second > paragraph here: > > https://github.com/tmux/tmux/issues/4699 > > > > > > On Wed, 27 May 2026, 08:27 Liam Huang, <[email protected]> wrote: > >> Hi Thomas, >> >> Thank you for your reply and pointers. I'm writing back with a correctio= n: >> after further investigation, I've determined that this is NOT an ncurses >> bug. I apologize for the noise. >> >> >> CORRECTION >> >> The "open terminal failed: not a terminal" error is caused by a tmux >> client/server version mismatch after a Homebrew upgrade, not by ncurses >> 6.6's setupterm(). ncurses is innocent here. >> >> >> WHAT WENT WRONG WITH MY ORIGINAL ANALYSIS >> >> My isolation test used LD_LIBRARY_PATH to swap ncurses versions at >> runtime. I believed this showed ncurses 6.6 as the regression boundary. >> However, the brew tmux binary has a hardcoded DT_RPATH (not DT_RUNPATH) >> pointing to its ncurses directory. DT_RPATH takes priority over >> LD_LIBRARY_PATH, so all three test groups were actually running with >> ncurses 6.6 =E2=80=94 the test was invalid. >> >> The real variable I failed to control was the tmux server version. Befor= e >> `brew upgrade`, an older tmux (3.5a) server was still running. After the >> upgrade, the new 3.6b client connected to the old server, triggering the >> failure. >> >> >> ACTUAL ROOT CAUSE >> >> tmux 3.6b upgraded its bundled OpenBSD imsg IPC library. The struct >> imsg_hdr layout changed in an ABI-incompatible way: >> >> Old (3.5a): type(u32) + len(u16) + flags(u16) + peerid(u32) + pid(u32) >> New (3.6b): type(u32) + len(u32) + peerid(u32) + pid(u32) >> >> The old library marks "message carries an fd" with flags & IMSGF_HASFD >> (bit 0 of the flags field). The new library uses len & 0x80000000 >> (bit 31 of the widened len field), with the flags field removed entirely= . >> >> When the new client sends MSG_IDENTIFY_STDIN (carrying the terminal fd), >> the old server misparses the header: it reads len=3D0x0010, flags=3D0x80= 00, >> and since 0x8000 & IMSGF_HASFD(0x0001) =3D=3D 0, it never dequeues the f= d. >> The client's terminal fd is lost, isatty() fails on a stale fd, and tmux >> reports "not a terminal". >> >> I confirmed this with strace (ioctl(TCGETS) returns ENOTTY on the >> received fd) and ncurses trace (setupterm() is never called =E2=80=94 th= e failure >> occurs before ncurses is involved). >> >> >> CLARIFICATIONS ON YOUR REPLY >> >> A few small points, respectfully: >> >> 1. "Homebrew isn't Ubuntu - it's MacOS" >> >> Homebrew has officially supported Linux since 2019 (the former >> Linuxbrew project merged into mainline). It is widely used on Linux >> servers and in CI environments. My setup =E2=80=94 Homebrew on Ubuntu= =E2=80=94 is a >> supported configuration, not an exotic one. >> >> 2. "This isn't the Ubuntu bug-reporting system" >> >> Understood. I was not reporting an Ubuntu/Debian packaging issue. I w= as >> reporting what I believed to be a regression in ncurses 6.6 itself, >> tested with a from-source build using the same configure flags. As it >> turns out, the bug was not in ncurses at all, so this is moot. >> >> 3. The 2026-02 report you referenced (bug-ncurses/2026-02/msg00025) >> >> That earlier report may or may not be the same issue. If the reporter >> was also hitting a tmux version mismatch after an upgrade, it would >> explain why no reproducer was provided =E2=80=94 the problem disappea= rs once >> the old server is killed. It might be worth asking them whether they >> had recently upgraded tmux. >> >> >> SUMMARY >> >> - ncurses 6.6 setupterm() works correctly. I verified this with both a >> minimal C reproducer (SCM_RIGHTS fd passing + setupterm()) and ncurses >> trace output. >> - The bug is in tmux's cross-version compatibility (or lack thereof) >> after an imsg library ABI change. >> - I've updated the Homebrew issue (homebrew-core#284568) with the >> correction. >> >> ONE QUESTION >> >> Since the root cause is in tmux (the imsg library ABI change breaks >> cross-version client/server compatibility without bumping >> PROTOCOL_VERSION), >> would you recommend I report this to the tmux project? If so, could you >> suggest the appropriate channel =E2=80=94 is it the GitHub issue tracker= at >> https://github.com/tmux/tmux/issues, or the tmux-users mailing list? >> >> Again, I apologize for the incorrect report and for taking your time. >> Thank you for maintaining ncurses. >> >> Best regards, >> Liam >> >> >> 2026=E5=B9=B45=E6=9C=8825=E6=97=A5 22:42=EF=BC=8CThomas Dickey <dickey@i= nvisible-island.net> =E5=86=99=E9=81=93=EF=BC=9A >> >> On Mon, May 25, 2026 at 10:15:16AM +0800, Liam Huang wrote: >> >> Hi, >> >> I've found a regression in ncurses 6.6 (patchdate 20251230) where >> setupterm() fails on valid Linux pseudo-terminals, causing tmux's >> tty_init() to fail and "open terminal failed: not a terminal" on attach. >> >> >> This sounds like a bug which was