feat: libc - add SYS_GETCHAR_NB and a non-blocking stdin readiness probe

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NQRTGoYgnZQsh7uCh6Jsxm

programs/include/Api/Syscall.hpp

@@ -228,6 +228,7 @@ namespace montauk::abi {
     static constexpr uint64_t SYS_WIFI_RESULTS        = 163;  // (WifiNetwork*, maxCount) -> count, no radio work
     static constexpr uint64_t SYS_WIFI_CONNECT_ASYNC  = 164;  // (ssid, password) -> 0 accepted, <0 on error
     static constexpr uint64_t SYS_NETIFS              = 165;  // (NetIfInfo*, maxCount) -> count
+    static constexpr uint64_t SYS_GETCHAR_NB          = 166;  // () -> ascii, 0 if nothing pending; never blocks
 
     // Tunable parameters (for SYS_SDR_SETPARAM / SYS_SDR_GETPARAM).
     static constexpr int SDR_PARAM_FREQ        = 0;  // center frequency, Hz

programs/include/libc/montauk.h

@@ -189,6 +189,7 @@ extern "C" {
 #define MTK_SYS_WIFI_RESULTS       163
 #define MTK_SYS_WIFI_CONNECT_ASYNC 164
 #define MTK_SYS_NETIFS             165
+#define MTK_SYS_GETCHAR_NB         166
 /* @SYSCALLS-END */
 
 #define MTK_SOCK_TCP 1

programs/include/libc/stdio.h

@@ -95,6 +95,17 @@ int    ungetc(int c, FILE *stream);
 char  *fgets(char *s, int size, FILE *stream);
 int    fputs(const char *s, FILE *stream);
 
+/* MontaukOS extension: non-blocking readiness probe for stdin.
+   Returns 1 when a complete line is buffered, so a following fgets/fgetc on
+   stdin returns without blocking; 0 otherwise. Partially typed input is kept.
+   This is what a select()-style poll over stdin has to be built on -- stdio
+   descriptors are not waitable kernel objects. */
+int    montauk_stdin_ready(void);
+
+/* Blocks until a complete line is buffered on stdin, then returns 1. Same
+   buffer as montauk_stdin_ready(); nothing is consumed either way. */
+int    montauk_stdin_wait(void);
+
 void   perror(const char *s);
 FILE  *tmpfile(void);
 char  *tmpnam(char *s);

programs/include/montauk/syscall.h

@@ -197,6 +197,8 @@ namespace montauk {
     inline bool is_key_available() { return (bool)syscall0(montauk::abi::SYS_ISKEYAVAILABLE); }
     inline void getkey(montauk::abi::KeyEvent* out) { syscall1(montauk::abi::SYS_GETKEY, (uint64_t)out); }
     inline char getchar() { return (char)syscall0(montauk::abi::SYS_GETCHAR); }
+    // Returns 0 rather than blocking when no character is pending.
+    inline char getchar_nb() { return (char)syscall0(montauk::abi::SYS_GETCHAR_NB); }
     inline uint64_t input_wait(uint64_t observedSerial, uint64_t timeoutMs) {
         return (uint64_t)syscall2(montauk::abi::SYS_INPUT_WAIT, observedSerial, timeoutMs);
     }

programs/lib/libc/libc.c

@@ -101,6 +101,7 @@ static inline long _zos_syscall4(long nr, long a1, long a2, long a3, long a4) {
 #define SYS_FREE    12
 #define SYS_GETMILLISECONDS 14
 #define SYS_GETCHAR 18
+#define SYS_GETCHAR_NB 166
 #define SYS_SPAWN   20
 #define SYS_WAITPID 23
 #define SYS_GETARGS 25
@@ -1478,9 +1479,23 @@ FILE *stdin  = &_stdin_file;
 FILE *stdout = &_stdout_file;
 FILE *stderr = &_stderr_file;
 
