Index: usr.bin/xargs/xargs.1
===================================================================
--- usr.bin/xargs/xargs.1
+++ usr.bin/xargs/xargs.1
@@ -33,7 +33,7 @@
 .\" $FreeBSD$
 .\" $xMach: xargs.1,v 1.2 2002/02/23 05:23:37 tim Exp $
 .\"
-.Dd March 16, 2012
+.Dd May 21, 2015
 .Dt XARGS 1
 .Os
 .Sh NAME
@@ -207,6 +207,11 @@
 invocations of
 .Ar utility
 at once.
+If
+.Ar maxprocs
+is negative or 0,
+.Nm
+will run as many processes as possible at a time.
 .It Fl p
 Echo each command to be executed and ask the user whether it should be
 executed.
Index: usr.bin/xargs/xargs.c
===================================================================
--- usr.bin/xargs/xargs.c
+++ usr.bin/xargs/xargs.c
@@ -48,6 +48,8 @@
 
 #include <sys/param.h>
 #include <sys/wait.h>
+#include <sys/types.h>
+#include <sys/sysctl.h>
 
 #include <err.h>
 #include <errno.h>
@@ -63,6 +65,8 @@
 
 #include "pathnames.h"
 
+#define BASE_DECIMAL	10
+
 static void	parse_input(int, char *[]);
 static void	prerun(int, char *[]);
 static int	prompt(void);
@@ -100,6 +104,7 @@
 	long arg_max;
 	int ch, Jflag, nargs, nflag, nline;
 	size_t linelen;
+	size_t proclen;
 	char *endptr;
 
 	inpline = replstr = NULL;
@@ -159,8 +164,14 @@
 			oflag = 1;
 			break;
 		case 'P':
-			if ((maxprocs = atoi(optarg)) <= 0)
-				errx(1, "max. processes must be >0");
+			maxprocs = strtol(optarg, &endptr, BASE_DECIMAL);
+			if (*endptr != '\0')
+				errx(1, "max processes must be a number");
+			if (maxprocs > nargs)
+				errx(1, "max processes must be %d or less", nargs);
+			if (maxprocs < 1)
+				proclen = sizeof(maxprocs);
+				sysctlbyname("kern.maxproc", &maxprocs, &proclen, NULL, 0);
 			break;
 		case 'p':
 			pflag = 1;