After one too many PRs on the subject, bite the bullet and define IOV_MAX

and its associated constants.  Implement _SC_IOV_MAX in the usual way.
Be a bit sloppy about the namespace question; this should get cleared up
in time for 5.0.

MFC after:	1 month

include/limits.h

@@ -62,6 +62,7 @@
 #define	_POSIX2_LINE_MAX	2048
 #define	_POSIX2_RE_DUP_MAX	255
 
+#define	_XOPEN_IOV_MAX		16
 
 #ifdef _P1003_1B_VISIBLE
 

lib/libc/gen/sysconf.c

@@ -38,6 +38,11 @@
 static char sccsid[] = "@(#)sysconf.c	8.2 (Berkeley) 3/20/94";
 #endif /* LIBC_SCCS and not lint */
 
+#if defined(LIBC_RCS) && !defined(lint)
+static const char rcsid[] =
+  "$FreeBSD$";
+#endif /* LIBC_RCS and not lint */
+
 #include <sys/_posix.h>
 #include <sys/param.h>
 #include <sys/time.h>
@@ -285,6 +290,13 @@ sysconf(name)
 		goto yesno;
 #endif /* _P1003_1B_VISIBLE */
 
+#ifdef _SC_IOV_MAX
+	case _SC_IOV_MAX:
+		mib[0] = CTL_KERN;
+		mib[1] = KERN_IOV_MAX;
+		break;
+#endif
+
 yesno:		if (sysctl(mib, 2, &value, &len, NULL, 0) == -1)
 			return (-1);
 		if (value == 0)

sys/kern/kern_subr.c

@@ -48,6 +48,7 @@
 #include <sys/proc.h>
 #include <sys/malloc.h>
 #include <sys/resourcevar.h>
+#include <sys/sysctl.h>
 #include <sys/vnode.h>
 
 #include <vm/vm.h>
@@ -56,6 +57,9 @@
 
 static void	uio_yield __P((void));
 
+SYSCTL_INT(_kern, KERN_IOV_MAX, iov_max, CTLFLAG_RD, NULL, UIO_MAXIOV, 
+	"Maximum number of elements in an I/O vector; sysconf(_SC_IOV_MAX)");
+
 int
 uiomove(cp, n, uio)
 	register caddr_t cp;

sys/sys/sysctl.h

@@ -328,7 +328,8 @@ TAILQ_HEAD(sysctl_ctx_list, sysctl_ctx_entry);
 #define	KERN_PS_STRINGS		32	/* int: address of PS_STRINGS */
 #define	KERN_USRSTACK		33	/* int: address of USRSTACK */
 #define	KERN_LOGSIGEXIT		34	/* int: do we log sigexit procs? */
-#define KERN_MAXID		35      /* number of valid kern ids */
+#define	KERN_IOV_MAX		35	/* int: value of UIO_MAXIOV */
+#define KERN_MAXID		36      /* number of valid kern ids */
 
 #define CTL_KERN_NAMES { \
 	{ 0, 0 }, \
@@ -366,6 +367,7 @@ TAILQ_HEAD(sysctl_ctx_list, sysctl_ctx_entry);
 	{ "ps_strings", CTLTYPE_INT }, \
 	{ "usrstack", CTLTYPE_INT }, \
 	{ "logsigexit", CTLTYPE_INT }, \
+	{ "iov_max", CTLTYPE_INT }, \
 }
 
 /*

sys/sys/syslimits.h

@@ -51,6 +51,7 @@
 #endif
 #define	PATH_MAX		 1024	/* max bytes in pathname */
 #define	PIPE_BUF		  512	/* max bytes for atomic pipe writes */
+#define	IOV_MAX			 1024	/* max elements in i/o vector */
 
 #define	BC_BASE_MAX		   99	/* max ibase/obase values in bc(1) */
 #define	BC_DIM_MAX		 2048	/* max array elements in bc(1) */

sys/sys/uio.h

@@ -70,6 +70,14 @@ struct uio {
 
 /*
  * Limits
+ *
+ * N.B.: UIO_MAXIOV must be no less than IOV_MAX from <sys/syslimits.h>
+ * which in turn must be no less than _XOPEN_IOV_MAX from <limits.h>.  If
+ * we ever make this tunable (probably pointless), then IOV_MAX should be
+ * removed from <sys/syslimits.h> and applications would be expected to use
+ * sysconf(3) to find out the correct value, or else assume the worst
+ * (_XOPEN_IOV_MAX).  Perhaps UIO_MAXIOV should be simply defined as
+ * IOV_MAX.
  */
 #define UIO_MAXIOV	1024		/* max 1K of iov's */
 #define UIO_SMALLIOV	8		/* 8 on stack, else malloc */

sys/sys/unistd.h

@@ -198,6 +198,8 @@
 
 #endif /* _P1003_1B_VISIBLE */
 
+#define	_SC_IOV_MAX		56
+
 #ifndef _POSIX_SOURCE
 /*
  * rfork() options.