reported in tmux without providing a >> way to reproduce it - >> >> https://lists.gnu.org/archive/html/bug-ncurses/2026-02/msg00025.html >> >> The issue does NOT occur with ncurses 6.5 (patchdate 20240427) or >> the Ubuntu system ncurses 6.4. >> >> >> This isn't the Ubuntu bug-reporting system. Since Ubuntu only >> mirrors ncurses updates from Debian, the place to report Ubuntu issues >> is in Debian. >> >> >> ENVIRONMENT >> >> OS: Ubuntu 24.04.4 LTS >> Kernel: 6.8.0-107-generic x86_64 >> tmux: 3.6b (Homebrew) >> >> >> Homebrew isn't Ubuntu - it's MacOS. >> >> ncurses: 6.6.20251230 (Homebrew, wide-char build) >> 6.5.20240427 (built from source, same configure flags) >> 6.4+20240113 (Ubuntu system /lib/x86_64-linux-gnu) >> >> >> That's 3 versions of ncurses, one self-built, and (Homebrew isn't >> far from that, either). If you're going to do all of that, you >> should be able to configure ncurses for traces and provide that >> information. >> >> REPRODUCTION >> >> # Using tmux 3.6b linked against ncurses 6.6: >> $ tmux new -d -s test >> $ tmux attach -t test >> open terminal failed: not a terminal >> >> This happens in: >> - Interactive SSH sessions (ssh user@host, then tmux a) >> - VS Code Remote integrated terminal >> - PTYs created by script(1) >> >> The ONLY working case is ssh -t host "tmux a -t test" (forced PTY >> with terminal size forwarding from the SSH client). >> >> >> ISOLATION >> >> Same tmux 3.6b binary, different ncurses library at runtime: >> >> | ncurses | method | result | >> |-------------------|-------------------------------------|---------| >> | 6.6.20251230 | default brew linkage | FAIL | >> | 6.5.20240427 | LD_LIBRARY_PATH=3D.../ncurses-6.5/lib | OK | >> | 6.4 (system) | LD_LIBRARY_PATH=3D/lib/x86_64-linux-gnu | OK | >> >> Commands to reproduce the isolation test: >> >> # FAIL with 6.6 >> script -qc 'tmux -S /tmp/t66 new -d -s x && \ >> tmux -S /tmp/t66 a -t x' /dev/null >> >> # OK with 6.5 >> script -qc 'LD_LIBRARY_PATH=3D/path/to/ncurses-6.5/lib \ >> tmux -S /tmp/t65 new -d -s x && \ >> LD_LIBRARY_PATH=3D/path/to/ncurses-6.5/lib \ >> tmux -S /tmp/t65 a -t x' /dev/null >> >> >> MECHANISM >> >> tmux's server receives the client's PTY fd via SCM_RIGHTS, then >> calls tty_init() -> setupterm(). With ncurses 6.6, setupterm() >> apparently rejects the terminal, causing tty_init() to return -1. >> >> >> "apparently" can be used with lots of supporting data (actual >> system calls used, along with their results). >> >> This leaves the CLIENT_TERMINAL flag unset, and server_client_open() >> returns "not a terminal". >> >> strace shows the PTY is valid =E2=80=94 TCGETS succeeds, the fd resolve= s >> to /dev/pts/N, and permissions are correct (crw--w---- owner:tty). >> >> >> I don't see a trace (and in any case, this report deals with a large >> system, >> in which things can be misconfigured) >> >> The tmux source code for this path is identical between 3.4 and >> 3.6b, confirming the behavioral change is in ncurses, not tmux. >> >> >> BUILD FLAGS (both 6.5 and 6.6 built with): >> >> ./configure --prefix=3D... --with-shared --with-widec \ >> --enable-pc-files --enable-sigwinch --enable-ext-colors \ >> --without-debug --without-ada >> >> >> I haven't bisected the exact ncurses patch between 6.5 and 6.6 yet. >> Happy to test patches or provide additional traces if helpful. >> >> >> That's about 18 months of changes :-) >> >> -- >> Thomas E. Dickey <[email protected]> >> https://invisible-island.net >> >> >> > --0000000000000041d60652c9fc3b Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable <div dir=3D"auto"><div>Normally we could bump the protocol version but this= wasn't really noticed until after it was too late, and I'm not cer= tain it would have worked anyway.</div><div dir=3D"auto"><br></div><div dir= =3D"auto">I wouldn't mind if we did refuse to attach to older versions = but people do hate it and it is a but harsh - it would mean for example tha= t if you did the upgrade inside tmux you couldn't reattach to see the r= esult even if there is in fact no incompatible changes (and that is more co= mmon than not).</div><div dir=3D"auto"><br></div><div dir=3D"auto"><br><br>= <div class=3D"gmail_quote gmail_quote_container" dir=3D"auto"><div dir=3D"l= tr" class=3D"gmail_attr">On Wed, 27 May 2026, 10:22 Liam Huang, <<a href= =3D"mailto:[email protected]">[email protected]</a>> wrote:<br></div><blockquote cla= ss=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid = rgb(204,204,204);padding-left:1ex"><div style=3D""><div>Hi Nicholas,</div><= div><br></div><div>Thank you for chiming in =E2=80=94 I certainly didn'= t expect to draw the</div><div>attention of tmux's author on the ncurse= s mailing list!</div><div><br></div><div>You're right, and I apologize = for the noise. I should have read the</div><div>3.6 release notes more care= fully before going down this rabbit hole.