copy_file_range: New function to copy file data

The semantics are based on the Linux system call, but a very close
emulation in user space is provided.

sysdeps/unix/sysv/linux/copy_file_range.c

@@ -0,0 +1,46 @@
+/* Linux implementation of copy_file_range.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU 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 2.1 of the License, or (at your option) any later version.
+
+   The GNU 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 GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <sysdep-cancel.h>
+#include <unistd.h>
+
+/* Include the fallback implementation.  */
+#ifndef __ASSUME_COPY_FILE_RANGE
+#define COPY_FILE_RANGE_DECL static
+#define COPY_FILE_RANGE copy_file_range_compat
+#include <io/copy_file_range-compat.c>
+#endif
+
+ssize_t
+copy_file_range (int infd, __off64_t *pinoff,
+                 int outfd, __off64_t *poutoff,
+                 size_t length, unsigned int flags)
+{
+#ifdef __NR_copy_file_range
+  ssize_t ret = SYSCALL_CANCEL (copy_file_range, infd, pinoff, outfd, poutoff,
+                                length, flags);
+# ifndef __ASSUME_COPY_FILE_RANGE
+  if (ret == -1 && errno == ENOSYS)
+    ret = copy_file_range_compat (infd, pinoff, outfd, poutoff, length, flags);
+# endif
+  return ret;
+#else  /* !__NR_copy_file_range */
+  return copy_file_range_compat (infd, pinoff, outfd, poutoff, length, flags);
+#endif
+}