How to clear input buffer in C

Question

I have the following program   int main int argc  char  argv        char ch1  ch2    printf  Input the first character        Line 1   scanf   c    amp ch1      printf  Input the second character        Line 2   ch2   getchar       printf  ch1  c  ASCII code    d n   ch1  ch1     printf  ch2  c  ASCII code    d n   ch2  ch2      system  PAUSE        return 0      As the author of the above code have explained  The program will not work properly because at Line 1  when the user presses Enter  it will leave in the input buffer 2 character  Enter key  ASCII code 13  and  n  ASCII code 10   Therefore  at Line 2  it will read the  n and will not wait for the user to enter a character   OK  I got this  But my first question is  Why the second getchar    ch2   getchar     does not read the Enter key  13   rather than  n character   Next  the author proposed 2 ways to solve such probrems    use fflush   write a function like this   void clear  void          while   getchar        n          This code worked actually  But I cannot explain myself how it works  Because in the while statement  we use getchar        n   that means read any single character except   n   if so  in the input buffer still remains the   n  character

User · Accepted Answer

The program will not work properly because at Line 1, when the user presses Enter, it will leave in the input buffer 2 character: Enter key (ASCII code 13) and \n (ASCII code 10). Therefore, at Line 2, it will read the \n and will not wait for the user to enter a character.

The behavior you see at line 2 is correct, but that's not quite the correct explanation. With text-mode streams, it doesn't matter what line-endings your platform uses (whether carriage return (0x0D) + linefeed (0x0A), a bare CR, or a bare LF). The C runtime library will take care of that for you: your program will see just '\n' for newlines.

If you typed a character and pressed enter, then that input character would be read by line 1, and then '\n' would be read by line 2. See I'm using scanf %c to read a Y/N response, but later input gets skipped. from the comp.lang.c FAQ.

As for the proposed solutions, see (again from the comp.lang.c FAQ):

which basically state that the only portable approach is to do:

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

Your getchar() != '\n' loop works because once you call getchar(), the returned character already has been removed from the input stream.

Also, I feel obligated to discourage you from using scanf entirely: Why does everyone say not to use scanf? What should I use instead?

User · Answer

Try this  stdin- gt  IO read ptr   stdin- gt  IO read end

User · Answer

I am surprised nobody mentioned this  scanf  quot      n  quot

User · Answer

you can try  scanf   c  c    amp ch1     where   c accepts and ignores the newline  one more method instead of fflush stdin  which invokes undefined behaviour you can write  while  getchar       n      don t forget the semicolon after while loop

User · Answer

A portable way to clear up to the end of a line that you ve already tried to read partially is   int c   while    c   getchar         n   amp  amp  c    EOF         This reads and discards characters until it gets  n which signals the end of the file  It also checks against EOF in case the input stream gets closed before the end of the line  The type of c must be int  or larger  in order to be able to hold the value EOF   There is no portable way to find out if there are any more lines after the current line  if there aren t  then getchar will block for input

User · Answer

I encounter a problem trying to implement the solution   while   c   getchar         n   amp  amp  c    EOF        I post a little adjustment  Code B  for anyone who maybe have the same problem   The problem was that the program kept me catching the   n  character  independently from the enter character  here is the code that gave me the problem   Code A  int y   printf   nGive any alphabetic character in lowercase      while   y   getchar         n   amp  amp  y    EOF      continue    printf   n c n   toupper y      and the adjustment was to  catch  the  n-1  character just before the conditional in the while loop be evaluated  here is the code   Code B  int y  x   printf   nGive any alphabetic character in lowercase      while   y   getchar         n   amp  amp  y    EOF      x   y    printf   n c n   toupper x      The possible explanation is that for the while loop to break  it has to assign the value   n  to the variable y  so it will be the last assigned value   If I missed something with the explanation  code A or code B please tell me  I   m barely new in c   hope it helps someone

User · Answer

Short  portable and declared in stdio h  stdin   freopen NULL  r  stdin     Doesn t get hung in an infinite loop when there is nothing on stdin to flush like the following well know line   while   c   getchar         n   amp  amp  c    EOF        A little expensive so don t use it in a program that needs to repeatedly clear the buffer   Stole from a coworker

User · Answer

You can do it  also  this way   fseek stdin 0 SEEK END

User · Answer

The lines   int ch  while   ch   getchar         n   amp  amp  ch    EOF          doesn t read only the characters before the linefeed    n     It reads all the characters in the stream  and discards them  up to and including the next linefeed  or EOF is encountered    For the test to be true  it has to read the linefeed first  so when the loop stops  the linefeed was the last character read  but it has been read   As for why it reads a linefeed instead of a carriage return  that s because the system has translated the return to a linefeed   When enter is pressed  that signals the end of the line    but the stream contains a line feed instead since that s the normal end-of-line marker for the system   That might be platform dependent   Also  using fflush   on an input stream doesn t work on all platforms  for example it doesn t generally work on Linux

User · Answer

unsigned char a 0  if kbhit         a getch        while kbhit            getch      cout lt  lt hex lt  lt  static cast lt unsigned int - gt  a   amp  0xFF  lt  lt endl    -or-  use maybe use  getch nolock

User · Answer

But I cannot explain myself how it works  Because in the while statement  we use getchar        n   that means read any single character except   n    if so  in the input buffer still remains the   n  character      Am I misunderstanding something     The thing you may not realize is that the comparison happens after getchar   removes the character from the input buffer   So when you reach the   n   it is consumed and then you break out of the loop

User · Answer

Another solution not mentioned yet is to use  rewind stdin

[c] How to clear input buffer in C?

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