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); 72