Broken getchar() and putchar() macros?

Corinna Vinschen
Wed Aug 2 12:37:00 GMT 2017

On Aug  2 13:36, Sebastian Huber wrote:
> Hello,
> we have via <stdio.h>
> #define    getchar()    getc(stdin)
> #define    putchar(x)    putc(x, stdout)
> which is
> #ifndef __CYGWIN__
> #ifndef lint
> #define    getc(fp)    __sgetc_r(_REENT, fp)
> #define putc(x, fp)    __sputc_r(_REENT, x, fp)
> #endif /* lint */
> #endif /* __CYGWIN__ */
> #endif /* __cplusplus */
> which is mostly
> #define __sgetc_r(__ptr, __p) __sgetc_raw_r(__ptr, __p)
> #define __sputc_r(__ptr, __c, __p) __sputc_raw_r(__ptr, __c, __p)
> which is
> /*
>  * The __sfoo macros are here so that we can
>  * define function versions in the C library.
>  */
> #define       __sgetc_raw_r(__ptr, __f) (--(__f)->_r < 0 ? __srget_r(__ptr,
> __f) : (int)(*(__f)->_p++))
> _ELIDABLE_INLINE int __sputc_r(struct _reent *_ptr, int _c, FILE *_p) {
> #ifdef __SCLE
>     if ((_p->_flags & __SCLE) && _c == '\n')
>       __sputc_r (_ptr, '\r', _p);
> #endif
>     if (--_p->_w >= 0 || (_p->_w >= _p->_lbfsize && (char)_c != '\n'))
>         return (*_p->_p++ = _c);
>     else
>         return (__swbuf_r(_ptr, _c, _p));
> }
> which modifies FILE members without protection of the FILE lock!
> [...]
> What is the purpose of these macros?

If you search for them in newlib, you'll notice that the macros are used
from a couple of places in libc/stdio.

The fact that they are directly called on non-Cygwin platforms is probably
historical, as usual.  They are reentrant.  As such, and under the
assumption that we're on embedded platforms without preemptive
multi-tasking, the macros work nicely and are faster than the function

I can think of four ways to solve this:

1.  Just add RTEMS to the exceptions, same as Cygwin.

2. Replace

    #define    getc(fp)    __sgetc_r(_REENT, fp)
    #define putc(x, fp)    __sputc_r(_REENT, x, fp)


    #define    getc_unlocked(fp)    __sgetc_r(_REENT, fp)
    #define putc_unlocked(x, fp)    __sputc_r(_REENT, x, fp)

  and let getc/putc simply call the functions.

3. Add a newlib setting to sys/config.h and use that in stdio.h, rather
   than excluding targets explicitely.  Best is probably an opt-in

4. Move the macros to libc/stdio/local.h and don't use them from
   user space at all.  This may be a bit harsh on some embedded
   targets, but actually I don't know.  Most targets have space
   constraints and a function would be more prudent then, I guess.


Corinna Vinschen
Cygwin Maintainer
Red Hat
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