Date: Tue, 17 Jul 2007 01:45:52 +0200 From: "Heiko Wundram (Beenic)" <wundram@beenic.net> To: freebsd-hackers@freebsd.org Subject: Re: Large gap between fwrite and write, and fread and read Message-ID: <200707170145.52781.wundram@beenic.net> In-Reply-To: <469B5F61.1060805@u.washington.edu>
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On Monday 16 July 2007 14:06:57 Garrett Cooper wrote:
> I ran some tests and I noticed a large difference in the cumulative
> sums of fwrite(2) vs write(3) and fread(2) vs read(3) (3-fold
> differences on a real machine).
This difference is at least partially explained when looking
at /usr/include/stdio.h, which defines the FILE structure: read/write on a
file descriptor is (pretty much) a direct syscall with operating system
specific semantics on a wide range of behavior, such as buffering, flushing
and seeking in read/write-opened files, whereas fread/fwrite on FILE*'s is
an "abstraction" of file access for which the stdio-API defines semantics,
such as buffering, the time flushing takes place and seeking in
read/write-opened files, across different flavors of POSIX-compatible libc's
equally.
As the stdio-interface is a "wrapper" (with indirect calls calling the syscall
read at some point in time, see the FILE-structure definition), you'll have
to expect a difference in runtime, too.
(You mixed up f{read,write}(3) and {read,write}(2), just as a sidenote, which
is also indicative of the difference: man3 is indicative of a libc
implementation, whereas man2 generally contains syscall documentation)
--
Heiko Wundram
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