</div><div>The note in <a href=3D"= https://github.com/tmux/tmux/issues/4699" target=3D"_blank" rel=3D"noreferr= er">https://github.com/tmux/tmux/issues/4699</a> describes exactly</div><di= v>the symptom I was seeing.</div><div><br></div><div>For what it's wort= h, the investigation did eventually converge on the</div><div>same conclusi= on =E2=80=94 the imsg library protocol change between 3.5a and</div><div>3.= 6b makes old servers unable to receive fds from new clients. I just</div><d= iv>took the scenic route getting there, and incorrectly blamed ncurses</div= ><div>along the way.</div><div><br></div><div>One thought: would it be feas= ible for the tmux client to detect a</div><div>server version or protocol m= ismatch and print a more specific error</div><div>message (e.g., "serv= er was started by an older tmux version, please</div><div>run 'tmux kil= l-server' and retry")? The current "open terminal failed:</di= v><div>not a terminal" doesn't hint at the actual cause, which is = probably why</div><div>both I and the earlier 2026-02 reporter ended up loo= king in the wrong</div><div>places.</div><div><br></div><div>In any case, t= hank you for the clarification, and sorry again for the</div><div>misdirect= ed report.</div><div><br></div><div>Best,</div><div>Liam</div><div><br></di= v><div><br><blockquote type=3D"cite"><div>2026=E5=B9=B45=E6=9C=8827=E6=97= =A5 17:15=EF=BC=8CNicholas Marriott <<a href=3D"mailto:nicholas.marriott= @gmail.com" target=3D"_blank" rel=3D"noreferrer">[email protected]= m</a>> =E5=86=99=E9=81=93=EF=BC=9A</div><br><div><div dir=3D"auto"><div>= This was already mentioned in the tmux 3.6 release notes, see the second pa= ragraph here:</div><div dir=3D"auto"><br></div><div dir=3D"auto"><a href=3D= "https://github.com/tmux/tmux/issues/4699" target=3D"_blank" rel=3D"norefer= rer">https://github.com/tmux/tmux/issues/4699</a></div><div dir=3D"auto"><b= r></div><div dir=3D"auto"><br></div><div dir=3D"auto"><br></div><div dir=3D= "auto"><br><br><div class=3D"gmail_quote" dir=3D"auto"><div dir=3D"ltr" cla= ss=3D"gmail_attr">On Wed, 27 May 2026, 08:27 Liam Huang, <<a href=3D"mai= lto:[email protected]" target=3D"_blank" rel=3D"noreferrer">[email protected]</a>> w= rote:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0p= x 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div>= Hi Thomas,</div><div><br></div><div>Thank you for your reply and pointers. = I'm writing back with a correction:</div><div>after further investigati= on, I've determined that this is NOT an ncurses</div><div>bug. I apolog= ize for the noise.</div><div><br></div><div><br></div><div>CORRECTION</div>= <div><br></div><div>The "open terminal failed: not a terminal" er= ror is caused by a tmux</div><div>client/server version mismatch after a Ho= mebrew upgrade, not by ncurses</div><div>6.6's setupterm(). ncurses is = innocent here.</div><div><br></div><div><br></div><div>WHAT WENT WRONG WITH= MY ORIGINAL ANALYSIS</div><div><br></div><div>My isolation test used LD_LI= BRARY_PATH to swap ncurses versions at</div><div>runtime. I believed this s= howed ncurses 6.6 as the regression boundary.</div><div>However, the brew t= mux binary has a hardcoded DT_RPATH (not DT_RUNPATH)</div><div>pointing to = its ncurses directory. DT_RPATH takes priority over</div><div>LD_LIBRARY_PA= TH, so all three test groups were actually running with</div><div>ncurses 6= .6 =E2=80=94 the test was invalid.</div><div><br></div><div>The real variab= le I failed to control was the tmux server version. Before</div><div>`brew = upgrade`, an older tmux (3.5a) server was still running. After the</div><di= v>upgrade, the new 3.6b client connected to the old server, triggering the<= /div><div>failure.</div><div><br></div><div><br></div><div>ACTUAL ROOT CAUS= E</div><div><br></div><div>tmux 3.6b upgraded its bundled OpenBSD imsg IPC = library. The struct</div><div>imsg_hdr layout changed in an ABI-incompatibl= e way:</div><div><br></div><div>=C2=A0 Old (3.5a): type(u32) + len(u16) + f= lags(u16) + peerid(u32) + pid(u32)</div><div>=C2=A0 New (3.6b): type(u32) += len(u32) =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0+ peerid(u32) + p= id(u32)</div><div><br></div><div>The old library marks "message carrie= s an fd" with flags & IMSGF_HASFD</div><div>(bit 0 of the flags fi= eld). The new library uses len & 0x80000000</div><div>(bit 31 of the wi= dened len field), with the flags field removed entirely.</div><div><br></di= v><div>When the new client sends MSG_IDENTIFY_STDIN (carrying the terminal = fd),</div><div>the old server misparses the header: it reads len=3D0x0010, = flags=3D0x8000,</div><div>and since 0x8000 & IMSGF_HASFD(0x0001) =3D=3D= 0, it never dequeues the fd.</div><div>The client's terminal fd is los= t, isatty() fails on a stale fd, and tmux</div><div>reports "not a ter= minal".