1 files changed, 72 insertions, 0 deletions
diff --git a/lib/fadvise.h b/lib/fadvise.h
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+++ b/lib/fadvise.h
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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);