73 lines
2.7 KiB
C
73 lines
2.7 KiB
C
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/* Declare an access pattern hint for files.
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Copyright (C) 2010-2020 Free Software Foundation, Inc.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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#include <config.h>
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#include <stdio.h>
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#include <fcntl.h>
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#include <sys/types.h>
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/* There are a few hints one can provide, which have the
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following characteristics on Linux 2.6.31 at least.
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POSIX_FADV_SEQUENTIAL
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Doubles the size of read ahead done for file
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POSIX_FADV_WILLNEED
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_synchronously_ prepopulate the buffer cache with the file
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POSIX_FADV_NOREUSE
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Could lower priority of data in buffer caches,
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but currently does nothing.
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POSIX_FADV_DONTNEED
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Drop the file from cache.
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Note this is automatically done when files are unlinked.
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We use this enum "type" both to make it explicit that
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these options are mutually exclusive, and to discourage
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the passing of the possibly undefined POSIX_FADV_... values.
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Note we could #undef the POSIX_FADV_ values, but that would
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preclude using the posix_fadvise() function with its standard
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constants. Using posix_fadvise() might be required if the return
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value is needed, but it must be guarded by appropriate #ifdefs. */
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#if HAVE_POSIX_FADVISE
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typedef enum {
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FADVISE_NORMAL = POSIX_FADV_NORMAL,
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FADVISE_SEQUENTIAL = POSIX_FADV_SEQUENTIAL,
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FADVISE_NOREUSE = POSIX_FADV_NOREUSE,
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FADVISE_DONTNEED = POSIX_FADV_DONTNEED,
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FADVISE_WILLNEED = POSIX_FADV_WILLNEED,
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FADVISE_RANDOM = POSIX_FADV_RANDOM
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} fadvice_t;
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#else
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typedef enum {
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FADVISE_NORMAL,
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FADVISE_SEQUENTIAL,
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FADVISE_NOREUSE,
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FADVISE_DONTNEED,
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FADVISE_WILLNEED,
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FADVISE_RANDOM
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} fadvice_t;
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#endif
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/* We ignore any errors as these hints are only advisory.
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There is the chance one can pass invalid ADVICE, which will
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not be indicated, but given the simplicity of the interface
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this is unlikely. Also not returning errors allows the
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unconditional passing of descriptors to non standard files,
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which will just be ignored if unsupported. */
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void fdadvise (int fd, off_t offset, off_t len, fadvice_t advice);
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void fadvise (FILE *fp, fadvice_t advice);
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