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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)