lib/fwritable.c

/* Retrieve information about a FILE stream.
   Copyright (C) 2007-2026 Free Software Foundation, Inc.

   This file is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation, either version 3 of the
   License, or (at your option) any later version.

   This file is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

#include <config.h>

/* Specification.  */
#include "fwritable.h"

#include "stdio-impl.h"

#if defined EPLAN9                  /* Plan9 */
# include <fcntl.h>
#endif

/* This file is not used on systems that have the __fwritable function,
   namely glibc >= 2.2, Solaris >= 7, UnixWare >= 7.1.4.MP4, Cygwin >= 1.7.34,
   Android API >= 23, musl libc, Haiku >= hrev58760.  */

bool
fwritable (FILE *fp)
{
  /* Most systems provide FILE as a struct and the necessary bitmask in
     <stdio.h>, because they need it for implementing getc() and putc() as
     fast macros.  */
#if defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
  /* GNU libc, BeOS, Haiku, Linux libc5 */
  return (fp->_flags & _IO_NO_WRITES) == 0;
#elif defined __sferror || defined __OpenBSD__ || defined __DragonFly__ || defined __ANDROID__
  /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin < 1.7.34, Minix 3, Android */
  return (fp_->_flags & (__SRW | __SWR)) != 0;
#elif defined __EMX__               /* emx+gcc */
  return (fp->_flags & (_IORW | _IOWRT)) != 0;
#elif defined __minix               /* Minix */
  return (fp->_flags & _IOWRITE) != 0;
#elif defined _IOERR                /* AIX, HP-UX, Solaris, OpenServer, UnixWare, mingw, MSVC, NonStop Kernel, OpenVMS */
  return (fp_->_flag & (_IORW | _IOWRT)) != 0;
#elif defined __QNX__               /* QNX */
  return (fp->_Mode & 0x2 /* _MOPENW */) != 0;
#elif defined __MINT__              /* Atari FreeMiNT */
  return fp->__mode.__write;
#elif defined EPLAN9                /* Plan9 */
  int fd = fp->fd;
  if (fd >= 0)
    {
      int flags = fcntl (fd, F_GETFL, NULL);
      if (flags >= 0)
        {
          flags &= O_ACCMODE;
          return (flags == O_WRONLY || flags == O_RDWR);
        }
    }
  return 0;
#else
# error "Please port gnulib fwritable.c to your platform! Look at the definition of fopen, fdopen on your system, then report this to bug-gnulib."
#endif
}