gl/lib/fadvise.h

   1 /* Declare an access pattern hint for files.
   2    Copyright (C) 2010-2023 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation, either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 #include <stdio.h>
  18 #include <fcntl.h>
  19 #include <sys/types.h>
  20
  21 /* There are a few hints one can provide, which have the
  22    following characteristics on Linux 2.6.31 at least.
  23
  24    POSIX_FADV_SEQUENTIAL
  25      Doubles the size of read ahead done for file
  26    POSIX_FADV_WILLNEED
  27      _synchronously_ prepopulate the buffer cache with the file
  28    POSIX_FADV_NOREUSE
  29      Could lower priority of data in buffer caches,
  30      but currently does nothing.
  31    POSIX_FADV_DONTNEED
  32      Drop the file from cache.
  33      Note this is automatically done when files are unlinked.
  34
  35    We use this enum "type" both to make it explicit that
  36    these options are mutually exclusive, and to discourage
  37    the passing of the possibly undefined POSIX_FADV_... values.
  38    Note we could #undef the POSIX_FADV_ values, but that would
  39    preclude using the posix_fadvise() function with its standard
  40    constants. Using posix_fadvise() might be required if the return
  41    value is needed, but it must be guarded by appropriate #ifdefs.  */
  42
  43 #if HAVE_POSIX_FADVISE
  44 typedef enum {
  45   FADVISE_NORMAL =     POSIX_FADV_NORMAL,
  46   FADVISE_SEQUENTIAL = POSIX_FADV_SEQUENTIAL,
  47   FADVISE_NOREUSE =    POSIX_FADV_NOREUSE,
  48   FADVISE_DONTNEED =   POSIX_FADV_DONTNEED,
  49   FADVISE_WILLNEED =   POSIX_FADV_WILLNEED,
  50   FADVISE_RANDOM =     POSIX_FADV_RANDOM
  51 } fadvice_t;
  52 #else
  53 typedef enum {
  54   FADVISE_NORMAL,
  55   FADVISE_SEQUENTIAL,
  56   FADVISE_NOREUSE,
  57   FADVISE_DONTNEED,
  58   FADVISE_WILLNEED,
  59   FADVISE_RANDOM
  60 } fadvice_t;
  61 #endif
  62
  63 /* We ignore any errors as these hints are only advisory.
  64    There is the chance one can pass invalid ADVICE, which will
  65    not be indicated, but given the simplicity of the interface
  66    this is unlikely.  Also not returning errors allows the
  67    unconditional passing of descriptors to non standard files,
  68    which will just be ignored if unsupported.  */
  69
  70 void fdadvise (int fd, off_t offset, off_t len, fadvice_t advice);
  71 void fadvise (FILE *fp, fadvice_t advice);