feat: process exit codes, posix_spawn, 4 KiB command lines

Groundwork for the GCC driver: a compiler driver must spawn cc1/as/ld
and know whether each stage succeeded.

Kernel: the scheduler keeps an exit-code ledger (pid -> code; pids are
monotonic so entries never alias), published during teardown right
before waiters wake. SYS_EXIT records main()'s return value, SYS_KILL
records 256+SIGKILL, and the exception handler records 256+signal
mapped from the fault vector (#PF/#GP -> SIGSEGV, #DE/FP -> SIGFPE,
#UD -> SIGILL). SYS_WAITPID now returns the code: 0..255 for a normal
exit, 256+signal for a violent death. Process args grow from 256
bytes to 4 KiB (cc1 invocations do not fit in 256), with crt1 now
parsing up to 255 argv entries from a static buffer.

libc: new spawn.h with posix_spawn/posix_spawnp over SYS_SPAWN -
libiberty's pex layer has a posix_spawn backend, so GCC's driver works
without fork. argv is joined into the kernel args string (spaces in
arguments rejected; no kernel quoting), envp is not transferred, and
non-empty file actions fail loudly with ENOTSUP until the kernel can
redirect stdio on spawn. waitpid() now decodes real POSIX status and
the sys/wait.h macros distinguish exited from signaled children.

Shell: prints [exit code N] after nonzero exits and [terminated by
signal N] for killed or crashed children.

Verified on the OS: cat on a missing file reports exit code 1; a
window-close (clean exit 0) stays silent as it should.

Co-Authored-By: Claude Fable 5 <[email protected]>

programs/include/libc/errno.h

@@ -51,6 +51,7 @@ extern int errno;
 #define EWOULDBLOCK  EAGAIN
 #define EOVERFLOW    75
 #define ETIMEDOUT    110
+#define ENOTSUP      95
 
 #ifdef __cplusplus
 }

programs/include/libc/spawn.h

@@ -0,0 +1,60 @@
+#ifndef _LIBC_SPAWN_H
+#define _LIBC_SPAWN_H
+
+#pragma once
+
+#include <sys/types.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+    * posix_spawn for MontaukOS, layered on SYS_SPAWN.
+    *
+    * Limitations (kernel spawn model):
+    *  - argv is joined into a single args string; arguments containing
+    *    spaces are rejected with EINVAL (no quoting in the kernel).
+    *  - envp is ignored (no environment transfer on spawn).
+    *  - file actions must be empty: stdio redirection needs kernel
+    *    support that does not exist yet, so any recorded action makes
+    *    posix_spawn fail with ENOTSUP rather than misbehave silently.
+*/
+
+typedef struct {
+    short flags;
+} posix_spawnattr_t;
+
+typedef struct {
+    int action_count;
+} posix_spawn_file_actions_t;
+
+int posix_spawnattr_init(posix_spawnattr_t *attr);
+int posix_spawnattr_destroy(posix_spawnattr_t *attr);
+int posix_spawnattr_setflags(posix_spawnattr_t *attr, short flags);
+int posix_spawnattr_getflags(const posix_spawnattr_t *attr, short *flags);
+
+int posix_spawn_file_actions_init(posix_spawn_file_actions_t *actions);
+int posix_spawn_file_actions_destroy(posix_spawn_file_actions_t *actions);
+int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t *actions,
+                                     int fd, int newfd);
+int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t *actions,
+                                      int fd);
+int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t *actions,
+                                     int fd, const char *path, int oflag,
+                                     mode_t mode);
+
+int posix_spawn(pid_t *pid, const char *path,
+                const posix_spawn_file_actions_t *actions,
+                const posix_spawnattr_t *attr,
+                char *const argv[], char *const envp[]);
+int posix_spawnp(pid_t *pid, const char *file,
+                 const posix_spawn_file_actions_t *actions,
+                 const posix_spawnattr_t *attr,
+                 char *const argv[], char *const envp[]);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _LIBC_SPAWN_H */

programs/include/libc/sys/wait.h

@@ -9,12 +9,12 @@
 extern "C" {
 #endif
 
-/* Status decoding. The Montauk kernel does not yet report exit codes
-   through SYS_WAITPID, so waited children always decode as exit 0. */
+/* POSIX status decoding. waitpid() encodes a normal exit as code<<8
+   and a killed/crashed child as the signal number in the low bits. */
 #define WIFEXITED(s)    (((s) & 0x7F) == 0)
 #define WEXITSTATUS(s)  (((s) >> 8) & 0xFF)
-#define WIFSIGNALED(s)  (0)
-#define WTERMSIG(s)     (0)
+#define WIFSIGNALED(s)  (((s) & 0x7F) != 0)
+#define WTERMSIG(s)     ((s) & 0x7F)
 #define WIFSTOPPED(s)   (0)
 #define WSTOPSIG(s)     (0)
 

programs/include/montauk/syscall.h

@@ -321,7 +321,9 @@ namespace montauk {
     }
 
     // Process management
-    inline void waitpid(int pid) { syscall1(montauk::abi::SYS_WAITPID, (uint64_t)pid); }
+    // Blocks until pid exits. Returns 0..255 for a normal exit,
+    // 256+signal when the process was killed or crashed.
+    inline int waitpid(int pid) { return (int)syscall1(montauk::abi::SYS_WAITPID, (uint64_t)pid); }
 
     // Framebuffer
     inline void fb_info(montauk::abi::FbInfo* info) { syscall1(montauk::abi::SYS_FBINFO, (uint64_t)info); }