libc: add pexec() to execute a command in $PATH

sys/include/libc.h

@@ -601,6 +601,7 @@ extern	int	jehanne_brk(void*);
 extern	int	jehanne_execl(const char*, ...);
 extern	int	jehanne_forgivewkp(int64_t);
 extern	pid_t	jehanne_fork(void);
+extern	int	jehanne_pexec(const char* cmd, char* args[]);
 extern	int32_t	jehanne_readn(int, void*, int32_t);
 extern	void*	jehanne_segbrk(uint32_t);
 #define sbrk(incr) jehanne_segbrk(incr)
@@ -674,4 +675,3 @@ extern char *argv0;
 
 /* this is used by segbrk and brk,  it's a really bad idea to redefine it */
 extern	char	end[];
-

sys/src/lib/c/9sys/pexec.c

@@ -0,0 +1,41 @@
+/*
+ * This file is part of Jehanne.
+ *
+ * Copyright (C) 2017 Giacomo Tesio <giacomo@tesio.it>
+ *
+ * Jehanne is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 2 of the License.
+ *
+ * Jehanne is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Jehanne.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <u.h>
+#include <libc.h>
+#include <envvars.h>
+
+int
+jehanne_pexec(const char *f, char *args[])
+{
+	char *epath, *path, *entries[33];
+	int n, i;
+
+	if(f[0] == '/' || (epath = jehanne_getenv(ENV_PATH)) == nil)
+		return exec(f, (const char**)args);
+
+	n = jehanne_getfields(epath, entries, nelem(entries), 1, ":");
+	for(i = 0; i < n; ++i){
+		path = jehanne_smprint("%s/%s", entries[i], f);
+		exec(path, args);
+		jehanne_free(path);
+	}
+
+	jehanne_free(epath);
+	return -1;
+}

sys/src/lib/c/build.json

@@ -60,6 +60,7 @@
 			"9sys/nsec.c",
 			"9sys/nulldir.c",
 			"9sys/ocreate.c",
+			"9sys/pexec.c",
 			"9sys/pipe.c",
 			"9sys/postnote.c",
 			"9sys/privalloc.c",