SAS/C Library Reference, Vol. 1

ungetc -- Push Back an Input Character

SYNOPSIS

 #include <stdio.h>

 int ungetc(int c, FILE *f);

DESCRIPTION

Use ungetc to undo the effect of getc. The ungetc function backs up the file input pointer so that the next call to an input routine returns c. The call ungetc(EOF, f) is valid but has no effect.

For a binary file, a successful call to ungetc moves the file position back to the previous character, unless the file is positioned at the beginning.

RETURN VALUE

ungetc returns c if its operation was successful or EOF if c cannot be pushed back. You may not be able to push back more than a single character, depending on the file contents and attributes.

PORTABILITY

Portable use of ungetc is limited to one character. Using ungetc to push back multiple characters without an intervening read is not portable.

EXAMPLE

This example reads a line from the terminal and separates it into words using ungetc. Note that this operation could be done more easily using scanf:

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

  int wordcnt = 0;

  static int skipspace(void);
  static int printword(void);

  main()
  {
     char line[80];

     puts("Enter a short line of text:");
     for(;;) {
        if (skipspace() == 0)
           break;

           /* Skip white space; stop at end of line.             */
        if (printword() == 0)      /* Print the next word.       */
           break;
     }
     if (wordcnt == 0) puts("There were no words in that line.n");
        exit(0);
  }

  static int skipspace(void) {

     /* Read white space characters from standard input.  Use    */
     /* ungetc() to put back any nonwhite space character        */
     /* found. If a new line is read, stop reading and return 0. */

     int ch;

     for (;;) {
        ch = getchar();
        if (ch == EOF || ch == 'n') return 0;
        if (!isspace(ch)) break;
     }

        /* Put back nonspace for printword to read.              */
     ungetc(ch, stdin);
     return 1;
  }

  static int printword(void) {
     int ch;
     if (wordcnt == 0)
        puts("Words found in input:");
     ++wordcnt;
     for(;;) {
        ch = getchar();
        if (ch == EOF || ch == 'n') return 0;
        if (!isspace(ch)) putchar(ch);
        else return 1;
     }
  }

RELATED FUNCTIONS

fgetc, getc