</div><div><br></div><div>I confirmed this with strace (ioctl(T= CGETS) returns ENOTTY on the</div><div>received fd) and ncurses trace (setu= pterm() is never called =E2=80=94 the failure</div><div>occurs before ncurs= es is involved).</div><div><br></div><div><br></div><div>CLARIFICATIONS ON = YOUR REPLY</div><div><br></div><div>A few small points, respectfully:</div>= <div><br></div><div>1. "Homebrew isn't Ubuntu - it's MacOS&quo= t;</div><div><br></div><div>=C2=A0 =C2=A0Homebrew has officially supported = Linux since 2019 (the former</div><div>=C2=A0 =C2=A0Linuxbrew project merge= d into mainline). It is widely used on Linux</div><div>=C2=A0 =C2=A0servers= and in CI environments. My setup =E2=80=94 Homebrew on Ubuntu =E2=80=94 is= a</div><div>=C2=A0 =C2=A0supported configuration, not an exotic one.</div>= <div><br></div><div>2. "This isn't the Ubuntu bug-reporting system= "</div><div><br></div><div>=C2=A0 =C2=A0Understood. I was not reportin= g an Ubuntu/Debian packaging issue. I was</div><div>=C2=A0 =C2=A0reporting = what I believed to be a regression in ncurses 6.6 itself,</div><div>=C2=A0 = =C2=A0tested with a from-source build using the same configure flags. As it= </div><div>=C2=A0 =C2=A0turns out, the bug was not in ncurses at all, so th= is is moot.</div><div><br></div><div>3. The 2026-02 report you referenced (= bug-ncurses/2026-02/msg00025)</div><div><br></div><div>=C2=A0 =C2=A0That ea= rlier report may or may not be the same issue. If the reporter</div><div>= =C2=A0 =C2=A0was also hitting a tmux version mismatch after an upgrade, it = would</div><div>=C2=A0 =C2=A0explain why no reproducer was provided =E2=80= =94 the problem disappears once</div><div>=C2=A0 =C2=A0the old server is ki= lled. It might be worth asking them whether they</div><div>=C2=A0 =C2=A0had= recently upgraded tmux.</div><div><br></div><div><br></div><div>SUMMARY</d= iv><div><br></div><div>- ncurses 6.6 setupterm() works correctly. I verifie= d this with both a</div><div>=C2=A0 minimal C reproducer (SCM_RIGHTS fd pas= sing + setupterm()) and ncurses</div><div>=C2=A0 trace output.</div><div>- = The bug is in tmux's cross-version compatibility (or lack thereof)</div= ><div>=C2=A0 after an imsg library ABI change.</div><div>- I've updated= the Homebrew issue (homebrew-core#284568) with the</div><div>=C2=A0 correc= tion.</div><div><br></div><div>ONE QUESTION</div><div><br></div><div>Since = the root cause is in tmux (the imsg library ABI change breaks</div><div>cro= ss-version client/server compatibility without bumping PROTOCOL_VERSION),</= div><div>would you recommend I report this to the tmux project? If so, coul= d you</div><div>suggest the appropriate channel =E2=80=94 is it the GitHub = issue tracker at</div><div><a href=3D"https://github.com/tmux/tmux/issues" = rel=3D"noreferrer noreferrer" target=3D"_blank">https://github.com/tmux/tmu= x/issues</a>, or the tmux-users mailing list?</div><div><br></div><div>Agai= n, I apologize for the incorrect report and for taking your time.</div><div= >Thank you for maintaining ncurses.</div><div><br></div><div>Best regards,<= /div><div>Liam</div><div><br></div><div><br><blockquote type=3D"cite"><div>= 2026=E5=B9=B45=E6=9C=8825=E6=97=A5 22:42=EF=BC=8CThomas Dickey <<a href= =3D"mailto:[email protected]" rel=3D"noreferrer noreferrer" targe= t=3D"_blank">[email protected]</a>> =E5=86=99=E9=81=93=EF=BC= =9A</div><br><div><span style=3D"font-family:Helvetica;font-size:12px;font-= style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal= ;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wo= rd-spacing:0px;text-decoration:none;float:none;display:inline">On Mon, May = 25, 2026 at 10:15:16AM +0800, Liam Huang wrote:</span><br style=3D"font-fam= ily:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;fon= t-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-tr= ansform:none;white-space:normal;word-spacing:0px;text-decoration-line:none;= text-decoration-style:solid"><blockquote type=3D"cite" style=3D"font-family= :Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-w= eight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-trans= form:none;white-space:normal;word-spacing:0px;text-decoration-line:none;tex= t-decoration-style:solid">Hi,<br><br>I've found a regression in ncurses= 6.6 (patchdate 20251230) where<br>setupterm() fails on valid Linux pseudo-= terminals, causing tmux's<br>tty_init() to fail and "open terminal= failed: not a terminal" on attach.