fgetc.c

/* ################################################################### */
/* Copyright 2015, Pierre Gentile (p.gen.progs@gmail.com)              */
/*                                                                     */
/* This Source Code Form is subject to the terms of the Mozilla Public */
/* License, v. 2.0. If a copy of the MPL was not distributed with this */
/* file, You can obtain one at https://mozilla.org/MPL/2.0/.           */
/* ################################################################### */

/* ************************************************************************* */
/* Custom fgetc/ungetc implementation able to unget more than one character. */
/* ************************************************************************* */

#include <stdio.h>
#include "fgetc.h"

enum
{
  GETC_BUFF_SIZE = 16
};

static unsigned char getc_buffer[GETC_BUFF_SIZE] = { '\0' };

static long next_buffer_pos = 0; /* Next free position in the getc buffer. */

/* ======================================== */
/* Gets a (possibly pushed-back) character. */
/* ======================================== */
int
my_fgetc(FILE *input)
{
  int c;

  if (next_buffer_pos > 0)
    return getc_buffer[--next_buffer_pos];
  else
  {
    errno = 0;
    c     = fgetc(input);

    while (c == EOF && errno == EAGAIN)
    {
      errno = 0;
      c     = fgetc(input);
    }

    return c;
  }
}

/* =============================== */
/* Pushes character back on input. */
/* =============================== */
int
my_ungetc(int c, FILE *input)
{
  int rc;

  if (next_buffer_pos >= GETC_BUFF_SIZE)
    rc = EOF;
  else
  {
    rc = getc_buffer[next_buffer_pos++] = (unsigned char)c;

    if (feof(input))
      clearerr(input); /* No more EOF. */
  }

  return rc;
}