4 ms·

> I believe the reason cat gets interrupted when we press Ctrl+C is that the Linux kernel on the server side receives this \x03 character, recognizes that it me

by schoen 4y ago

> I believe the reason cat gets interrupted when we press Ctrl+C is that the Linux kernel on the server side receives this \x03 character, recognizes that it means “interrupt”, and then sends a SIGINT to the process that owns the pseudoterminal’s process group. So it’s handled in the kernel and not in userspace. Also interestingly, the use of \x03 for this purpose is a default, but it's not hardcoded. You can change it with the stty command. For example, if you run stty intr '^X' then your interrupt character sequence will be Ctrl+X instead of Ctrl+C (!). In order to make this change, the stty program actually has to call into the kernel with an ioctl call (TCSETS or a related ioctl, for "terminal control set settings"). You can learn more about this ioctl on Linux in the ioctl_tty(2) man page, and you can see the various settings that can be changed this way with stty -a (It's a little bit confusing which things are handled by the readline library and which things are handled by the kernel, as the kernel's ability to support some rudimentary line-editing features on an interactive terminal long predates libraries like readline. I think readline might actually disable kernel interpretation of some of these control characters when it starts accepting input, and then re-enable them when it stops, but I've never looked into that.)

formerly_proven 4y ago
> I think readline might actually disable kernel interpretation of some of these control characters when it starts accepting input, and then re-enable them when it stops, but I've never looked into that. That's the canonical mode bit (ICANON). Canonical mode means the kernel/terminal is line-buffered and line-editing is handled by it; non-canonical mode means user input is pushed to the application immediately.
tonnydourado 4y ago
> For example, if you run > stty intr '^X' > then your interrupt character sequence will be Ctrl+X instead of Ctrl+C (!). I have a feeling that if you actually do this in a real Linux system, a sysadmin will hunt you down and dismember you alive, but I might be wrong.
- deadbunny 4y ago
  Can confirm.
- jwilk 4y ago
  Fun fact: it doesn't have to be Ctrl+something. If you feel paricularly evil today, you might try this: stty intr y
  - psydvl 4y ago
    Is it fun enough to be 1st April joke?
  - Brian_K_White 4y ago
    It can be any single byte, except probably not 0x00. It cannot be a multi-byte sequence, which means it cannot be any keyboard key. On SCO systems intr/break is the Del key, which on a scoansi terminal emits ^? (I don't remember the ascii value just that ctrl-? is another way to produce it) So to break out of programs is the Del key instead of Ctrl-C. But on a vtxx terminal like the linux console or xterm, even if you were perverse enough to want to, you can not assign break/intr to the Del key like that, because on a vtxx terminal the Del key emits a multi byte escape sequence, not a single byte. hot keys like ctrl-c require multiple fingers, but what's produced is a single byte. (you can actually modify both the console and xterm to change what a key emits, but then the resulting terminal no longer matches the definition of a linux or xterm terminal)
    - jwilk 4y ago
      ^? = 0x7F
    - jwilk 4y ago
      > It can be any single byte, except probably not 0x00. At least on Linux, indeed you can't use 0x00 (aka ^@), because _POSIX_VDISABLE (the thing you use for disabling special characters) is 0.
    - kps 4y ago
dylan604 4y ago
The timing of HN can be spooky sometimes. Earlier this week I was just wondering it I could switch the ctrl-c interrupt to a single key for easier keyboard smashing to stop an errant poorly thought out script. My choice would have been the spacebar as that is second nature muscle memory from all of my days in an edit bay where the spacebar acted as the All Stop and was engraved with the phrase Awww Shit! as that was the typical utterance from the editor just before using the key
dundarious 4y ago
Anyone interested in the machinations of all of this terminal stuff should look at antirez’ kilo, a terminal text editor in under 1000 lines of code: https://github.com/antirez/kilo https://github.com/antirez/kilo There is a nice tutorial that walks through how one might write it from scratch: https://viewsourcecode.org/snaptoken/kilo/ https://viewsourcecode.org/snaptoken/kilo/
account42 4y ago
stty is useful when an interactive program died before it had a chance to restore the terminal mode so you end up with nothing displayed when you type - `stty sane` will fix that up.

by schoen 4y ago

formerly_proven 4y ago

> I think readline might actually disable kernel interpretation of some of these control characters when it starts accepting input, and then re-enable them when it stops, but I've never looked into that. That's the canonical mode bit (ICANON). Canonical mode means the kernel/terminal is line-buffered and line-editing is handled by it; non-canonical mode means user input is pushed to the application immediately.

tonnydourado 4y ago

> For example, if you run > stty intr '^X' > then your interrupt character sequence will be Ctrl+X instead of Ctrl+C (!). I have a feeling that if you actually do this in a real Linux system, a sysadmin will hunt you down and dismember you alive, but I might be wrong.

deadbunny 4y ago
Can confirm.
jwilk 4y ago
Fun fact: it doesn't have to be Ctrl+something. If you feel paricularly evil today, you might try this: stty intr y
- psydvl 4y ago
  Is it fun enough to be 1st April joke?
- Brian_K_White 4y ago
  It can be any single byte, except probably not 0x00. It cannot be a multi-byte sequence, which means it cannot be any keyboard key. On SCO systems intr/break is the Del key, which on a scoansi terminal emits ^? (I don't remember the ascii value just that ctrl-? is another way to produce it) So to break out of programs is the Del key instead of Ctrl-C. But on a vtxx terminal like the linux console or xterm, even if you were perverse enough to want to, you can not assign break/intr to the Del key like that, because on a vtxx terminal the Del key emits a multi byte escape sequence, not a single byte. hot keys like ctrl-c require multiple fingers, but what's produced is a single byte. (you can actually modify both the console and xterm to change what a key emits, but then the resulting terminal no longer matches the definition of a linux or xterm terminal)
  - jwilk 4y ago
    ^? = 0x7F
  - jwilk 4y ago
    > It can be any single byte, except probably not 0x00. At least on Linux, indeed you can't use 0x00 (aka ^@), because _POSIX_VDISABLE (the thing you use for disabling special characters) is 0.
  - kps 4y ago

dylan604 4y ago

The timing of HN can be spooky sometimes. Earlier this week I was just wondering it I could switch the ctrl-c interrupt to a single key for easier keyboard smashing to stop an errant poorly thought out script. My choice would have been the spacebar as that is second nature muscle memory from all of my days in an edit bay where the spacebar acted as the All Stop and was engraved with the phrase Awww Shit! as that was the typical utterance from the editor just before using the key

dundarious 4y ago

Anyone interested in the machinations of all of this terminal stuff should look at antirez’ kilo, a terminal text editor in under 1000 lines of code: https://github.com/antirez/kilo https://github.com/antirez/kilo There is a nice tutorial that walks through how one might write it from scratch: https://viewsourcecode.org/snaptoken/kilo/ https://viewsourcecode.org/snaptoken/kilo/

account42 4y ago

stty is useful when an interactive program died before it had a chance to restore the terminal mode so you end up with nothing displayed when you type - `stty sane` will fix that up.