ref: d02dde25b6faa53013cbbb9894f1539645093ee5
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; }