<br></blockquote><br style=3D"font-= family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;= font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text= -transform:none;white-space:normal;word-spacing:0px;text-decoration-line:no= ne;text-decoration-style:solid"><span style=3D"font-family:Helvetica;font-s= ize:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-= spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-s= pace:normal;word-spacing:0px;text-decoration:none;float:none;display:inline= ">This sounds like a bug which was reported in tmux without providing a</sp= an><br style=3D"font-family:Helvetica;font-size:12px;font-style:normal;font= -variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start= ;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;te= xt-decoration-line:none;text-decoration-style:solid"><span style=3D"font-fa= mily:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;fo= nt-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-t= ransform:none;white-space:normal;word-spacing:0px;text-decoration:none;floa= t:none;display:inline">way to reproduce it -<span>=C2=A0</span></span><br s= tyle=3D"font-family:Helvetica;font-size:12px;font-style:normal;font-variant= -caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-in= dent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decor= ation-line:none;text-decoration-style:solid"><br style=3D"font-family:Helve= tica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:= 400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:n= one;white-space:normal;word-spacing:0px;text-decoration-line:none;text-deco= ration-style:solid"><a href=3D"https://lists.gnu.org/archive/html/bug-ncurs= es/2026-02/msg00025.html" style=3D"font-family:Helvetica;font-size:12px;fon= t-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:norm= al;text-align:start;text-indent:0px;text-transform:none;white-space:normal;= word-spacing:0px" rel=3D"noreferrer noreferrer" target=3D"_blank">https://l= ists.gnu.org/archive/html/bug-ncurses/2026-02/msg00025.html</a><br style=3D= "font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:n= ormal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0p= x;text-transform:none;white-space:normal;word-spacing:0px;text-decoration-l= ine:none;text-decoration-style:solid"><br style=3D"font-family:Helvetica;fo= nt-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;let= ter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi= te-space:normal;word-spacing:0px;text-decoration-line:none;text-decoration-= style:solid"><blockquote type=3D"cite" style=3D"font-family:Helvetica;font-= size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter= -spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-= space:normal;word-spacing:0px;text-decoration-line:none;text-decoration-sty= le:solid">The issue does NOT occur with ncurses 6.5 (patchdate 20240427) or= <br>the Ubuntu system ncurses 6.4.<br></blockquote><br style=3D"font-family= :Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-w= eight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-trans= form:none;white-space:normal;word-spacing:0px;text-decoration-line:none;tex= t-decoration-style:solid"><span style=3D"font-family:Helvetica;font-size:12= px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacin= g:normal;text-align:start;text-indent:0px;text-transform:none;white-space:n= ormal;word-spacing:0px;text-decoration:none;float:none;display:inline">This= isn't the Ubuntu bug-reporting system.=C2=A0 Since Ubuntu only</span><= br style=3D"font-family:Helvetica;font-size:12px;font-style:normal;font-var= iant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;tex= t-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-d= ecoration-line:none;text-decoration-style:solid"><span style=3D"font-family= :Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-w= eight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-trans= form:none;white-space:normal;word-spacing:0px;text-decoration:none;float:no= ne;display:inline">mirrors ncurses updates from Debian, the place to report= Ubuntu issues</span><br style=3D"font-family:Helvetica;font-size:12px;font= -style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:norma= l;text-align:start;text-indent:0px;text-transform:none;white-space:normal;w= ord-spacing:0px;text-decoration-line:none;text-decoration-style:solid"><spa= n style=3D"font-family:Helvetica;font-size:12px;font-style:normal;font-vari= ant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text= -indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-de= coration:none;float:none;display:inline">is in Debian.