Large gap between fwrite and write, and fread and read

[Heiko Wundram (Beenic)]

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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