libc: add wait4()

wait4() is BSD/Linux-standard (used by toybox's "time" applet) but NuttX
only had waitpid()+getrusage() separately. Add it to libs/libc/unistd/
built on top of those two existing primitives, so it needs no syscall
plumbing of its own and works unmodified across flat/protected/kernel
build separation. Prototype added to include/sys/wait.h.

Signed-off-by: Alan C. Assis <acassis@gmail.com>
Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>

include/sys/wait.h

@@ -96,6 +96,15 @@ pid_t wait(FAR int *stat_loc);
 int   waitid(idtype_t idtype, id_t id, FAR siginfo_t *info, int options);
 pid_t waitpid(pid_t pid, FAR int *stat_loc, int options);
 
+/* wait4() is waitpid() plus the reaped child's struct rusage in one call.
+ * struct rusage is forward-declared here (not #include <sys/resource.h>)
+ * since it is only ever used as a pointer type in this prototype.
+ */
+
+struct rusage;
+pid_t wait4(pid_t pid, FAR int *stat_loc, int options,
+            FAR struct rusage *rusage);
+
 #undef EXTERN
 #if defined(__cplusplus)
 }

libs/libc/unistd/CMakeLists.txt

@@ -73,6 +73,10 @@ set(SRCS
     lib_confstr.c
     lib_ulimit.c)
 
+if(CONFIG_SCHED_WAITPID)
+  list(APPEND SRCS lib_wait4.c)
+endif()
+
 if(NOT CONFIG_SCHED_USER_IDENTITY)
   list(
     APPEND

libs/libc/unistd/Make.defs

@@ -36,6 +36,10 @@ CSRCS += lib_getsid.c lib_setpgid.c lib_setsid.c
 CSRCS += lib_lockf.c lib_flock.c lib_getpass.c
 CSRCS += lib_chdir.c lib_fchdir.c lib_confstr.c lib_ulimit.c
 
+ifeq ($(CONFIG_SCHED_WAITPID),y)
+CSRCS += lib_wait4.c
+endif
+
 ifneq ($(CONFIG_SCHED_USER_IDENTITY),y)
 CSRCS += lib_setuid.c lib_setgid.c lib_getuid.c lib_getgid.c
 CSRCS += lib_seteuid.c lib_setegid.c lib_geteuid.c lib_getegid.c

libs/libc/unistd/lib_wait4.c

@@ -0,0 +1,64 @@
+/****************************************************************************
+ * libs/libc/unistd/lib_wait4.c
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include <sys/wait.h>
+#include <sys/resource.h>
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: wait4
+ *
+ * Description:
+ *   wait4() is waitpid() plus the reaped child's resource usage in one
+ *   call. Built entirely on top of the existing waitpid()/getrusage()
+ *   primitives (each already correctly proxied across build separation --
+ *   waitpid() is a real syscall, getrusage() is plain libc), so this needs
+ *   no syscall plumbing of its own and works unmodified in flat,
+ *   protected, and kernel builds alike.
+ *
+ *   NuttX's getrusage() only supports RUSAGE_SELF/RUSAGE_CHILDREN (not a
+ *   specific pid), so the returned usage is exact only when the caller has
+ *   a single outstanding child at a time.
+ *
+ ****************************************************************************/
+
+pid_t wait4(pid_t pid, FAR int *stat_loc, int options,
+            FAR struct rusage *rusage)
+{
+  pid_t ret = waitpid(pid, stat_loc, options);
+
+  if (ret > 0 && rusage != NULL)
+    {
+      getrusage(RUSAGE_CHILDREN, rusage);
+    }
+
+  return ret;
+}