setvbuf didn't work in linux?

0_0 · 2012-02-14 04:14:26

Hi,all of you!
I wrote some codes to test setvbuf function in linux.Here are the codes:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

char buf[10];

int main()
{
	
	if((setvbuf(stdout,buf,_IOFBF,10)) != 0) {
		fprintf(stderr,"setvbuf() failed: %s\n",strerror(errno));
		exit(1);
	} 
	printf("123456789");
	_exit(0);
}

As you can see,it should not output anything on screen.But,unfortunately,it outputs "123456789".I test the codes with archlinux,ubuntu,opensuse,and it's all the same.As a matter of fact,it WORKED in freebsd.I just wonder if there is something wrong with my codes or the function setvbuf in linux,it confused me for a long time.
I will appreciate any comments/suggestions,and thanks in advance.

sujoy · 2012-02-16 02:21:40

Try printing the buffer to test the result.

Last edited by sujoy (2012-02-16 02:23:27)

ewaller · 2012-02-16 04:39:03

Does this passage from man setvbuf have any relevance?

       You must make sure that the space that buf points to still exists by the time stream is closed, which also happens at program termination.  For example, the follow‐
       ing is invalid:

       #include <stdio.h>

       int
       main(void)
       {
           char buf[BUFSIZ];
           setbuf(stdin, buf);
           printf("Hello, world!\n");
           return 0;
       }

I'm not certain myself; It is something to play with.

tavianator · 2012-02-16 05:54:28

Simply put, the implementation is free to flush the stream whenever it wants. On my box, printf("1"); doesn't print anything, but printf("12") does. See this:

http://www.gnu.org/software/libc/manual … g-Concepts

Characters written to or read from an unbuffered stream are transmitted individually to or from the file as soon as possible.
Characters written to a line buffered stream are transmitted to the file in blocks when a newline character is encountered.
Characters written to or read from a fully buffered stream are transmitted to or from the file in blocks of arbitrary size.

0_0 · 2012-02-17 02:37:09

Thanks for all responses.
Hi,ewaller.I've already noticed that passage yet,and i allocated buf statically.See my codes above.

Simply put, the implementation is free to flush the stream whenever it wants.

Characters written to or read from a fully buffered stream are transmitted to or from the file in blocks of arbitrary size.

Well,tavianator.I think the arbitrary size means the size of buffer allocated,that is,the buffer's size is arbitrary.
I don't think the implementation is free to flush the stream whenever it wants,if it's so,parameter size is no need in servbuf.

Last edited by 0_0 (2012-02-17 02:39:07)

skunktrader · 2012-02-17 02:52:30

Nothing to see here...please delete this post

Last edited by skunktrader (2012-02-17 02:54:53)

Blµb · 2012-02-17 15:24:05

Maybe a buffer size of 16 isn't considered useful... For me, the text stops being printed at a buffer size of >= 128.

0_0 · 2012-02-18 07:43:10

Blµb wrote:

Maybe a buffer size of 16 isn't considered useful... For me, the text stops being printed at a buffer size of >= 128.

Thank you for testing the codes.It seems you are right,if the buffer have a size of >= 128,setvbuf works normally.That's instreasting.

Last edited by 0_0 (2012-02-18 07:43:58)

tavianator · 2012-02-19 02:06:56

0_0 wrote:

Well,tavianator.I think the arbitrary size means the size of buffer allocated,that is,the buffer's size is arbitrary.
I don't think the implementation is free to flush the stream whenever it wants,if it's so,parameter size is no need in servbuf.

The C99 Standard, 7.19.3.3, Files wrote:

When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled. ... Support for these characteristics is implementation-defined, and may be affected via the setbuf and setvbuf functions.

The C99 Standard, 7.19.5.6, The setvbuf function wrote:

If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by the setvbuf function, and the argument size specifies the size of the array

Note the use of may in two key places. The C standard does not enforce any particular behaviour here.

0_0 · 2012-02-19 02:44:17

Oh,thank you .Could you tell me how to use that function correctly?

tavianator · 2012-02-21 08:02:09

Well, what are you trying to use it for? If you're trying to redirect output from stdout to somewhere else, use freopen() or dup2().

0_0 · 2012-02-23 11:50:21

Sorry,busy these days.I use it for nothing,just learning its usage.

Arch Linux

#1 2012-02-14 04:14:26

setvbuf didn't work in linux?

#2 2012-02-16 02:21:40

Re: setvbuf didn't work in linux?

#3 2012-02-16 04:39:03

Re: setvbuf didn't work in linux?

#4 2012-02-16 05:54:28

Re: setvbuf didn't work in linux?

#5 2012-02-17 02:37:09

Re: setvbuf didn't work in linux?

#6 2012-02-17 02:52:30

Re: setvbuf didn't work in linux?

#7 2012-02-17 15:24:05

Re: setvbuf didn't work in linux?

#8 2012-02-18 07:43:10

Re: setvbuf didn't work in linux?

#9 2012-02-19 02:06:56

Re: setvbuf didn't work in linux?

#10 2012-02-19 02:44:17

Re: setvbuf didn't work in linux?

#11 2012-02-21 08:02:09

Re: setvbuf didn't work in linux?

#12 2012-02-23 11:50:21

Re: setvbuf didn't work in linux?

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