ref: 5ac3a0d4c1a9d7e4d3912c4770084b885007aca6
dir: /src/libc/stdio/ungetc.c/
#include <stdio.h>
/**
* ungetc() - push back one character to a input stream
*
* Context: A stream just opened has rp, wp, lp and buf pointing to NULL,
* in the same way that a closed stream. In both cases we have to
* return EOF, so the check fp->rp == fp->buf detects both cases.
* An input stream can be either a read only input or a read and
* write input in read state, and we can detect the write state
* when wp does not point to the beginning of buf. _IOSTRG is used
* in sprintf/sscanf functions, where it is possible rp points to
* a constant string, so we cannot write back c, but it is safe
* because in those functions we are going to push back always
* the same character that the one contained in the string.
*/
int
ungetc(int c, FILE *fp)
{
if (c == EOF)
return EOF;
if ((fp->flags & _IOWRITE) != 0)
return EOF;
if (fp->rp == fp->buf || fp->wp != fp->buf)
return EOF;
--fp->rp;
if ((fp->flags & _IOSTRG) == 0)
*fp->rp = c;
fp->flags &= ~_IOEOF;
return c;
}