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
– Jesse Good, Commented Dec 5, 2011 at 3:32
You guys know there's an awesome field down below labeled "Your Answer" where you can post this stuff — Michael Mrozek
– Michael Mrozek, Commented Dec 5, 2011 at 3:35
I'd need to find a whole 10 more characters to post it as an actual answer! — Kevin
– Kevin, Commented Dec 5, 2011 at 4:10

AusCBloke · Accepted Answer · 2011-12-05 03:35:11Z

19

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;
}

answered Dec 5, 2011 at 3:35

AusCBloke

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1 Comment

Roman Byshko Over a year ago

+2, but it was impossible, so +1 only.

juliomalegria · Accepted Answer · 2011-12-05 03:37:43Z

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 5, 2011 at 3:37

answered Dec 5, 2011 at 3:31

juliomalegria

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9 Comments

Roman Byshko Over a year ago

You are referencing to a C++ header, although the question is tagged with C.

juliomalegria Over a year ago

@Beginner: it's exactly the same stuff .. but I guess you're right, I'll add a warning

Roman Byshko Over a year ago

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.

AusCBloke Over a year ago

@Beginner it is the same, at least in this case, and especially since: C++ : Reference : C Library : cstdio (stdio.h) : fflush

Roman Byshko Over a year ago

@AusCBloke I just find this misleading (because of C++ there). This reference would be much better I think pubs.opengroup.org/onlinepubs/007904875/functions/fflush.html The link contains cstdio which is different from string.h.

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