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

sync, syncfs - commit filesystem caches to disk

#include <unistd.h>
void sync(void);
int syncfs(int fd);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE

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.

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

sync() is always successful.
syncfs() can fail for at least the following reason:
fd is not a valid file descriptor.

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

sync(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
syncfs() is Linux-specific.

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.

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

sync(1), fdatasync(2), fsync(2)
2017-09-15 Linux