sync(8) [redhat man page]

SYNC(8) 						      System Manager's Manual							   SYNC(8)

NAME

       sync - synchronize data on disk with memory

SYNOPSYS

       sync [--help] [--version]

DESCRIPTION

       sync  writes  any data buffered in memory out to disk.  This can include (but is not limited to) modified superblocks, modified inodes, and
       delayed reads and writes.  This must be implemented by the kernel; The sync program does nothing but exercise the sync(2) system call.

       The kernel keeps data in memory to avoid doing (relatively slow) disk reads and writes.	This improves performance,  but  if  the  computer
       crashes, data may be lost or the filesystem corrupted as a result.  sync ensures that everything in memory is written to disk.

       sync  should  be  called before the processor is halted in an unusual manner (e.g., before causing a kernel panic when debugging new kernel
       code).  In general, the processor should be halted using the shutdown(8) or reboot(8) or halt(8) commands, which will attempt  to  put  the
       system  in a quiescent state before calling sync(2).  (Various implementations of these commands exist; consult your documentation; on some
       systems one should not call reboot(8) and halt(8) directly.)

GNU STANDARD OPTIONS

       --help Print a usage message on standard output and exit successfully.

       --version
	      Print version information on standard output, then exit successfully.

       --     Terminate option list.

ENVIRONMENT

       The variables LANG, LC_ALL, LC_CTYPE, and LC_MESSAGES have the usual meaning.

CONFORMING TO

       POSIX 1003.2

NOTES

       On Linux, sync is only guaranteed to schedule the dirty blocks for writing; it can actually take a short time before  all  the  blocks  are
       finally written.  The reboot(8) and halt(8) commands take this into account by sleeping for a few seconds after calling sync(2).

       This  page  describes  sync  as	found in the fileutils-4.0 package; other versions may differ slightly.  Mail corrections and additions to
       aeb@cwi.nl.  Report bugs in the program to fileutils-bugs@gnu.ai.mit.edu.

SEE ALSO

       sync(2), halt(8), reboot(8), update(8)

GNU fileutils 4.0						      1998-11								   SYNC(8)

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 versions <= 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 reason:

       EBADF  fd is not a valid file descriptor.

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.

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)

COLOPHON

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15								   SYNC(2)