</span><br style=3D"f= ont-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:nor= mal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;= text-transform:none;white-space:normal;word-spacing:0px;text-decoration-lin= e:none;text-decoration-style:solid"><br style=3D"font-family:Helvetica;font= -size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;lette= r-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white= -space:normal;word-spacing:0px;text-decoration-line:none;text-decoration-st= yle:solid"><blockquote type=3D"cite" style=3D"font-family:Helvetica;font-si= ze:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-s= pacing:normal;text-align:start;text-indent:0px;text-transform:none;white-sp= ace:normal;word-spacing:0px;text-decoration-line:none;text-decoration-style= :solid"><br>ENVIRONMENT<br><br>=C2=A0OS: =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0Ubuntu 24.04.4 LTS<br>=C2=A0Kernel: =C2=A0=C2=A06.8.0-107-generic x86_64= <br>=C2=A0tmux: =C2=A0=C2=A0=C2=A0=C2=A03.6b (Homebrew)<br></blockquote><br= style=3D"font-family:Helvetica;font-size:12px;font-style:normal;font-varia= nt-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-= indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-dec= oration-line:none;text-decoration-style:solid"><span style=3D"font-family:H= elvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-wei= ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transfo= rm:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none= ;display:inline">Homebrew isn't Ubuntu - it's MacOS.</span><br styl= e=3D"font-family:Helvetica;font-size:12px;font-style:normal;font-variant-ca= ps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-inden= t:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decorati= on-line:none;text-decoration-style:solid"><br style=3D"font-family:Helvetic= a;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400= ;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none= ;white-space:normal;word-spacing:0px;text-decoration-line:none;text-decorat= ion-style:solid"><blockquote type=3D"cite" style=3D"font-family:Helvetica;f= ont-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;le= tter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;wh= ite-space:normal;word-spacing:0px;text-decoration-line:none;text-decoration= -style:solid">=C2=A0ncurses: =C2=A06.6.20251230 (Homebrew, wide-char build)= <br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A06.5.2= 0240427 (built from source, same configure flags)<br>=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A06.4+20240113 (Ubuntu system /l= ib/x86_64-linux-gnu)<br></blockquote><br style=3D"font-family:Helvetica;fon= t-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;lett= er-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whit= e-space:normal;word-spacing:0px;text-decoration-line:none;text-decoration-s= tyle:solid"><span style=3D"font-family:Helvetica;font-size:12px;font-style:= normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-= align:start;text-indent:0px;text-transform:none;white-space:normal;word-spa= cing:0px;text-decoration:none;float:none;display:inline">That's 3 versi= ons of ncurses, one self-built, and (Homebrew isn't</span><br style=3D"= font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:no= rmal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px= ;text-transform:none;white-space:normal;word-spacing:0px;text-decoration-li= ne:none;text-decoration-style:solid"><span style=3D"font-family:Helvetica;f= ont-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;le= tter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;wh= ite-space:normal;word-spacing:0px;text-decoration:none;float:none;display:i= nline">far from that, either).=C2=A0 If you're going to do all of that,= you</span><br style=3D"font-family:Helvetica;font-size:12px;font-style:nor= mal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-ali= gn:start;text-indent:0px;text-transform:none;white-space:normal;word-spacin= g:0px;text-decoration-line:none;text-decoration-style:solid"><span style=3D= "font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:n= ormal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0p= x;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:n= one;float:none;display:inline">should be able to configure ncurses for trac= es and provide that information.</span><br style=3D"font-family:Helvetica;f= ont-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;le= tter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;wh= ite-space:normal;word-spacing:0px;text-decoration-line:none;text-decoration= -style:solid"><br style=3D"font-family:Helvetica;font-size:12px;font-style:= normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-= align:start;text-indent:0px;text-transform:none;white-space:normal;word-spa= cing:0px;text-decoration-line:none;text-decoration-style:solid"><blockquote= type=3D"cite" style=3D"font-family:Helvetica;font-size:12px;font-style:nor= mal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-ali= gn:start;text-indent:0px;text-transform:none;white-space:normal;word-spacin= g:0px;text-decoration-line:none;text-decoration-style:solid">REPRODUCTION<b= r><br>=C2=A0# Using tmux 3.6b linked against ncurses 6.6:<br>=C2=A0$ tmux n= ew -d -s test<br>=C2=A0$ tmux attach -t test<br>=C2=A0open terminal failed:= not a terminal<br><br>=C2=A0This happens in:<br>=C2=A0- Interactive SSH se= ssions (ssh user@host, then tmux a)<br>=C2=A0- VS Code Remote integrated te= rminal<br>=C2=A0- PTYs created by script(1)<br><br>=C2=A0The ONLY working c= ase is ssh -t host "tmux a -t test" (forced PTY<br>=C2=A0with ter= minal size forwarding from the SSH client).