+/* ------------------------------------------------------------------------
+   stdin line editing.
+
+   Input is gathered a line at a time so backspace can still take a character
+   back. The gather is incremental: _stdin_pump() may be called without
+   blocking, in which case a half-typed line simply stays in the buffer until
+   the next call. That is what lets montauk_stdin_ready() answer "is a line
+   available?" without stealing keystrokes -- see the select() shim in the
+   NetSurf port, which is the reason this is not just a blocking read.
+
+   _stdin_line_ready distinguishes "buffer holds a complete line" from "buffer
+   holds a partial one"; readers must never look at the buffer without it.
+   ------------------------------------------------------------------------ */
 static char _stdin_linebuf[512];
 static size_t _stdin_line_len = 0;
 static size_t _stdin_line_pos = 0;
+static int _stdin_line_ready = 0;
 
 static void _stdin_echo_char(char c) {
     if (c == '\b') {
@@ -1492,13 +1507,25 @@ static void _stdin_echo_char(char c) {
     _zos_syscall1(SYS_PUTCHAR, (unsigned char)c);
 }
 
-static int _stdin_fill_line(void) {
-    _stdin_line_len = 0;
-    _stdin_line_pos = 0;
+/* Gather keystrokes into _stdin_linebuf. Returns 1 when a complete line is
+   buffered. With block == 0, returns 0 as soon as input runs dry, keeping
+   whatever has been typed so far for the next call. */
+static int _stdin_pump(int block) {
+    if (_stdin_line_ready) {
+        if (_stdin_line_pos < _stdin_line_len) return 1;
+        /* Previous line fully read; start gathering the next one. */
+        _stdin_line_ready = 0;
+        _stdin_line_len = 0;
+        _stdin_line_pos = 0;
+    }
 
-    while (_stdin_line_len + 1 < sizeof(_stdin_linebuf)) {
-        int c = (int)_zos_syscall0(SYS_GETCHAR);
-        if (c <= 0) continue;
+    for (;;) {
+        int c = block ? (int)_zos_syscall0(SYS_GETCHAR)
+                      : (int)_zos_syscall0(SYS_GETCHAR_NB);
+        if (c <= 0) {
+            if (block) continue;
+            return 0;
+        }
 
         if (c == '\r') c = '\n';
 
@@ -1513,6 +1540,7 @@ static int _stdin_fill_line(void) {
         if (c == '\n') {
             _stdin_linebuf[_stdin_line_len++] = (char)c;
             _stdin_echo_char((char)c);
+            _stdin_line_ready = 1;
             return 1;
         }
 
@@ -1520,15 +1548,27 @@ static int _stdin_fill_line(void) {
             _stdin_linebuf[_stdin_line_len++] = (char)c;
             _stdin_echo_char((char)c);
         }
-    }
 
-    if (_stdin_line_len > 0) {
-        _stdin_linebuf[_stdin_line_len++] = '\n';
-        _stdin_echo_char('\n');
-        return 1;
+        /* Full buffer: break the line here rather than dropping input. */
+        if (_stdin_line_len + 1 >= sizeof(_stdin_linebuf)) {
+            _stdin_linebuf[_stdin_line_len++] = '\n';
+            _stdin_echo_char('\n');
+            _stdin_line_ready = 1;
+            return 1;
+        }
     }
+}
 
-    return 0;
+static int _stdin_fill_line(void) {
+    return _stdin_pump(1);
+}
+
+int montauk_stdin_ready(void) {
+    return _stdin_pump(0);
+}
+
+int montauk_stdin_wait(void) {
+    return _stdin_pump(1);
 }
 
 FILE *fopen(const char *path, const char *mode) {
@@ -1742,7 +1782,7 @@ int fgetc(FILE *stream) {
     }
 
     if (stream->is_std == 1) {
-        if (_stdin_line_pos >= _stdin_line_len) {
+        if (!_stdin_line_ready || _stdin_line_pos >= _stdin_line_len) {
             if (!_stdin_fill_line()) {
                 stream->eof = 1;
                 return EOF;

programs/man/syscalls.2

@@ -233,6 +233,12 @@
     Block until a printable character is typed.
         char montauk::getchar();
 
+.B SYS_GETCHAR_NB (166)
+    Return the next printable character, or 0 when none is
+    pending. Never blocks; key events carrying no ascii are
+    drained rather than left to stall a following getchar.
+        char montauk::getchar_nb();
+
 .B SYS_INPUT_WAIT (123)
     Block until the input serial number differs from
     observedSerial or the timeout elapses; used to sleep