glib-unix: New Unix-specific API

GLib historically has been designed to be "mostly" portable; there
are some functions only available on Unix like g_io_channel_unix_new(),
but these are typically paired with obvious counterparts for Win32.

However, as GLib is used not only by portable software, but components
targeting Unix (or even just Linux), there are a few cases where it
would be very convenient if GLib shipped built-in functionality.

This initial patch is a basic wrapper around pipe2(), including
fallbacks for older kernels.  This pairs well with the
existing g_spawn_*() API and its child_setup functionality.

However, in the future, I want to add a signal() wrapper here,
complete with proxying the signal to a mainloop.  I have initial code
for this, but doing it sanely (including factoring out gmain.c's
private worker thread), is a complex task, and I don't want to block
on that.

See also gwin32.h for Win32 specific functionality.

https://bugzilla.gnome.org/show_bug.cgi?id=644941

glib/glib-unix.h

@@ -0,0 +1,70 @@
+/* glib-unix.h - Unix specific integration
+ * Copyright (C) 2011 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This 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
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#ifndef __G_UNIX_H__
+#define __G_UNIX_H__
+
+/* We need to include the UNIX headers needed to use the APIs below,
+ * but we also take this opportunity to include a wide selection of
+ * other UNIX headers.  If one of the headers below is broken on some
+ * system, work around it here (or better, fix the system or tell
+ * people to use a better one).
+ */
+#ifndef _GNU_SOURCE
+#define _G_GNU_SOURCE_TEMPORARILY_DEFINED
+#define _GNU_SOURCE
+#endif
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+#include <errno.h>
+#include <sys/wait.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#ifdef _G_GNU_SOURCE_TEMPORARILY_DEFINED
+#undef _GNU_SOURCE
+#undef _G_GNU_SOURCE_TEMPORARILY_DEFINED
+#endif
+
+#include <glib.h>
+
+/**
+ * G_UNIX_ERROR:
+ * 
+ * Error domain for API in the "g_unix_" namespace.  Note that there
+ * is no exported enumeration mapping "errno".  Instead, all functions
+ * ensure that "errno" is relevant.  The code for all G_UNIX_ERROR is
+ * always 0, and the error message is always generated via
+ * g_strerror().
+ *
+ * It is expected that most code will not look at "errno" from these
+ * APIs. Important cases where one would want to differentiate between
+ * errors are already covered by existing cross-platform GLib API,
+ * such as e.g. GFile wrapping "ENOENT".  However, it is provided for
+ * completeness, at least.
+ */
+#define G_UNIX_ERROR (g_unix_error_quark())
+
+GQuark g_unix_error_quark (void);
+
+gboolean g_unix_pipe_flags (int      *fds,
+			    int       flags,
+			    GError  **error);
+
+#endif