fflush() is not working in Linux

Question

I used the fflush() in Linux GCC but it did not work. Are there any alternatives for that function? Here is my code:

#include<stdio.h>
void main()
{
  char ch='y';
  while(ch=='y')
  {
    int a;
    printf("Enter some value:");
    scanf("%d",&a);
    fflush(stdin);
    printf("Do you want to continue?");
    scanf("%c",&ch)
  }

The output that I got is:

Enter some value: 10

Then the program ends. That's all. What can I do in Linux? Is there an alternative function?

Remember that flushing `stdin` is actually undefined behavior. Don't do it. — Some programmer dude, Jun 26 '13 at 11:34
Don't `fflush(stdin)`. Instead change `scanf()` to `fgets()`. — , Jun 26 '13 at 11:36
@Stefan I just tried to flush the input stream using fflush(). In windows it works, but in linux it does not work... — sundar, Jun 26 '13 at 11:37
@sundar Yes, but why do you want to flush the input stream? It's impossible to recommend an alternative unless you say what you need the programme to do. — Stefan, Jun 26 '13 at 11:40
`fflush()` is an undefined operation on input streams. Some compilers may, _as an extension_, provide support for this undefined behavior. I would not recommend relying on this. — This isn't my real name, Jun 27 '13 at 01:26
Possible duplicate of [Using fflush(stdin)](http://stackoverflow.com/questions/2979209/using-fflushstdin) — Thomas Dickey, Aug 07 '16 at 00:49

Mathuin · Accepted Answer · 2013-07-03T18:42:22.910

21

Don't use fflush, use this function instead:

#include <stdio.h>
void clean_stdin(void)
{
    int c;
    do {
        c = getchar();
    } while (c != '\n' && c != EOF);
}

fflush(stdin) depends of the implementation, but this function always works. In C, it is considered bad practice to use fflush(stdin).

edited Jul 03 '13 at 18:42

answered Jun 26 '13 at 11:46

Mathuin

790
1
6
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6

`fflush(stdin)` is a bad practice, not `fflush`. ;) – md5 Jul 03 '13 at 18:26
@md5 what do you mean "not fflush". do you mean fflush is good practice? or fflush with no parameters doesn't exist? – barlop Dec 19 '15 at 16:09
1

@barlop: `fflush` might be useful, e.g. with an output stream: `fflush(stdout)` forces the output to be written (see http://paste.awesom.eu/4dym). Here it is _not_ an undefined behavior. – md5 Dec 19 '15 at 17:04
3

Can you explain how this works instead of just saying "dont use that, use this, it works"? – NOP da CALL Feb 16 '18 at 05:47
1

While this will work if the buffer has some input. If it will be empty getchar() will force the console to ask for additional input meaning you will need to click additional "enter" key to create a newline. Maybe fflush(stdin) is a bad practice, but it prevents this problem – Denis_LT Sep 28 '21 at 11:27

ataraxic · Answer 2 · 2014-05-27T09:00:43.053

7

One that always works on Linux:

#include <termios.h>
#include <unistd.h>

void clean_stdin()
{
        int stdin_copy = dup(STDIN_FILENO);
        /* remove garbage from stdin */
        tcdrain(stdin_copy);
        tcflush(stdin_copy, TCIFLUSH);
        close(stdin_copy);
}

You can use tcdrain and tcflush not only for in/out/err fd.

edited May 27 '14 at 09:00

answered May 27 '14 at 08:54

ataraxic

2,157
2
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10

Why are you creating a duplicate of STDIN_FILENO? – user4035 Nov 09 '22 at 18:44

score 5 · Answer 3 · answered Jun 26 '13 at 12:41

The behavior of fflush is not defined for input streams (online 2011 standard):

7.21.5.2 The fflush function

Synopsis

1
    #include <stdio.h>
    int fflush(FILE *stream);
Description

2 If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

3 If stream is a null pointer, the fflush function performs this flushing action on all streams for which the behavior is defined above.

Returns

4 The fflush function sets the error indicator for the stream and returns EOF if a write error occurs, otherwise it returns zero.

`fflush` on input streams is not defined in POSIX.1-2001, but **it is** defined in POSIX.1-2008 — iBug, Apr 17 '19 at 14:51

score 2 · Answer 4 · edited Mar 26 '18 at 16:36

2

I faced the same problem while working on LINUX and an alternative solution of this problem can be that you define a dummy character lets say char dummy; and put a scanf() to scan it just before your actual input takes place. This worked for me. I hope it would work for you too.

edited Mar 26 '18 at 16:36

Nikhil Wagh

1,376
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answered Jun 27 '15 at 14:15

Akshdeep Shubham

21
2

Aaron Digulla · Answer 5 · 2013-06-26T15:03:33.270

1

fflush() doesn't do much for input streams but since scanf() never returns this doesn't matter. scanf() blocks because the terminal window doesn't send anything to the C program until you press Enter

You have two options:

Type 10 Enter
Put the terminal into raw mode.

The second option has many drawbacls like you will lose editing capabilities, so I suggest to read the input line by line.

edited Jun 26 '13 at 15:03

answered Jun 26 '13 at 11:38

Aaron Digulla

321,842
108
597
820

score 1 · Answer 6 · answered Apr 27 '14 at 21:59

1

You must include and use __fpurge(whatever you want) instead.

Salute from argentina

answered Apr 27 '14 at 21:59

user3579333

11
1

1

You should explain why you need to include that, instead of just saying you do. – phyrrus9 Apr 27 '14 at 22:17

score 0 · Answer 7 · answered Oct 10 '18 at 19:17

0

Use getchar() instead, after scanf

answered Oct 10 '18 at 19:17

Ahlecksz9

1

score 0 · Answer 8 · answered Jul 10 '20 at 05:38

0

#include<stdio.h>
int main()
{
char ans='y';
int a;
while(ans=='y'||ans=='Y')
    {

     printf("Type a number:-");
     scanf("%d",&a);
     printf("square of number = %d\nwant to enter 
     number again(y/n)?\nANS=",a*a); 
     scanf("%s",&ans);//use %s in place of %c
     }
return 0;
}

answered Jul 10 '20 at 05:38

pheonix

1

1

Please add some description with your answer – SayAz Jul 10 '20 at 07:04

score -1 · Answer 9 · edited Aug 07 '16 at 00:46

-1

By using bzero(); system call in Linux we can flush the previous stored value.
Please read the manual page of bzero(); by typing in terminal man bzero. try this example

#include<stdio.h>
#include<string.h>

int main()
{
  char buf[]={'y'};
  int num;
  while(buf[0]=='y')
  {
    printf("enter number");
    scanf("%d",&num);
    printf("square of %d is %d\n",num,num*num);
    bzero(buf, 1);
    printf("want to enter y/n");
    scanf("%s",&buf[0]);
  }
  return 0;
}

edited Aug 07 '16 at 00:46

Tomer Cohen

374
6
13

answered Jun 06 '16 at 04:58

Raj Ravi

7
2

This code will cause a buffer overflow on the `scanf("%s"` line. Also, `buf[0] = 0;` would be less error-prone than `bzero(buf, 1)`. – M.M Feb 11 '17 at 04:34

fflush() is not working in Linux

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