1 files changed, 69 insertions, 2 deletions
diff --git a/nuttx/configs/README.txt b/nuttx/configs/README.txt
index babdf7330..1b78567a3 100644
--- a/nuttx/configs/README.txt
+++ b/nuttx/configs/README.txt
@@ -334,12 +334,79 @@ defconfig -- This is a configuration file similar to the Linux
     CONFIG_TASK_NAME_SIZE - Specifies that maximum size of a
       task name to save in the TCB.  Useful if scheduler
       instrumentation is selected.  Set to zero to disable.
-    CONFIG_SCHED_HAVE_PARENT - Remember the ID of the parent thread
-      when a new child thread is created.  This support enables some
+    CONFIG_SCHED_HAVE_PARENT - Remember the ID of the parent task
+      when a new child task is created.  This support enables some
       additional features (such as SIGCHLD) and modifies the behavior
       of other interfaces.  For example, it makes waitpid() more
       standards complete by restricting the waited-for tasks to the
       children of the caller. Default: disabled.
+    CONFIG_SCHED_CHILD_STATUS
+      If this option is selected, then the exit status of the child task
+      will be retained after the child task exits.  This option should be
+      selected if you require knowledge of a child process' exit status.
+      Without this setting, wait(), waitpid() or waitid() may fail.  For
+      example, if you do:
+
+        1) Start child task
+        2) Wait for exit status (using wait(), waitpid(), or waitid()).
+
+      This can fail because the child task may run to completion before
+      the wait begins.  There is a non-standard work-around in this case:
+      The above sequence will work if you disable pre-emption using
+      sched_lock() prior to starting the child task, then re-enable pre-
+      emption with sched_unlock() after the wait completes.  This works
+      because the child task is not permitted to run until the wait is in
+      place.
+
+      The standard solution would be to enable CONFIG_SCHED_CHILD_STATUS.  In
+      this case the exit status of the child task is retained after the
+      child exits and the wait will successful obtain the child task's
+      exit status whether it is called before the child task exits or not.
+
+      Warning:  If you enable this feature, then your application must
+      either (1) take responsibility for reaping the child status with wait(),
+      waitpid(), or waitid(), or (2) suppress retention of child status.
+      If you do not reap the child status, then you have a memory leak and
+      your system will eventually fail.
+
+      Retention of child status can be suppressed on the parent using logic like:
+
+        struct sigaction sa;
+
+        sa.sa_handler = SIG_IGN;
+        sa.sa_flags = SA_NOCLDWAIT;
+        int ret = sigaction(SIGCHLD, &sa, NULL);
+
+    CONFIG_PREALLOC_CHILDSTATUS
+      To prevent runaway child status allocations and to improve
+      allocation performance, child task exit status structures are pre-
+      allocated when the system boots.  This setting determines the number
+      of child status structures that will be pre-allocated.  If this
+      setting is not defined or if it is defined to be zero then a value
+      of 2*MAX_TASKS is used.
+
+      Note that there cannot be more that CONFIG_MAX_TASKS tasks in total.
+      However, the number of child status structures may need to be
+      significantly larger because this number includes the maximum number
+      of tasks that are running PLUS the number of tasks that have exit'ed
+      without having their exit status reaped (via wait(), waitid(), or
+      waitpid()).
+
+      Obviously, if tasks spawn children indefinitely and never have the
+      exit status reaped, then you may have a memory leak!  If you enable
+      the SCHED_CHILD_STATUS feature, then your application must take
+      responsibility for either (1) reaping the child status with wait(),
+      waitpid(), or waitid() or it must (2) suppress retention of child
+      status.  Otherwise, your system will eventually fail.
+
+      Retention of child status can be suppressed on the parent using logic like:
+
+        struct sigaction sa;
+
+        sa.sa_handler = SIG_IGN;
+        sa.sa_flags = SA_NOCLDWAIT;
+        int ret = sigaction(SIGCHLD, &sa, NULL);
+
     CONFIG_START_YEAR, CONFIG_START_MONTH, CONFIG_START_DAY -
       Used to initialize the internal time logic.
     CONFIG_GREGORIAN_TIME - Enables Gregorian time conversions.