Advanced Programming in the UNIX® Environment

ptg10805159

Section 3.13 sync,fsync,andfdatasyncFunctions 81

1. dup2is an atomic operation, whereas the alternate form involves two function
   calls. It is possible in the latter case to have a signal catcher called between the
   closeand thefcntlthat could modify the file descriptors. (Wedescribe
   signals in Chapter 10.) The same problem could occur if a different thread
   changes the file descriptors. (Wedescribe threads in Chapter 11.)


2. Thereare someerrnodifferences betweendup2andfcntl.

Thedup2system call originated with Version 7 and propagated through the BSD releases. The

fcntlmethod for duplicating file descriptors appeared with System III and continued with

System V.SVR3.2 picked up thedup2function, and 4.2BSD picked up thefcntlfunction and

theF_DUPFDfunctionality.POSIX.1 requires bothdup2and theF_DUPFDfeatureoffcntl.

3.13 sync,fsync,and fdatasyncFunctions

Tr aditional implementations of the UNIX System have a buffer cache or page cache in

the kernel through which most disk I/O passes. When we write data to a file, the data

is normally copied by the kernel into one of its buffers and queued for writing to disk at

some later time. This is calleddelayed write.(Chapter 3 of Bach[ 1986 ]discusses this

buffer cache in detail.)

The kernel eventually writes all the delayed-write blocks to disk, normally when it

needs to reuse the buffer for some other disk block. To ensureconsistency of the file

system on disk with the contents of the buffer cache, thesync,fsync,andfdatasync

functions areprovided.

#include <unistd.h>

int fsync(intfd);

int fdatasync(intfd);

Returns: 0 if OK,−1 on error

void sync(void);

Thesyncfunction simply queues all the modified block buffers for writing and returns;

it does not wait for the disk writes to take place.

The functionsyncis normally called periodically (usually every 30 seconds) from a

system daemon, often calledupdate.This guarantees regular flushing of the kernel’s

block buffers. The commandsync( 1 )also calls thesyncfunction.

The functionfsyncrefers only to a single file, specified by the file descriptorfd,

and waits for the disk writes to complete beforereturning. This function is used when

an application, such as a database, needs to be surethat the modified blocks have been

written to the disk.

Thefdatasyncfunction is similar tofsync,but it affects only the data portions of

afile. Withfsync,the file’s attributes arealso updated synchronously.

All four of the platforms described in this book supportsyncandfsync.However,FreeBSD

8.0 does not supportfdatasync.