CODEWITHSUNDEEP
cscanf
Žiga Gazvoda
Žiga Gazvoda

Reputation: 173

Printf not working with scanf

Hi i am new to the programing and for example in my code:

#include <stdio.h>

int main (void){
   int a;
   printf("Write a number: ");
   scanf("%d", &a);
   printf("Your written number was: %d", a);
   return 0;
}

Printf does not write "write a number" in console when i start the program but only after i already inserted the number and pressed enter. I have already done some research and found out for this code:

setvbuf(stdout, NULL, _IONBF, 0);

when i paste this into my program it works as it should but i am wondering why do i have to do that?

Upvotes: 1

Views: 238

Answers (4)

Spaceghost
Spaceghost

Reputation: 6995

Add linefeed "\n" to your printf lines like so:

printf("Write a number: \n");

Upvotes: 0

P.P
P.P

Reputation: 121427

when i paste this into my program it works as it should but i am wondering why do i have to do that?

It's because printf() is usually line-buffered when attached to a terminal. So disabling the buffering with the call to setvbuf() makes stdio library to not buffer at all.

You can also use fflush(stdout); after the printf() call to flush out the buffered output. The same can be done with setbuf(stdout, NULL); as well.

You can also add a \n at the end of printf() statement to force the flushing. But this will work only if the output goes to a terminal device.

For example, if you do (on a unix-like system): 

./a.out > output_file

then the \n will not flush the buffer.

Out of the two options (setbuf() and fflush()),fflush(stdout); is probably the better option in most cases. Since disabling the buffering completely can have negative impact on performance (which is the primary reason for buffering in the first place) whereas fflush() can be judiciously used at the right place when you think it's necessary.

Upvotes: 1

DarthNoodles
DarthNoodles

Reputation: 370

You probably want \n in each of those printf statements.

Upvotes: 0

Ed Heal
Ed Heal

Reputation: 60037

printf has a buffer. It is a mechanism to make code run faster by not having to switch between the user context and the kernel context. To get over this you can tell the code to flush the buffer - i.e. send it to the operating system. This can be done by

fflush(stdout);

After a printf. If the printf contains a new line this is done automatically.

Upvotes: 1

Related Questions