c - 在文件读/写操作返回 -1 后调用 fsync

FSYNC(2)                                            Linux Programmer's Manual                                            FSYNC(2)

NAME
       fsync, fdatasync - synchronize a file's in-core state with storage device

SYNOPSIS
       #include <unistd.h>

       int fsync(int fd);

       int fdatasync(int fd);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fsync():
           Glibc 2.16 and later:
               No feature test macros need be defined
           Glibc up to and including 2.15:
               _BSD_SOURCE || _XOPEN_SOURCE
                   || /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
       fdatasync():
           _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500

DESCRIPTION
       fsync() transfers ("flushes") all modified in-core data of (i.e., modified buffer cache pages for) the file referred to by
       the file descriptor fd to the disk device (or other permanent storage device) so that all changed information can  be  re‐
       trieved  even  if  the  system crashes or is rebooted.  This includes writing through or flushing a disk cache if present.
       The call blocks until the device reports that the transfer has completed.

       As well as flushing the file data, fsync() also flushes the metadata information associated with the file (see inode(7)).

       Calling fsync() does not necessarily ensure that the entry in the directory containing the file  has  also  reached  disk.
       For that an explicit fsync() on a file descriptor for the directory is also needed.

       fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a
       subsequent data retrieval to be correctly handled.  For example, changes to st_atime or st_mtime  (respectively,  time  of
       last access and time of last modification; see inode(7)) do not require flushing because they are not necessary for a sub‐
       sequent data read to be handled correctly.  On the other hand, a change to the file size (st_size, as made by  say  ftrun‐
       cate(2)), would require a metadata flush.

       The  aim  of  fdatasync()  is to reduce disk activity for applications that do not require all metadata to be synchronized
       with the disk.

RETURN VALUE
       On success, these system calls return zero.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       EBADF  fd is not a valid open file descriptor.

       EIO    An error occurred during synchronization.  This error may relate to data written to some other file  descriptor  on
              the  same  file.  Since Linux 4.13, errors from write-back will be reported to all file descriptors that might have
              written the data which triggered the error.  Some filesystems (e.g., NFS) keep  close  track  of  which  data  came
              through  which  file descriptor, and give more precise reporting.  Other filesystems (e.g., most local filesystems)
              will report errors to all file descriptors that where open on the file when the error was recorded.

       ENOSPC Disk space was exhausted while synchronizing.

       EROFS, EINVAL
              fd is bound to a special file (e.g., a pipe, FIFO, or socket) which does not support synchronization.

       ENOSPC, EDQUOT
              fd is bound to a file on NFS or another filesystem which does not allocate space at the time of a  write(2)  system
              call, and some previous write failed due to insufficient storage space.

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 4.3BSD.

AVAILABILITY
       On  POSIX  systems  on  which fdatasync() is available, _POSIX_SYNCHRONIZED_IO is defined in <unistd.h> to a value greater
       than 0.  (See also sysconf(3).)

NOTES
       On some UNIX systems (but not Linux), fd must be a writable file descriptor.

       In Linux 2.2 and earlier, fdatasync() is equivalent to fsync(), and so has no performance advantage.

       The fsync() implementations in older kernels and lesser used filesystems does not know how to flush disk caches.  In these
       cases disk caches need to be disabled using hdparm(8) or sdparm(8) to guarantee safe operation.

SEE ALSO
       sync(1),  bdflush(2), open(2), posix_fadvise(2), pwritev(2), sync(2), sync_file_range(2), fflush(3), fileno(3), hdparm(8),
       mount(8)

COLOPHON
       This page is part of release 4.16 of the Linux man-pages project.  A description of the project, information about report‐
       ing bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux                                                       2017-09-15 FSYNC(2) RTFM
c - 在文件读/写操作返回 -1 后调用 fsync

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