Document sign extension of si_code by the kernel.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@5123

diff --git a/coregrind/m_signals.c b/coregrind/m_signals.c
index f11269bee46f5d157cf11de3b790012c6db436df..438262dde73f46f8ffdde3abfe0cb00379b59229 100644 (file)
--- a/coregrind/m_signals.c
+++ b/coregrind/m_signals.c
@@ -1358,7 +1358,11 @@ void async_signalhandler ( Int sigNo, vki_siginfo_t *info, struct vki_ucontext *
    /* The linux kernel uses the top 16 bits of si_code for it's own
       use and only exports the bottom 16 bits to user space - at least
       that is the theory, but it turns out that there are some kernels
-      around that forget to mask out the top 16 bits so we do it here. */
+      around that forget to mask out the top 16 bits so we do it here.
+
+      The kernel treats the bottom 16 bits as signed and (when it does
+      mask them off) sign extends them when exporting to user space so
+      we do the same thing here. */
    info->si_code = (Short)info->si_code;
 #endif
 
@@ -1475,7 +1479,11 @@ void sync_signalhandler ( Int sigNo, vki_siginfo_t *info, struct vki_ucontext *u
    /* The linux kernel uses the top 16 bits of si_code for it's own
       use and only exports the bottom 16 bits to user space - at least
       that is the theory, but it turns out that there are some kernels
-      around that forget to mask out the top 16 bits so we do it here. */
+      around that forget to mask out the top 16 bits so we do it here.
+
+      The kernel treats the bottom 16 bits as signed and (when it does
+      mask them off) sign extends them when exporting to user space so
+      we do the same thing here. */
    info->si_code = (Short)info->si_code;
 #endif