Output of printf out of order

Question

I wrote the following:

#include <stdlib.h>
#include <stdio.h>
void ExecAsRoot (char* str);
int main ()
{
  printf ("Host real ip is:");
  ExecAsRoot("ip addr | grep 'state UP' -A2 | tail -n1 | awk '{print $2}' | cut -f1  -d'/'");
  return 0;
 }

void ExecAsRoot (char* str) {
  system (str);
}

My expected output is:

Host real ip is:7.17.11.29

While the actual output is:

7.17.11.29
Host real ip is:

Why is this?

dbush · Accepted Answer · 2017-08-04T14:17:57.223

12

The output of printf is being buffered because the string being printed does not contain a newline. As a result, the buffer does not get flushed until the program ends, and therefore appears after the output of the system command.

To flush the buffer, use fflush:

printf ("Host real ip is:");
fflush(stdout);
ExecAsRoot("ip addr | grep 'state UP' -A2 | tail -n1 | awk '{print $2}' | cut -f1  -d'/'");

If you want all writes to stdout to be unbuffered, you can use setvbuf to disable buffering:

setvbuf(stdout, NULL, _IONBF, 0);     // _IONBF = unbuffered

Or more simply:

setbuf(stdout, NULL);

Then all writes to stdout will appear immediately.

edited Aug 04 '17 at 14:17

answered Aug 02 '17 at 18:10

dbush

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Is there a way to implicitly "declare" that I need fflush on every printf? Other than wrapping it in a function which contains fflush? – Joel G Mathew Aug 02 '17 at 18:13
1

@Droidzone You can't have executable statements, such as a function call, outside of a function. You need to put the call to `setbuf` in a function, preferably at the start of `main`. – dbush Aug 02 '17 at 18:28
Worked with `setvbuf(stdout, NULL, _IONBF, 0);` – Joel G Mathew Aug 02 '17 at 18:36
@Droidzone Right! Edited. – dbush Aug 02 '17 at 18:38
@Droidzone note that it disables buffering, not that it'll call `fflush` on every `printf` – phuclv Aug 03 '17 at 12:04
use printf ("Host real ip is:\n"); to avoid this misbehavior – EsmaeelE Aug 04 '17 at 14:11
@EsmaeelE Obviously if I do that IP gets printed on another line, which wasnt what I wanted. – Joel G Mathew Aug 04 '17 at 21:11
@Droidzone Thanks for good question that i learn some weird thing on C. fflush() the output solve it. I know what is the problem but i cant find out what is the application of it. – EsmaeelE Aug 04 '17 at 22:03

Output of printf out of order

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