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sync, syncfs - commit buffer cache to disk
#include <unistd.h> void sync(void); void syncfs(int fd); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): sync(): _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED syncfs(): _GNU_SOURCE
sync() causes all buffered modifications to file metadata and data to be written to the underlying file systems. syncfs() is like sync(), but synchronizes just the file system 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: EBADF fd is not a valid file descriptor.
syncfs() first appeared in Linux 2.6.39.
sync(): SVr4, 4.3BSD, POSIX.1-2001. syncfs() is Linux-specific.
Since glibc 2.2.2 the Linux prototype for sync() is as listed above, following the various standards. In libc4, libc5, and glibc up to 2.2.1 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, since version 1.3.20 Linux does actually wait. (This still does not guarantee data integrity: modern disks have large caches.)
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