Question

urgent: getc

Asked by: zizi21

hi,

if i use getc to read from file , can i assume it to be slower than fgets or fread or it does not matter? thanks a million..

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Asked On
2008-07-16 at 02:53:49ID23568992
Topic

C Programming Language

Participating Experts
4
Points
500
Comments
18

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Answers

 

by: Infinity08Posted on 2008-07-16 at 02:56:55ID: 22014486

It's equivalent. Usually, getc is even implemented as a macro that uses fgetc.

 

by: sunnycoderPosted on 2008-07-16 at 02:57:33ID: 22014489

Hello zizi21,

It may not be significantly sloerw but it is not going to be faster.
I/O operations are typically buffered. So even though you fetch a char, system would fetch an entire disk block. Further requests would be satisfied from this block. So the extra time spent would be in calls and not I/O.
This is implementation dependent and not standard but should be fairly common behavior.

Regards,
sunnycoder

 

by: evilrixPosted on 2008-07-16 at 02:57:43ID: 22014492

>> can i assume it to be slower than fgets or fread
Well they do different things to it's a bit like comparing apples and oranges but I don't think you van safely assume anything since nothing is defined in the standards docs regarding performance. If performance is a critical issue you should use a profiling tool to determine which, on the platforms you need to support, is better performant.

>> or it does not matter
That depends upon your requirements :)

 

by: Infinity08Posted on 2008-07-16 at 02:58:24ID: 22014496

>> can i assume it to be slower than fgets or fread or it does not matter?

Ah, I misread. I thought you were talking about fgetc.

If you mean that you read one character at a time in a loop, as opposed to reading all characters at the same time using fgets, then yes, it will be slower, since you have the overhead of the loop.

 

by: Infinity08Posted on 2008-07-16 at 03:00:25ID: 22014501

>> then yes, it will be slower

should be : "then yes, it will most probably be slower".


Btw, the accepted post is about fgetc, not fgets ...

 

by: sunnycoderPosted on 2008-07-16 at 03:01:04ID: 22014503

Either ways, it is never going to be equivalent.

 

by: evilrixPosted on 2008-07-16 at 03:02:31ID: 22014504

Of course, it all depends upon what's being done.. hence my suggestion that the only way to be sure is to profile the code.

 

by: Infinity08Posted on 2008-07-16 at 03:03:42ID: 22014512

>> Either ways, it is never going to be equivalent.

If you are referring to my first post which compared getc and fgetc, then yes they are equivalent as per the standard ;)

        "The getc function is equivalent to fgetc, except that if it is implemented as a macro, it
        may evaluate stream more than once, so the argument should never be an expression
        with side effects."

 

by: sunnycoderPosted on 2008-07-16 at 03:05:04ID: 22014518

What you quoted is functionality and not speed.

 

by: Infinity08Posted on 2008-07-16 at 03:09:31ID: 22014529

>> What you quoted is functionality and not speed.

I see no reason for getc and fgetc having different performance on any system I know. But yes, technically you are right :)

Anyway, that's beside the topic, which is about getc compared to fgets ... zizi21, did you notice that you accepted a reply that doesn't answer your question ?

 

by: evilrixPosted on 2008-07-21 at 06:28:50ID: 22050280

>> Please leave your specific (Comment ID) recommendations below my post

My suggestion is thus...
PAQ: http:#22014489 sunnycoder
PAQ: http:#22014492 evilrix
PAQ: http:#22014496 infinity08

As all three provide a reasonable answer for the OP to consider.

 

by: PaulCaswellPosted on 2008-07-21 at 08:58:12ID: 22051699

>> Please leave your specific (Comment ID) recommendations below my post

My 2c.

There is a subtle problem here. Although profiling should ALWAYS be used to determine the correct processes to optimise, asker wanted to 'assume' not 'determine' or 'measure'. In this respect, sadly, the suggestion to use a profiler should be judged off-topic.

My suggestion is thus...
SPLIT:
http:#22014489 sunnycoder
http:#22014496 infinity08

In my opinion ER's point that performance should be optimized through careful profiling, NOT assumptions, is actually more valuable for the PAQ than the answers provided. Unfortunately only the assumption was in question and was therefore covered by the other two answers.

Paul <ZA>

 

by: evilrixPosted on 2008-07-21 at 09:04:35ID: 22051752

Hi Paul,

That's fine by me.. no objections :)

-Rx.

 

by: evilrixPosted on 2008-07-21 at 09:05:56ID: 22051771

^^^ although I did point out the assumtion wasn't valid, "I don't think you can safely assume anything since nothing is defined in the standards docs" and I think that might be worth consideration for PAQ :)

 

by: zizi21Posted on 2008-07-22 at 13:06:02ID: 22063107

hi,

sorry, this question was suppose to be closed..maybe some problem with the internet connection at that time..... i don't know...

20120131-EE-VQP-002

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