Implement POSIX.1b shared memory objects. In this implementation,

shared memory objects are regular files; the shm_open(3) routine
uses fcntl(2) to set a flag on the descriptor which tells mmap(2)
to automatically apply MAP_NOSYNC.

Not objected to by: bde, dillon, dufault, jasone

1 files changed, 18 insertions, 2 deletions

diff --git a/sys/sys/fcntl.h b/sys/sys/fcntl.h
index d5b29e6..6bbf156 100644
--- a/sys/sys/fcntl.h
+++ b/sys/sys/fcntl.h
@@ -106,12 +106,12 @@
 /* bits to save after open */
 #define	FMASK		(FREAD|FWRITE|FAPPEND|FASYNC|FFSYNC|FNONBLOCK)
 /* bits settable by fcntl(F_SETFL, ...) */
-#define	FCNTLFLAGS	(FAPPEND|FASYNC|FFSYNC|FNONBLOCK)
+#define	FCNTLFLAGS	(FAPPEND|FASYNC|FFSYNC|FNONBLOCK|FPOSIXSHM)
 #endif
 
 /*
  * The O_* flags used to have only F* names, which were used in the kernel
- * and by fcntl.  We retain the F* names for the kernel f_flags field
+ * and by fcntl.  We retain the F* names for the kernel f_flag field
  * and for backward compatibility for fcntl.
  */
 #ifndef _POSIX_SOURCE
@@ -124,6 +124,22 @@
 #endif
 
 /*
+ * We are out of bits in f_flag (which is a short).  However,
+ * the flag bits not set in FMASK are only meaningful in the
+ * initial open syscall.  Those bits can thus be given a
+ * different meaning for fcntl(2).
+ */
+#ifndef _POSIX_SOURCE
+
+/*
+ * Set by shm_open(3) to get automatic MAP_ASYNC behavior
+ * for POSIX shared memory objects (which are otherwise
+ * implemented as plain files).
+ */
+#define	FPOSIXSHM	O_NOFOLLOW
+#endif
+
+/*
  * Constants used for fcntl(2)
  */