Add funopen() function

This is a wrapper over the glibc fopencookie() function.

We diverge from the FreeBSD, OpenBSD and DragonFlyBSD declarations,
because seekfn() there wrongly uses fpos_t, assuming it's an integral
type, and any code using that on a system where fpos_t is a struct
(such as GNU-based systems or NetBSD) will fail to build. In which case,
as the code has to be modified anyway, we might just as well use the
correct declaration.

src/funopen.c

@@ -0,0 +1,142 @@
+/*
+ * Copyright © 2011, 2013 Guillem Jover
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
+ * AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
+ * THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+ * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+ * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+#include <sys/types.h>
+#include <errno.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#ifdef HAVE_FOPENCOOKIE
+struct funopen_cookie {
+	void *orig_cookie;
+
+	int (*readfn)(void *cookie, char *buf, int size);
+	int (*writefn)(void *cookie, const char *buf, int size);
+	off_t (*seekfn)(void *cookie, off_t offset, int whence);
+	int (*closefn)(void *cookie);
+};
+
+static ssize_t
+funopen_read(void *cookie, char *buf, size_t size)
+{
+	struct funopen_cookie *cookiewrap = cookie;
+
+	if (cookiewrap->readfn == NULL) {
+		errno = EBADF;
+		return -1;
+	}
+
+	return cookiewrap->readfn(cookiewrap->orig_cookie, buf, size);
+}
+
+static ssize_t
+funopen_write(void *cookie, const char *buf, size_t size)
+{
+	struct funopen_cookie *cookiewrap = cookie;
+
+	if (cookiewrap->writefn == NULL)
+		return EOF;
+
+	return cookiewrap->writefn(cookiewrap->orig_cookie, buf, size);
+}
+
+static int
+funopen_seek(void *cookie, off64_t *offset, int whence)
+{
+	struct funopen_cookie *cookiewrap = cookie;
+	off_t soff = *offset;
+
+	if (cookiewrap->seekfn == NULL) {
+		errno = ESPIPE;
+		return -1;
+	}
+
+	soff = cookiewrap->seekfn(cookiewrap->orig_cookie, soff, whence);
+	*offset = soff;
+
+	return *offset;
+}
+
+static int
+funopen_close(void *cookie)
+{
+	struct funopen_cookie *cookiewrap = cookie;
+	int rc;
+
+	if (cookiewrap->closefn == NULL)
+		return 0;
+
+	rc = cookiewrap->closefn(cookiewrap->orig_cookie);
+
+	free(cookiewrap);
+
+	return rc;
+}
+
+FILE *
+funopen(const void *cookie,
+        int (*readfn)(void *cookie, char *buf, int size),
+        int (*writefn)(void *cookie, const char *buf, int size),
+        off_t (*seekfn)(void *cookie, off_t offset, int whence),
+        int (*closefn)(void *cookie))
+{
+	struct funopen_cookie *cookiewrap;
+	cookie_io_functions_t funcswrap = {
+		.read = funopen_read,
+		.write = funopen_write,
+		.seek = funopen_seek,
+		.close = funopen_close,
+	};
+	const char *mode;
+
+	if (readfn) {
+		if (writefn == NULL)
+			mode = "r";
+		else
+			mode = "r+";
+	} else if (writefn) {
+		mode = "w";
+	} else {
+		errno = EINVAL;
+		return NULL;
+	}
+
+	cookiewrap = malloc(sizeof(*cookiewrap));
+	if (cookiewrap == NULL)
+		return NULL;
+
+	cookiewrap->orig_cookie = (void *)cookie;
+	cookiewrap->readfn = readfn;
+	cookiewrap->writefn = writefn;
+	cookiewrap->seekfn = seekfn;
+	cookiewrap->closefn = closefn;
+
+	return fopencookie(cookiewrap, mode, funcswrap);
+}
+#else
+#error "Function funopen() needs to be ported."
+#endif