Index: xargs.1
===================================================================
--- xargs.1	(revision 283275)
+++ xargs.1	(working copy)
@@ -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: xargs.c
===================================================================
--- xargs.c	(revision 283275)
+++ xargs.c	(working copy)
@@ -46,8 +46,10 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#include <sys/param.h>
 #include <sys/wait.h>
+#include <sys/types.h>
+#include <sys/time.h>
+#include <sys/resource.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 +104,8 @@
 	long arg_max;
 	int ch, Jflag, nargs, nflag, nline;
 	size_t linelen;
+	struct rlimit reslimit;
+	int proclim;
 	char *endptr;
 
 	inpline = replstr = NULL;
@@ -106,6 +112,7 @@
 	ep = environ;
 	eofstr = "";
 	Jflag = nflag = 0;
+	proclim = 1;
 
 	(void)setlocale(LC_ALL, "");
 
@@ -159,8 +166,15 @@
 			oflag = 1;
 			break;
 		case 'P':
-			if ((maxprocs = atoi(optarg)) <= 0)
-				errx(1, "max. processes must be >0");
+			if (getrlimit(RLIMIT_NPROC , &reslimit) == 0)
+				proclim = reslimit.rlim_cur;
+			maxprocs = strtol(optarg, &endptr, BASE_DECIMAL);
+			if (*endptr != '\0')
+				errx(1, "max processes must be a number");
+			if (maxprocs > proclim)
+				errx(1, "max processes must be less than RLIMIT_NPROC: %d", proclim);
+			if (maxprocs < 1)
+				maxprocs = proclim;
 			break;
 		case 'p':
 			pflag = 1;