How to clear buffer in receiving multiple strings?

Question

If I enter str1 longer than length 10, the rest of it remains in the buffer and gets entered into my str2. How to clear the buffer before str2, so I can input it?

#include <stdio.h>
int main(void)
{
    char str1[10];
    char str2[10]; 
    fgets(str1,10,stdin);
    fgets(str2,10,stdin);

    puts(str1);
    puts(str2);
    return 0;
}

`fflush` is not defined for `stdin`. See http://www.c-faq.com/stdio/stdinflush.html and http://www.c-faq.com/stdio/stdinflush2.html — jamesdlin, Jul 13 '13 at 09:54

Nithin Bhaskar · Answer 1 · 2013-07-13T12:15:34.963

2

After fgets(str1,10,stdin); do

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

This would clear the input buffer after 'str1' is read.

So your code should be

#include <stdio.h>
int main()
{
    char str1[10];
    char str2[10]; 
    int c;
    str1[0]=0;
    str2[0]=0;
    fgets(str1,10,stdin);
    if( (str1[0]!=0) && (!strrchr(str1,'\n')) )
        while((c = getchar()) != '\n' && c != EOF);
    fgets(str2,10,stdin);
    puts(str1);
    puts(str2);
    return 0;
}

edited Jul 13 '13 at 12:15

answered Jul 13 '13 at 10:02

Nithin Bhaskar

686
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What if there's a `'\n'` within the first 10 chars of input? – Jim Balter Jul 13 '13 at 10:28
@JimBalter Thanks for pointing it out. Made the necessary edit. – Nithin Bhaskar Jul 13 '13 at 11:07
Still, you don't test the return from `fgets`; you may access an uninitialized `str1` (e.g. for empty input). – Jens Jul 13 '13 at 11:23
@Jens Thanks for the heads up. Fixed it. – Nithin Bhaskar Jul 13 '13 at 12:16
@NithinBhaskar is that ` ; ` after while loop intentional ? clear me if I am missing something silly here – exexzian Jul 13 '13 at 12:33
@Bingo the ';' will cause the program to loop repeatedly in the while loop as long as the condition is satisfied. In this code, this will make sure that the characters remaining in the buffer after 'str1' is read will be flushed out of the buffer. – Nithin Bhaskar Jul 13 '13 at 12:39
@NithinBhaskar ooppss I have been taking it other way for years :) thanks and have already upvoted you +1 – exexzian Jul 13 '13 at 12:54
@Bingo refer this link for more info http://stackoverflow.com/questions/8706139/what-happens-when-one-places-a-semi-colon-after-a-while-loops-condition – Nithin Bhaskar Jul 13 '13 at 13:03

wildplasser · Answer 2 · 2013-07-13T14:42:44.370

A different way: avoid fgets(), and read characters one by one. This allows you to handle all the conditions inside one loop:

int main(void)
{
    char str1[12];
    char str2[13];
    size_t pos;
    int ch;

    for (pos=0;;) {
      ch = getc(stdin);
      if (ch == '\n' || ch == EOF ) break;
      if (pos < sizeof str1 -1) str1[pos++] = ch;
      }
    str1[pos] = 0;

    for (pos=0;;) {
      ch = getc(stdin);
      if (ch == '\n' || ch == EOF ) break;
      if (pos < sizeof str2 -1) str2[pos++] = ch;
      }
    str2[pos] = 0;

    printf( "str1='%s', str2=%s'\n", str1, str2);

    return 0;
}

score -1 · Answer 3 · answered Jul 13 '13 at 11:56

#include <stdio.h>

#define MAX_LEN 9
#define READBUF_LEN 4092

int main(void)
{
    char str1[MAX_LEN+1];
    char str2[MAX_LEN+1]; 
    char readbuf[READBUF_LEN+1];

    fgets(readbuf,READBUF_LEN,stdin);
    strncpy(str1, readbuf,MAX_LEN);
    str1[MAX_LEN]='\0';

    fgets(readbuf,READBUF_LEN,stdin);
    strncpy(str2, readbuf,MAX_LEN);
    str2[MAX_LEN]='\0';

    puts(str1);
    puts(str2);
    return 0;
}

Yeah, I know, what if someone enters more than 4092 characters... There's a big chance of that happening.

How to clear buffer in receiving multiple strings?

3 Answers3