Add freopen(3).

libc/Makefile

@@ -172,6 +172,7 @@ fdio.o \
 fileno.o \
 fork.o \
 fpipe.o \
+freopen.o \
 fsm_bootstraprootfd.o \
 fsm_closechannel.o \
 fsm_closeserver.o \

libc/freopen.cpp

@@ -0,0 +1,57 @@
+/*******************************************************************************
+
+    Copyright(C) Jonas 'Sortie' Termansen 2013.
+
+    This file is part of the Sortix C Library.
+
+    The Sortix C Library is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or (at your
+    option) any later version.
+
+    The Sortix C Library 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 Lesser General Public
+    License for more details.
+
+    You should have received a copy of the GNU Lesser General Public License
+    along with the Sortix C Library. If not, see <http://www.gnu.org/licenses/>.
+
+    freopen.cpp
+    Changes the file with which a FILE is associated.
+
+*******************************************************************************/
+
+#include <errno.h>
+#include <stdio.h>
+
+#include "fdio.h"
+
+extern "C" FILE* freopen(const char* path, const char* mode, FILE* fp)
+{
+	// If no path is given, simply try to change the mode of the file.
+	if ( !path )
+	{
+		// TODO: Determine whether the file should be closed in case we fail
+		// here to change the mode. A quick look at POSIX doesn't mention that
+		// it should be closed, so we'll assume it shouldn't!
+		if ( !fp->reopen_func )
+			return errno = EBADF, (FILE*) NULL;
+		if ( !fp->reopen_func(fp->user, mode) )
+			return NULL;
+		return fp;
+	}
+
+	// Uninstall the current backend of the FILE (after flushing) and return the
+	// FILE to the default state (like a new object from fnewfile).
+	fshutdown(fp);
+
+	// Attempt to open the new path and install that into our FILE object.
+	if ( fdio_install_path(fp, path, mode) != 0 )
+	{
+		fclose(fp);
+		return NULL;
+	}
+
+	return fp;
+}