问题描述
我正在执行文件读取/写入操作，以将大小为900 MB的文件从目录复制到USB（/ mnt / usb）。 读/写完成后，我将按照以下步骤进行fflush和fsync，
FILE *filename;
/* file read/write operations */
fflush(filename);
fsync(fileno(filename));
在上面的代码中，fsync返回-1。是什么原因以及如何检查它。预先感谢。
解决方法
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