Pseudo-terminals, and what tmux and htop needed around them

- /dev/ptmx and /dev/pts/N: Unix98 pairs with a full line discipline,
  window sizes, job control and hangup (kernel/dev/pty.c).
- The console speaks scroll regions, insert/delete line, SU/SD, RI/IND/NEL,
  DECSC/DECRC; swallows charset designations and private CSI sequences;
  and LF only returns the carriage under ONLCR.
- setsid() drops the controlling terminal, so a pane's pty becomes its ctty.
- pipe2() honours O_NONBLOCK and O_CLOEXEC (libevent's signal pipe).
- File modes are stored and enforced: chmod, fchmod(at), umask, creation
  modes; /tmp is 1777.
- /proc/stat has real user/system/idle; /proc/<pid>/task; kthread flag.
- SO_PEERCRED, IOV_MAX 1024.
- /proc/strace: name a program and its syscalls go to the kernel log.
- World gains htop, btop, tmux, mc, ncdu, fastfetch, games, X toys, NetSurf.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/fs/devfs.c

@@ -38,6 +38,7 @@
 #include "fs/file.h"
 #include "uapi/errno.h"
 #include "fs/blkdev.h"
+#include "dev/pty.h"
 
 // Inode numbering. Two directories and a flat table of nodes, so the number is
 // either a directory id or an index into devnodes[] offset past them.
@@ -49,6 +50,12 @@
 // device number, and inode DEVFS_BLK0 plus that number. Not in devnodes[],
 // because they come and go: a stick plugged in appears, pulled out vanishes.
 #define DEVFS_BLK0     4096ULL
+// Pseudo-terminals: the /dev/pts directory, its slaves by pair number, and the
+// masters, which have inodes so that an open file can hold one but appear in no
+// directory -- the only way to one is opening /dev/ptmx.
+#define DEVFS_PTS      3ULL
+#define DEVFS_PTS0     8192ULL
+#define DEVFS_PTM0     12288ULL
 
 // Which directory a node lives in. Only two exist and a third would want a real
 // tree rather than a flag, which is the point at which this file should grow
@@ -110,6 +117,8 @@ static const struct devnode devnodes[] = {
   // Audio, spelled the way UNIX spells it. See include/dev/audio.h.
   { "dsp",     12, 0, D_ROOT },
   { "mouse",   3, 1,  D_ROOT },
+  // Opening it makes a new pseudo-terminal pair; see kernel/dev/pty.c.
+  { "ptmx",    PTMX_MAJOR, 0, D_ROOT },
 
   // Raw disks. Created whether or not a drive is present: opening one with no
   // disk behind it fails with ENXIO, which is a better answer than a missing
@@ -143,9 +152,22 @@ devfs_iload(struct inode *ip)
   ip->major = ip->minor = 0;
   ip->size  = 0;
 
-  if (ip->inum == DEVFS_ROOT || ip->inum == DEVFS_INPUT) {
+  if (ip->inum == DEVFS_ROOT || ip->inum == DEVFS_INPUT ||
+      ip->inum == DEVFS_PTS) {
     ip->type  = T_DIR;
     ip->nlink = 2;
+  } else if (ip->inum >= DEVFS_PTS0 && ip->inum < DEVFS_PTS0 + NPTY &&
+             pty_used((int)(ip->inum - DEVFS_PTS0))) {
+    ip->type  = T_DEV;
+    ip->nlink = 1;
+    ip->major = PTS_MAJOR;
+    ip->minor = (short)(ip->inum - DEVFS_PTS0);
+    ip->mode  = 0620;
+  } else if (ip->inum >= DEVFS_PTM0 && ip->inum < DEVFS_PTM0 + NPTY) {
+    ip->type  = T_DEV;
+    ip->nlink = 1;
+    ip->major = PTM_MAJOR;
+    ip->minor = (short)(ip->inum - DEVFS_PTM0);
   } else if (i >= 0) {
     ip->type  = T_DEV;
     ip->nlink = 1;
@@ -196,6 +218,32 @@ devfs_dirlookup(struct inode *dp, char *name, uint *poff)
 
