write() to stdout and printf output not interleaved?

Question

#include <stdio.h>
#define MAXLEN 256

int main() {
  int n;
  char buf[MAXLEN];
  while((n = read(0,buf,sizeof(buf))) != 0){
    printf("n: %d:",n);
    write(1,buf,n);
  }
  return 1;
}

The output of the program (where the first read and first write is typed by the user and echoed by the terminal) is:

read
read
write
write
n: 5:n: 6:

The output of printf comes after pressing Ctrl+D at the standard input and not along with the subsequent reads. Why does this happen?

score 22 · Accepted Answer · answered Aug 07 '09 at 05:26

22

Printf is buffered.

You can force printf to 'flush' its buffer using the fflush call:

#include <stdio.h>
#define MAXLEN 256

int main() {
  int n;
  char buf[MAXLEN];
  while((n = read(0,buf,sizeof(buf))) != 0){
    printf("n: %d:",n);
    fflush(stdout); /* force it to go out */
    write(1,buf,n);
  }
  return 1;
}

In general, printf() being buffered is a good thing. Unbuffered output, particularly to visible consoles that require screen updates and such, is slow. Slow enough that an application that is printf'ing a lot can be directly slowed down by it (especially on the Windows platform; Linux and unixes are typically impacted less).

However, printf() being buffered does bite you if you also fprintf(stderr,) - stderr is deliberately unbuffered. As a consequence, you may get your messages with some printf() missing; if you write to another FILE handle that is also associated with the terminal, and might be unbuffered, make sure you first explicitly fflush(stdout).

answered Aug 07 '09 at 05:26

Will

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7

You can also change the buffering mode with setvbuf() before doing any IO. – AProgrammer Aug 07 '09 at 06:51
What does "`printf()` is [buffered](http://en.wikipedia.org/wiki/Data_buffer)" mean? – ma11hew28 Apr 26 '14 at 17:12
1

@MattDiPasquale printf writes to stdout, and stdout is buffered (by default). It is only flushed e.g. on line-feeds. – Will Apr 26 '14 at 19:50
1

Should also mention that a newline ('\n') will cause stdout to be flushed – carefulnow1 Nov 26 '17 at 14:24
@carefulnow1: Depends on where it's pointing. If it's going to a file, it's typically block buffered, it's only when it goes to a terminal that it's typically line buffered. – ShadowRanger Mar 07 '20 at 17:06

score 2 · Answer 2 · answered Aug 07 '09 at 12:18

2

The manpage for fgets tells me:

It is not advisable to mix calls to input functions from the stdio library with low-level calls to read(2) for the file descriptor associ‐ ated with the input stream; the results will be undefined and very probably not what you want.

So the best solution would be not to to use write and printf on the same descriptor.

answered Aug 07 '09 at 12:18

Kasper

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POSIX specifies very carefully when the results are undefined and when they're well-defined based on an abstract notion of the "active handle". See http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_05_01 – R.. GitHub STOP HELPING ICE Jan 08 '11 at 17:23

score 1 · Answer 3 · answered Aug 07 '09 at 05:11

1

Printf is using stdio and it is buffered. Push it out by sending a changing to "n: %d:\n"

answered Aug 07 '09 at 05:11

Tim Williscroft

3,705
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Either that, or don't mix output channels -- that is, use one and the same function to output all content. – Kim Gräsman Aug 07 '09 at 05:26
1

\n is not guaranteed to flush it. – EFraim Aug 07 '09 at 05:34
1

stdout is line buffered unless it it directed to an non interactive device. – AProgrammer Aug 07 '09 at 06:53

score 1 · Answer 4 · answered Oct 08 '12 at 13:33

You can use the std fflush() function to flush the std out buffer or you can make use of an additional \n at the end of the control string inside the printf. Something like this

printf("\n :%d:\n",n);

Its always better to use the write() & read() functions in C instead of printf() and scanf(). Printf and scanf have got some problems like printf stores the string parameter in stdout buffer. So a manual flush is required which is done through fflush function or by means of \n. In a small hello world printing program you will not find such a problem as the stdout buffer is flushed at the end of the program execution. Better use write() which works fine. scanf also have the problem of reading spaces and a lot of other problems related to stdin buffer.

For example in the code below:

main()  {   char a; int i=0,c; for(;i<2;i++) { scanf("%d",&c); scanf("%c",&a);} }

The above program as got the problem of reading \n into stdin on pressing enter. We could resolve this but not flushing the stdin buffer or making use of \n character. Always better to use the read() and write() functions.

Hope that helps....

score 0 · Answer 5 · answered Aug 07 '09 at 19:01

0

Use fwrite (streams version) rather than write.

Note that, while is associated with file number 1, it isn't the same thing.

answered Aug 07 '09 at 19:01

write() to stdout and printf output not interleaved?

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