### ungetc -- Push Character onto Input Stream

Format

>     #include <stdio.h>
>     int ungetc(int c, FILE *stream);

Language Level: ANSI, POSIX, XPG4
ungetc pushes the unsigned character c back onto the given input stream. However, only one sequential character or consecutive character is guaranteed to be pushed back onto the input stream if you call ungetc consecutively. The stream must be open for reading. A subsequent read operation on the stream starts with c. The character c cannot be the EOF character.

Characters placed on the stream by ungetc will be erased if fseek, fsetpos, rewind, or fflush is called before the character is read from the stream.

Return Value
ungetc returns the integer argument c converted to an unsigned char, or EOF if c cannot be pushed back.

Example
In this example, the while statement reads decimal digits from an input data stream by using arithmetic statements to compose the numeric values of the numbers as it reads them. When a nondigit character appears before the end of the file, ungetc replaces it in the input stream so that later input functions can process it.

    #include <stdio.h>
    #include <ctype.h>

    int main(void)
    {
       FILE *stream;
       int ch;
       unsigned int result = 0;

       while ((ch = getc(stdin)) != EOF && isdigit(ch))
          result = result * 10 + ch - '0';
       if (ch != EOF)
          ungetc(ch,stdin);
             /* Put the nondigit character back */
       printf("Input number : %d\n", result);
       return 0;

       /************************************************
          For the following input:

          12345s

          The output should be:

          Input number : 12345
       ************************************************/
    }

getc - getchar -- Read a Character
fflush -- Write Buffer to File
fseek -- Reposition File Position
fsetpos -- Set File Position
putc - putchar -- Write a Character
rewind -- Adjust Current File Position
_ungetch -- Push Character Back to Keyboard
<stdio.h>