how can i be certain that a file has copied exactly?
keramida at freebsd.org
Sat Dec 27 21:58:51 UTC 2008
On Sat, 27 Dec 2008 13:35:51 -0800, Gary Kline <kline at thought.org> wrote:
> On Sat, Dec 27, 2008 at 02:58:06PM +0200, Giorgos Keramidas wrote:
>> On Sat, 27 Dec 2008 01:40:13 -0800, Gary Kline <kline at thought.org> wrote:
>> > howdy,
>> > in a word, YES, /usr/bin/cmp saved the save before i unlinked the
>> > oldfile. here is the strangeness. maybe you know, giorgos, or
>> > somebody else on-list. At first--before i got smart and used your
>> > snprintf to simply /bin/cp and then unlink---yes, or /bin/mv, or
>> > simply rename()--- Before, while i creating via fgets/fputs a new
>> > file, everything went fine until i ran out of buffer space. i
>> > increased to buf to buf. more files were successfully
>> > copied from dos\;5 to .dos/*.htm, actually. suddenly, cmp caught a
>> > mismatch and the program exited. a careful diff showed the err a
>> > something like line 3751. my copy was missing a byte near the EOF:
>> > </body></html
>> > minus the closing ">"
> Your code copies flawlessly. I noticed late last night that cmp uses
> the same byte-by-byte cp and IIRC checks each to make certain they
> bytes are identical. My copyFile() function simply used fopen, fgets,
> and fputs. I yanked it from a program that copied files from ~/Mail
> where the lines were around 80 bytes rather than in the thousands.
> With few newlines. The gotcha got me, in other words! Thanks much
> for the function!
That's good news, because I didn't even compile it. I just wrote it in
my mailer and hit send. I'm glad it worked :)
For what it's worth, if you are not handling *text* files, fgets() and
fputs() are probably a bad idea. They are line oriented, and they
depend on the presence of '\n' characters. The concept of ``lines'' is,
at best, ill defined for binary files. So it makes more sense to use
either byte-for-byte copies and rely on stdio to do buffering, or to use
some sort of custom buffer and fread()/fwrite() or plain read()/write().
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