C programming: write to file without buffer

Question

I am using fputs to write strings to file, but under the debug mode, the content is not written to disk after the statement fputs. I think there is some buffer. But I would like to debug to check whether the logic is correct by viewing the content directly. Is there anyway to disable the buffer? Thanks.

Yes, set the second parameter of `setbuf` to NULL like this `setbuf(myfilepointer, NULL);`. — Jesse Good, Dec 05 '11 at 03:32
You guys know there's an awesome field down below labeled "Your Answer" where you can post this stuff — Michael Mrozek, Dec 05 '11 at 03:35
I'd need to find a whole 10 more characters to post it as an actual answer! — Kevin, Dec 05 '11 at 04:10

score 19 · Accepted Answer · answered Dec 05 '11 at 03:35

You have a couple of alternatives:

fflush(f); to flush the buffer at a certain point.
setbuf(f, NULL); to disable buffering.

Where f is obviously your FILE*.

ie.

#include <stdio.h>

int main(void)
{
   char s[100];

   FILE *f = fopen("test.txt", "w");
   setbuf(f, NULL);

   while (fgets(s, 100, stdin))
      fputs(s, f);

   return 0;
}

OR

#include <stdio.h>

int main(void)
{
   char s[100];

   FILE *f = fopen("test.txt", "w");

   while (fgets(s, 100, stdin)) {
      fputs(s, f);
      fflush(f);
   }

   return 0;
}

+2, but it was impossible, so +1 only. – Roman Byshko Dec 05 '11 at 03:40 — Roman Byshko, Dec 05 '11 at 03:40

juliomalegria · Answer 2 · 2011-12-05T03:37:43.760

0

I don't know if you can't disable the buffer, but you can force it to write in disk using fflush

More about it: (C++ reference, but just the same as in C): http://www.cplusplus.com/reference/clibrary/cstdio/fflush/

edited Dec 05 '11 at 03:37

answered Dec 05 '11 at 03:31

juliomalegria

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You are referencing to a C++ header, although the question is tagged with C. – Roman Byshko Dec 05 '11 at 03:32
@Beginner: it's exactly the same stuff .. but I guess you're right, I'll add a warning – juliomalegria Dec 05 '11 at 03:36
It's not... believe me :) Make better reference to POSIX standard. But actually it would be even better to delete your answer, as I did it with mine... @AusCBloke has give a *full* answer. – Roman Byshko Dec 05 '11 at 03:38
@Beginner it is the same, at least in this case, and especially since: `C++ : Reference : C Library : cstdio (stdio.h) : fflush` – AusCBloke Dec 05 '11 at 03:40
@AusCBloke I just find this misleading (because of C++ there). This reference would be much better I think http://pubs.opengroup.org/onlinepubs/007904875/functions/fflush.html The link contains `cstdio` which is different from `string.h`. – Roman Byshko Dec 05 '11 at 03:43
@Beginner: I agree that is a better reference, another is http://man7.org/linux/man-pages/man3/fflush.3.html, but the ones at cplusplus.com aren't bad either since they include small examples, and the example there for `fflush` is completely C. – AusCBloke Dec 05 '11 at 03:44
@Beginner I'm pretty sure is the same because it is inside the C Library (http://www.cplusplus.com/reference/clibrary/). I don't like deleting my answers, at least not if they are correct. – juliomalegria Dec 05 '11 at 03:47
@julio.alegria: I don't see anything wrong with your answer, I use cplusplus.com a lot for C stuff. – AusCBloke Dec 05 '11 at 03:48
You can track this question, that I asked just now: http://stackoverflow.com/questions/8380805/difference-between-string-h-and-cstring – Roman Byshko Dec 05 '11 at 03:53

C programming: write to file without buffer

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