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+/* Declare an access pattern hint for files.
+ Copyright (C) 2010-2022 Free Software Foundation, Inc.
+
+ This program 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, either version 3 of the License, or
+ (at your option) any later version.
+
+ This program 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 this program. If not, see <https://www.gnu.org/licenses/>. */
+
+#include <config.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <sys/types.h>
+
+/* There are a few hints one can provide, which have the
+ following characteristics on Linux 2.6.31 at least.
+
+ POSIX_FADV_SEQUENTIAL
+ Doubles the size of read ahead done for file
+ POSIX_FADV_WILLNEED
+ _synchronously_ prepopulate the buffer cache with the file
+ POSIX_FADV_NOREUSE
+ Could lower priority of data in buffer caches,
+ but currently does nothing.
+ POSIX_FADV_DONTNEED
+ Drop the file from cache.
+ Note this is automatically done when files are unlinked.
+
+ We use this enum "type" both to make it explicit that
+ these options are mutually exclusive, and to discourage
+ the passing of the possibly undefined POSIX_FADV_... values.
+ Note we could #undef the POSIX_FADV_ values, but that would
+ preclude using the posix_fadvise() function with its standard
+ constants. Using posix_fadvise() might be required if the return
+ value is needed, but it must be guarded by appropriate #ifdefs. */
+
+#if HAVE_POSIX_FADVISE
+typedef enum {
+ FADVISE_NORMAL = POSIX_FADV_NORMAL,
+ FADVISE_SEQUENTIAL = POSIX_FADV_SEQUENTIAL,
+ FADVISE_NOREUSE = POSIX_FADV_NOREUSE,
+ FADVISE_DONTNEED = POSIX_FADV_DONTNEED,
+ FADVISE_WILLNEED = POSIX_FADV_WILLNEED,
+ FADVISE_RANDOM = POSIX_FADV_RANDOM
+} fadvice_t;
+#else
+typedef enum {
+ FADVISE_NORMAL,
+ FADVISE_SEQUENTIAL,
+ FADVISE_NOREUSE,
+ FADVISE_DONTNEED,
+ FADVISE_WILLNEED,
+ FADVISE_RANDOM
+} fadvice_t;
+#endif
+
+/* We ignore any errors as these hints are only advisory.
+ There is the chance one can pass invalid ADVICE, which will
+ not be indicated, but given the simplicity of the interface
+ this is unlikely. Also not returning errors allows the
+ unconditional passing of descriptors to non standard files,
+ which will just be ignored if unsupported. */
+
+void fdadvise (int fd, off_t offset, off_t len, fadvice_t advice);
+void fadvise (FILE *fp, fadvice_t advice);