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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)