<br><br><br>ISOLATION<br><br>=C2= =A0Same tmux 3.6b binary, different ncurses library at runtime:<br><br>=C2= =A0| ncurses =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0| = method =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0| result =C2=A0|<br>=C2=A0|---------------= ----|-------------------------------------|---------|<br>=C2=A0| 6.6.202512= 30 =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0| default brew linkage =C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0| F= AIL =C2=A0=C2=A0=C2=A0|<br>=C2=A0| 6.5.20240427 =C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0| LD_LIBRARY_PATH=3D.../ncurses-6.5/lib | OK =C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0|<br>=C2=A0| 6.4 (system) =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0| LD_LIBRARY_PAT= H=3D/lib/x86_64-linux-gnu | OK =C2=A0=C2=A0=C2=A0|<br><br>=C2=A0Commands to= reproduce the isolation test:<br><br>=C2=A0=C2=A0=C2=A0# FAIL with 6.6<br>= =C2=A0=C2=A0=C2=A0script -qc 'tmux -S /tmp/t66 new -d -s x && \= <br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0tmux -S /tmp/t66 a -t x' /dev/null<br><br>= =C2=A0=C2=A0=C2=A0# OK with 6.5<br>=C2=A0=C2=A0=C2=A0script -qc 'LD_LIB= RARY_PATH=3D/path/to/ncurses-6.5/lib \<br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0tmux -S /tmp= /t65 new -d -s x && \<br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0LD_LIBRARY_PATH=3D/pa= th/to/ncurses-6.5/lib \<br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0tmux -S /tmp/t65 a -t x'= ; /dev/null<br><br><br>MECHANISM<br><br>=C2=A0tmux's server receives th= e client's PTY fd via SCM_RIGHTS, then<br>=C2=A0calls tty_init() -> = setupterm(). With ncurses 6.6, setupterm()<br>=C2=A0apparently rejects the = terminal, causing tty_init() to return -1.<br></blockquote><br style=3D"fon= t-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:norma= l;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;te= xt-transform:none;white-space:normal;word-spacing:0px;text-decoration-line:= none;text-decoration-style:solid"><span style=3D"font-family:Helvetica;font= -size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;lette= r-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white= -space:normal;word-spacing:0px;text-decoration:none;float:none;display:inli= ne">"apparently" can be used with lots of supporting data (actual= </span><br style=3D"font-family:Helvetica;font-size:12px;font-style:normal;= font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:s= tart;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0p= x;text-decoration-line:none;text-decoration-style:solid"><span style=3D"fon= t-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:norma= l;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;te= xt-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;= float:none;display:inline">system calls used, along with their results).</s= pan><br style=3D"font-family:Helvetica;font-size:12px;font-style:normal;fon= t-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:star= t;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;t= ext-decoration-line:none;text-decoration-style:solid"><br style=3D"font-fam= ily:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;fon= t-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-tr= ansform:none;white-space:normal;word-spacing:0px;text-decoration-line:none;= text-decoration-style:solid"><blockquote type=3D"cite" style=3D"font-family= :Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-w= eight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-trans= form:none;white-space:normal;word-spacing:0px;text-decoration-line:none;tex= t-decoration-style:solid">=C2=A0This leaves the CLIENT_TERMINAL flag unset,= and server_client_open()<br>=C2=A0returns "not a terminal".<br><= br>=C2=A0strace shows the PTY is valid =E2=80=94 TCGETS succeeds, the fd re= solves<br>=C2=A0to /dev/pts/N, and permissions are correct (crw--w---- owne= r:tty).<br></blockquote><br style=3D"font-family:Helvetica;font-size:12px;f= ont-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:no= rmal;text-align:start;text-indent:0px;text-transform:none;white-space:norma= l;word-spacing:0px;text-decoration-line:none;text-decoration-style:solid"><= span style=3D"font-family:Helvetica;font-size:12px;font-style:normal;font-v= ariant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;t= ext-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text= -decoration:none;float:none;display:inline">I don't see a trace (and in= any case, this report deals with a large system,</span><br style=3D"font-f= amily:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;f= ont-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-= transform:none;white-space:normal;word-spacing:0px;text-decoration-line:non= e;text-decoration-style:solid"><span