printf() seemingly not working as expected

Question

Printf() on following code may behave incorrectly on some machine (including mine).

printf("\n number of factor %d\n", sum); 
fflush(stdout);
if(sum == 0) 
{   
   printf("\n The  number %d is prime", p);
}

The code first prints out the variable ``sum'', and if, the sum is zero then it prints out the number.

While first printf prints correctly but second printf statement doesn't print. Am I missing something here?

If the second `printf` doesn't print, then you have discovered a bug in your compiler. More likely `sum` is non-zero. — David Heffernan, Mar 04 '12 at 20:08
You can check the value of sum right before the if statement to check if it is actually 0. — Diego Allen, Mar 04 '12 at 20:08
"printf() not doing its job". You need to have a little faith on the primitives you use... — UmNyobe, Mar 04 '12 at 20:10
I'm printing the sum right before the "if" block. And it does print sum variable to 0. — piyush_sao, Mar 04 '12 at 21:30
Possible duplicate of [Why does printf not flush after the call unless a newline is in the format string?](https://stackoverflow.com/questions/1716296/why-does-printf-not-flush-after-the-call-unless-a-newline-is-in-the-format-strin) — phuclv, Mar 22 '19 at 15:24

Basile Starynkevitch · Answer 1 · 2012-03-04T20:25:19.930

1

You are either missing a newline at the end of the printf format string, or another call to fflush

example why `fflush` is important

// file exflu.c
#include <stdio.h>
#include <unistd.h>
int main(int argc, char**argv) {
   int i=0;
   printf ("without newline from %s", argv[0]);
   // add perhaps a fflush(stdout); here
   sleep(5);
   scanf ("%d", &i);
   printf(" i=%d\n", i);
   return 0;
}

Observe the above program with and without fflush; without it, the message appears after five seconds (because the scanf is probably doing a fflush(NULL) implicitly).

edited Mar 04 '12 at 20:25

answered Mar 04 '12 at 20:04

Basile Starynkevitch

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Not flushing won't prevent a message from showing up though. It just won't show right away. – sepp2k Mar 04 '12 at 20:06
1

Except if the program don't exit immediately, then the output is still inside the `FILE` internal buffer. For instance, if something makes a read, the output won't appear before without `fflush` calls. – Basile Starynkevitch Mar 04 '12 at 20:08
I've never seen what you describe. I never call `fflush` in my simple C programs. Output always appears on the console when the program terminates. Do you really think this is a more likely explanation than sum being non-zero? – David Heffernan Mar 04 '12 at 20:15
Try the example I just added. – Basile Starynkevitch Mar 04 '12 at 20:19
I edited my example (and discovered that `scanf` might do a `fflush(NULL)` with it). – Basile Starynkevitch Mar 04 '12 at 20:35
\n at the end of printf did the job. Thanks. – piyush_sao Mar 04 '12 at 21:39
A good beginner's rule is that almost every `printf` should end by `\n` when you want the output to be seen. – Basile Starynkevitch Mar 04 '12 at 22:31

printf() seemingly not working as expected

1 Answers1

example why fflush is important

example why `fflush` is important