I am not able to flush stdin. How can I flush stdin in C?

Question

How to flush the stdin??

Why is it not working in the following code snippet?

#include <string.h> #include <stdio.h> #include <malloc.h> #include <fcntl.h> int main() { int i = 0, j = 0, sat; char arg[256]; char *argq; argq = malloc(sizeof(char) * 10); printf("Input the line\n"); i = read(0, arg, sizeof(char) * 9); arg[i - 1] = '\0'; fflush(stdin); i = read(0, argq, sizeof(char) * 5); argq[i - 1] = '\0'; puts(arg); puts(argq); return 0; }

Now if I give the input as 11 characters, only 9 should be read but the remaining two characters in the stdin are not flushed and read again in the argq. Why?

Input: 123 456 789

Output:

123 456 89

Why am I getting this 89 as the output?

You can always create your own function for discarding input characters. If you name it ignore, then you could be closer to the C++ streams. ;-) — Thomas Matthews
– Thomas Matthews, Commented Feb 3, 2010 at 0:26
Does this answer your question? How to completely clear stdin and the \n before scanf() — Toby Speight
– Toby Speight, Commented Jan 19, 2024 at 11:00

jschmier · Accepted Answer · 2010-02-02 21:08:28Z

46

I believe fflush is only used with output streams.

You might try fpurge or __fpurge on Linux. Note that fpurge is nonstandard and not portable. It may not be available to you.

From a Linux fpurge man page: Usually it is a mistake to want to discard input buffers.

The most portable solution for flushing stdin would probably be something along the lines of the following:

int c; while ((c = getchar()) != '\n' && c != EOF);

edited Feb 2, 2010 at 21:08

answered Feb 2, 2010 at 20:37

jschmier

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5 Comments

mob Over a year ago

Nice - didn't know about fpurge()

R.. GitHub STOP HELPING ICE Over a year ago

Or, easier: scanf("%*[^\n]%*c"); You can of course merge this onto the end of your existing scanf format string to discard the remainder of a line after processing part of it with scanf, too..

Ungeheuer Over a year ago

@R.. perhaps you could explain the provided regex in more detail, even though it's been 5 years. Or provide a link to material that can clarify it.

R.. GitHub STOP HELPING ICE Over a year ago

@Ungeheuer: It's not regex. It's the scanf %[ conversion specifier that should be documented with scanf.

chqrlie Over a year ago

@R..GitHubSTOPHELPINGICE: scanf("%*[^\n]%*c"); will not discard the newline if there is no pending character to match the [^\n] character class, ie: if there is just a newline pending in stdin.

jamesdlin · Accepted Answer · 2010-02-02 20:40:45Z

From the comp.lang.c FAQ, see:

How can I flush pending input so that a user's typeahead isn't read at the next prompt? Will fflush(stdin) work?
If fflush won't work, what can I use to flush input?

user1831086 · Accepted Answer · 2012-01-27 01:02:56Z

11

int c; while((c = getchar()) != '\n' && c != EOF);

Is how I'd clear the input buffer.

answered Jan 27, 2012 at 1:02

user1831086

1 Comment

J.G Over a year ago

There might be a potential downside if doing so. Say afterward you want to use fgets read from stdin, which is empty, then you have to firstly type the enter key before the actual typing.

dirkgently · Accepted Answer · 2010-02-02 20:36:13Z

How to flush the stdin??

Flushing input streams is invoking Undefined Behavior. Don't try it.

You can only flush output streams.

Nikolai Fetissov · Accepted Answer · 2010-02-02 20:50:48Z

You are overriding the last element of the input in arg with '\0'. That line should be arg[i]='\0'; instead (after error and boundary checking you are missing.)

Other's already commented of the flushing part.

Zopazz · Accepted Answer · 2022-06-22 14:05:29Z

If you use GLIBC you can just mess with the stdin ptr manually

void flush_stdin(){ unsigned long* tmpstdin = (unsigned long*)stdin; unsigned long* oldbuf = (unsigned long*)*(tmpstdin+4); free((void*)oldbuf); *tmpstdin=(unsigned long)0xfbad2088; tmpstdin+=1; memset(tmpstdin,'\x00',64); }

Sunny127 · Accepted Answer · 2017-12-24 16:18:31Z

You can't clean stdin in Linux without bumping into scenarios that the command will start waiting for input in some cases. The way to solve it is to replace all std::cin with readLineToStdString():

void readLine(char* input , int nMaxLenIncludingTerminatingNull ) { fgets(input, nMaxLenIncludingTerminatingNull , stdin); int nLen = strlen(input); if ( input[nLen-1] == '\n' ) input[nLen-1] = '\0'; } std::string readLineToStdString(int nMaxLenIncludingTerminatingNull) { if ( nMaxLenIncludingTerminatingNull <= 0 ) return ""; char* input = new char[nMaxLenIncludingTerminatingNull]; readLine(input , nMaxLenIncludingTerminatingNull ); string sResult = input; delete[] input; input = NULL; return sResult; }

This will also allow you to enter spaces in std::cin string.

Ngô Thái Sơn · Accepted Answer · 2016-06-03 14:06:18Z

-2

In Windows you can use rewind(stdin) fuction.

answered Jun 3, 2016 at 14:06

Ngô Thái Sơn

71 bronze badge

1 Comment

HeavenHM Over a year ago

why don't you post tested code snippet along with your code. Because rewind is not intended to do what OP want achieve.

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