- Check and report write(2) errors.

- Issue a single writev(2) call instead of multiple write(2)s.
  This change improves the inefficiencies introduced when echo
  went on an stdio diet.

The following figures are for echoing 1000 arguments.
original stdio-based echo:
        0.01 real         0.01 user         0.00 sys
before:
        0.05 real         0.00 user         0.04 sys
after:
        0.01 real         0.00 user         0.00 sys

bin/echo/echo.c

@@ -45,23 +45,56 @@ static char sccsid[] = "@(#)echo.c	8.1 (Berkeley) 5/31/93";
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#include <unistd.h>
-#include <string.h>
+#include <sys/types.h>
+#include <sys/uio.h>
 
-/* ARGSUSED */
+#include <assert.h>
+#include <errno.h>
+#include <limits.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+/*
+ * Report an error and exit.
+ * Use it instead of err(3) to avoid linking-in stdio.
+ */
+static void
+errexit(const char *prog, const char *reason)
+{
+	char *errstr = strerror(errno);
+	write(STDERR_FILENO, prog, strlen(prog));
+	write(STDERR_FILENO, ": ", 2);
+	write(STDERR_FILENO, reason, strlen(reason));
+	write(STDERR_FILENO, ": ", 2);
+	write(STDERR_FILENO, errstr, strlen(errstr));
+	write(STDERR_FILENO, "\n", 1);
+	exit(1);
+}
+	
 int
-main(int argc __unused, char *argv[])
+main(int argc, char *argv[])
 {
 	int nflag;	/* if not set, output a trailing newline. */
+	int veclen;	/* number of writev arguments. */
+	struct iovec *iov, *vp; /* Elements to write, current element. */
+	char space[] = " ";
+	char newline[] = "\n";
+	char *progname = argv[0];
 
 	/* This utility may NOT do getopt(3) option parsing. */
 	if (*++argv && !strcmp(*argv, "-n")) {
 		++argv;
+		--argc;
 		nflag = 1;
-	}
-	else
+	} else
 		nflag = 0;
 
+	veclen = (argc >= 2) ? (argc - 2) * 2 + 1 : 0;
+
+	if ((vp = iov = malloc((veclen + 1) * sizeof(struct iovec))) == NULL)
+		errexit(progname, "malloc");
+
 	while (argv[0] != NULL) {
 		size_t len;
 		
@@ -82,11 +115,27 @@ main(int argc __unused, char *argv[])
 				nflag = 1;
 			}
 		}
-		write(STDOUT_FILENO, argv[0], len);
-		if (*++argv)
-			write(STDOUT_FILENO, " ", 1);
+		vp->iov_base = *argv;
+		vp++->iov_len = len;
+		if (*++argv) {
+			vp->iov_base = space;
+			vp++->iov_len = 1;
+		}
+	}
+	if (!nflag) {
+		veclen++;
+		vp->iov_base = newline;
+		vp++->iov_len = 1;
+	}
+	/* assert(veclen == (vp - iov)); */
+	while (veclen) {
+		int nwrite;
+
+		nwrite = (veclen > IOV_MAX) ? IOV_MAX : veclen;
+		if (writev(STDOUT_FILENO, iov, nwrite) == -1)
+			errexit(progname, "write");
+		iov += nwrite;
+		veclen -= nwrite;
 	}
-	if (!nflag)
-		write(STDOUT_FILENO, "\n", 1);
 	return 0;
 }