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

NAME
       sync, syncfs - commit filesystem caches to disk

SYNOPSIS
       #include <unistd.h>

       void sync(void);

       int syncfs(int fd);

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

       sync():
           _XOPEN_SOURCE >= 500
               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
               || /* Glibc <= 2.19: */ _BSD_SOURCE

       syncfs():
           _GNU_SOURCE

DESCRIPTION
       sync() causes all pending modifications to filesystem metadata and cached file data to be written to the underlying filesystems.

       syncfs() is like sync(), but synchronizes just the filesystem containing file referred to by the open file descriptor fd.

RETURN VALUE
       syncfs() returns 0 on success; on error, it returns -1 and sets errno to indicate the error.

ERRORS
       sync() is always successful.

       syncfs() can fail for at least the following reasons:

       EBADF  fd is not a valid file descriptor.

       EIO    An  error  occurred during synchronization.  This error may relate to data written to any file on the filesystem, or on metadata related to the filesystem
              itself.

       ENOSPC Disk space was exhausted while synchronizing.

       ENOSPC, EDQUOT
              Data was written 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.

VERSIONS
       syncfs() first appeared in Linux 2.6.39; library support was added to glibc in version 2.14.

CONFORMING TO
       sync(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

       syncfs() is Linux-specific.

NOTES
       Since  glibc 2.2.2, the Linux prototype for sync() is as listed above, following the various standards.  In glibc 2.2.1 and earlier, it was "int sync(void)", and
       sync() always returned 0.

       According to the standard specification (e.g., POSIX.1-2001), sync() schedules the writes, but may return before the actual writing is done.  However Linux waits
       for I/O completions, and thus sync() or syncfs() provide the same guarantees as fsync() called on every file in the system or filesystem respectively.

       In mainline kernel versions prior to 5.8, syncfs() will fail only when passed a bad file descriptor (EBADF).  Since Linux 5.8, syncfs() will also report an error
       if one or more inodes failed to be written back since the last syncfs() call.

BUGS
       Before version 1.3.20 Linux did not wait for I/O to complete before returning.

SEE ALSO
       sync(1), fdatasync(2), fsync(2)

Linux                                                                          2021-03-22                                                                        SYNC(2)