support -n option

This makes parallel stuff more than one argument to the subcommands.
It's very useful when subcommands are slow to start and are able to
process multiple files at once, that tasks are small but plenty.

For example:
    $ ./parallel -n 3 echo -- a b c d e f g h
    a b c
    d e f
    g h

Signed-off-by: Pierre Habouzit <madcoder@debian.org>

2 files changed, 39 insertions, 15 deletions

diff --git a/parallel.c b/parallel.c
index 8045e22..99a9727 100644
--- a/parallel.c
+++ b/parallel.c
@@ -38,7 +38,7 @@ void usage() {
 	exit(1);
 }
 
-void exec_child(char **command, char *argument, int replace_cb) {
+void exec_child(char **command, char **arguments, int replace_cb, int nargs) {
 	char **argv;
 	int argc = 0;
 	int i;
@@ -48,14 +48,14 @@ void exec_child(char **command, char *argument, int replace_cb) {
 	}
 	if (replace_cb == 0)
 		argc++;
-	argv = calloc(sizeof(char*), argc+1);
+	argv = calloc(sizeof(char*), argc + nargs);
 	for (i = 0; i < argc; i++) {
 		argv[i] = command[i];
 		if (replace_cb && (strcmp(argv[i], "{}") == 0))
-			argv[i] = argument;
+			argv[i] = arguments[0];
 	}
 	if (replace_cb == 0)
-		argv[i-1] = argument;
+		memcpy(argv + i - 1, arguments, nargs * sizeof(char *));
 	if (fork() == 0) {
 		/* Child */
 		execvp(argv[0], argv);
@@ -81,16 +81,18 @@ int main(int argc, char **argv) {
 	int maxjobs = -1;
 	int curjobs = 0;
 	int maxload = -1;
+	int argsatonce = 1;
 	int opt;
 	char **command = calloc(sizeof(char*), argc);
 	char **arguments = NULL;
 	int argidx = 0;
+	int arglen = 0;
 	int cidx = 0;
 	int returncode = 0;
 	int replace_cb = 0;
 	char *t;
 
-	while ((opt = getopt(argc, argv, "+hij:l:")) != -1) {
+	while ((opt = getopt(argc, argv, "+hij:l:n:")) != -1) {
 		switch (opt) {
 		case 'h':
 			usage();
@@ -116,12 +118,26 @@ int main(int argc, char **argv) {
 				exit(2);
 			}
 			break;
+		case 'n':
+			errno = 0;
+			argsatonce = strtoul(optarg, &t, 0);
+			if (errno != 0 || argsatonce < 1 || (t-optarg) != strlen(optarg)) {
+				fprintf(stderr, "option '%s' is not a positive number\n",
+					optarg);
+				exit(2);
+			}
+			break;
 		default: /* ’?’ */
 			usage();
 			break;
 		}
 	}
 
+	if (replace_cb && argsatonce > 1) {
+		fprintf(stderr, "options -i and -n are incomaptible\n");
+		exit(2);
+	}
+
 	if (maxjobs < 0) {
 #ifdef _SC_NPROCESSORS_ONLN
 		maxjobs = sysconf(_SC_NPROCESSORS_ONLN);
@@ -134,18 +150,15 @@ int main(int argc, char **argv) {
 	while (optind < argc) {
 		if (strcmp(argv[optind], "--") == 0) {
 			int i;
-			size_t mem = (sizeof (char*)) * (argc - optind);
 
-			arguments = malloc(mem);
+			optind++;
+			arglen = argc - optind;
+			arguments = calloc(sizeof(char *), arglen);
 			if (! arguments) {
 				exit(1);
 			}
-			memset(arguments, 0, mem);
-			optind++; /* Skip the -- separator, skip after
-				   * malloc so we have a trailing
-				   * null */
 
-			for (i = 0; i < argc - optind; i++) {
+			for (i = 0; i < arglen; i++) {
 				arguments[i] = strdup(argv[optind + i]);
 			}
 			optind += i;
@@ -157,15 +170,18 @@ int main(int argc, char **argv) {
 		optind++;
 	}
 
-	while (arguments[argidx] != 0) {
+	while (argidx < arglen) {
 		double load;
 
 		getloadavg(&load, 1);
 
 		if ((maxjobs > 0 && curjobs < maxjobs) ||
 		    (maxload > 0 && load < maxload)) {
-			exec_child(command, arguments[argidx], replace_cb);
-			argidx++;
+			if (argsatonce > arglen - argidx)
+				argsatonce = arglen - argidx;
+			exec_child(command, arguments + argidx,
+				   replace_cb, argsatonce);
+			argidx += argsatonce;
 			curjobs++;
 		}
 
diff --git a/parallel.docbook b/parallel.docbook
index 2fa5a5c..6da57bb 100644
--- a/parallel.docbook
+++ b/parallel.docbook
@@ -84,6 +84,14 @@ Written by Joey Hess
 			</listitem>
 		</varlistentry>
 		
+		<varlistentry>
+			<term><option>-n</option></term>
+			<listitem>
+				<para>Number of arguments to pass to one command at
+                                  once. Incompatible with -i</para>
+			</listitem>
+		</varlistentry>
+		
 		</variablelist>
 		
 	</refsect1>