   if (dp->inum == DEVFS_ROOT && streq(name, "input"))
     return iget(DEVFS_DEV, DEVFS_INPUT);
+  if (dp->inum == DEVFS_ROOT && streq(name, "pts"))
+    return iget(DEVFS_DEV, DEVFS_PTS);
+
+  // /dev/pts/N: a pair that exists.
+  if (dp->inum == DEVFS_PTS) {
+    int n = 0;
+    const char *s = name;
+    if (!*s)
+      return 0;
+    for (; *s; s++) {
+      if (*s < '0' || *s > '9')
+        return 0;
+      n = n * 10 + (*s - '0');
+      if (n >= NPTY)
+        return 0;
+    }
+    return pty_used(n) ? iget(DEVFS_DEV, DEVFS_PTS0 + (uint64)n) : 0;
+  }
+
+  // /dev/tty is the caller's controlling terminal. When that is a pty, it is
+  // the slave itself -- what a program inside xterm or tmux opens to talk to
+  // its terminal must be that terminal, not the console.
+  if (dp->inum == DEVFS_ROOT && streq(name, "tty") && myproc() &&
+      myproc()->ctty >= PTY_CTTY && myproc()->ctty < PTY_CTTY + NPTY &&
+      pty_used(myproc()->ctty - PTY_CTTY))
+    return iget(DEVFS_DEV, DEVFS_PTS0 + (uint64)(myproc()->ctty - PTY_CTTY));
 
   short want = dp->inum == DEVFS_INPUT ? D_INPUT : D_ROOT;
   for (int i = 0; i < NDEVNODES; i++)
@@ -258,7 +306,8 @@ devfs_getdents(struct inode *dp, char *out, int cap, uint64 *poff)
   int used = 0;
   uint64 n = *poff;
 
-  if (dp->inum != DEVFS_ROOT && dp->inum != DEVFS_INPUT)
+  if (dp->inum != DEVFS_ROOT && dp->inum != DEVFS_INPUT &&
+      dp->inum != DEVFS_PTS)
     return -1;
 
   if (n == 0) {
@@ -271,15 +320,36 @@ devfs_getdents(struct inode *dp, char *out, int cap, uint64 *poff)
     n++;
   }
 
-  // /dev/input is a directory entry of the root, and it is emitted between the
-  // dots and the nodes so that its ordinal is stable.
+  // /dev/pts: the dots, then the pairs that exist. Ordinal 2 + N is pair N,
+  // whether or not the ones before it exist, so a pair opened between two
+  // calls does not shift the rest.
+  if (dp->inum == DEVFS_PTS) {
+    for (; n >= 2 && n < 2 + NPTY; n++) {
+      int k = (int)(n - 2);
+      char nm[4];
+      if (!pty_used(k))
+        continue;
+      nm[0] = k >= 10 ? '0' + k / 10 : '0' + k;
+      nm[1] = k >= 10 ? '0' + k % 10 : 0;
+      nm[2] = 0;
+      if (emit(out, cap, &used, DEVFS_PTS0 + (uint64)k, 2, nm) < 0) goto out;
+    }
+    goto out;
+  }
+
+  // /dev/input and /dev/pts are directory entries of the root, emitted
+  // between the dots and the nodes so that their ordinals are stable.
   if (dp->inum == DEVFS_ROOT && n == 2) {
     if (emit(out, cap, &used, DEVFS_INPUT, 4, "input") < 0) goto out;
     n++;
   }
+  if (dp->inum == DEVFS_ROOT && n == 3) {
+    if (emit(out, cap, &used, DEVFS_PTS, 4, "pts") < 0) goto out;
+    n++;
+  }
 
   short want = dp->inum == DEVFS_INPUT ? D_INPUT : D_ROOT;
-  uint64 base = dp->inum == DEVFS_ROOT ? 3 : 2;
+  uint64 base = dp->inum == DEVFS_ROOT ? 4 : 2;
 
   for (int i = 0; i < NDEVNODES; i++) {
     if (devnodes[i].dir != want)
@@ -339,3 +409,12 @@ devfs_mount(const char *target)
   iput(dp);
   return r < 0 ? -EBUSY : 0;
 }
+
+// The inode of pty master N, for ptmx_open() to hand to the file it opens.
+struct inode *
+devfs_pty_master(int n)
+{
+  if (n < 0 || n >= NPTY)
+    return 0;
+  return iget(DEVFS_DEV, DEVFS_PTM0 + (uint64)n);
+}