style=3D"font-family:Helvetica;font-si= ze:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-s= pacing:normal;text-align:start;text-indent:0px;text-transform:none;white-sp= ace:normal;word-spacing:0px;text-decoration:none;float:none;display:inline"= >in which things can be misconfigured)</span><br style=3D"font-family:Helve= tica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:= 400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:n= one;white-space:normal;word-spacing:0px;text-decoration-line:none;text-deco= ration-style:solid"><br style=3D"font-family:Helvetica;font-size:12px;font-= style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal= ;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wo= rd-spacing:0px;text-decoration-line:none;text-decoration-style:solid"><bloc= kquote type=3D"cite" style=3D"font-family:Helvetica;font-size:12px;font-sty= le:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;te= xt-align:start;text-indent:0px;text-transform:none;white-space:normal;word-= spacing:0px;text-decoration-line:none;text-decoration-style:solid">=C2=A0Th= e tmux source code for this path is identical between 3.4 and<br>=C2=A03.6b= , confirming the behavioral change is in ncurses, not tmux.<br><br><br>BUIL= D FLAGS (both 6.5 and 6.6 built with):<br><br>=C2=A0./configure --prefix=3D= ... --with-shared --with-widec \<br>=C2=A0=C2=A0=C2=A0--enable-pc-files --e= nable-sigwinch --enable-ext-colors \<br>=C2=A0=C2=A0=C2=A0--without-debug -= -without-ada<br><br><br>I haven't bisected the exact ncurses patch betw= een 6.5 and 6.6 yet.<br>Happy to test patches or provide additional traces = if helpful.<br></blockquote><br style=3D"font-family:Helvetica;font-size:12= px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacin= g:normal;text-align:start;text-indent:0px;text-transform:none;white-space:n= ormal;word-spacing:0px;text-decoration-line:none;text-decoration-style:soli= d"><span style=3D"font-family:Helvetica;font-size:12px;font-style:normal;fo= nt-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:sta= rt;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;= text-decoration:none;float:none;display:inline">That's about 18 months = of changes :-)</span><br style=3D"font-family:Helvetica;font-size:12px;font= -style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:norma= l;text-align:start;text-indent:0px;text-transform:none;white-space:normal;w= ord-spacing:0px;text-decoration-line:none;text-decoration-style:solid"><br = style=3D"font-family:Helvetica;font-size:12px;font-style:normal;font-varian= t-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-i= ndent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-deco= ration-line:none;text-decoration-style:solid"><span style=3D"font-family:He= lvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weig= ht:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transfor= m:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;= display:inline">--<span>=C2=A0</span></span><br style=3D"font-family:Helvet= ica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:4= 00;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:no= ne;white-space:normal;word-spacing:0px;text-decoration-line:none;text-decor= ation-style:solid"><span style=3D"font-family:Helvetica;font-size:12px;font= -style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:norma= l;text-align:start;text-indent:0px;text-transform:none;white-space:normal;w= ord-spacing:0px;text-decoration:none;float:none;display:inline">Thomas E. D= ickey <</span><a href=3D"mailto:[email protected]" style=3D"fo= nt-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:norm= al;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;t= ext-transform:none;white-space:normal;word-spacing:0px" rel=3D"noreferrer n= oreferrer" target=3D"_blank">[email protected]</a><span style=3D"= font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:no= rmal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px= ;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:no= ne;float:none;display:inline">></span><br style=3D"font-family:Helvetica= ;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;= letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;= white-space:normal;word-spacing:0px;text-decoration-line:none;text-decorati= on-style:solid"><a href=3D"https://invisible-island.net/" style=3D"font-fam= ily:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;fon= t-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-tr= ansform:none;white-space:normal;word-spacing:0px" rel=3D"noreferrer norefer= rer" target=3D"_blank">https://invisible-island.net</a></div></blockquote><= /div><br></div></blockquote></div></div></div> </div></blockquote></div><br></div></blockquote></div></div></div> --0000000000000041d60652c9fc3b--