@node open
@subsection @code{open}
@findex open

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9799919799/functions/open.html}

Gnulib module: open, fchdir
@mindex open
@mindex fchdir

Portability problems fixed by the Gnulib module @code{open}:
@itemize
@item
Some platforms do not support @code{O_CLOEXEC}:
@c See posix-headers/fcntl.texi for the list.
Mac OS X 10.6, FreeBSD 8.4, NetBSD 5.1, OpenBSD 4.9, Minix 3.1.8, AIX 7.1, HP-UX 11, Solaris 10, Cygwin 1.7.x, mingw, MSVC 14.
@item
Some platforms do not support @code{O_DIRECTORY}:
glibc 2.0, Mac OS X 10.4, FreeBSD 7.4, NetBSD 4.0.1, OpenBSD 4.9, HP-UX 11, Solaris 10, Cygwin 1.5.x, mingw, MSVC 14.
@item
On platforms where @code{off_t} is a 32-bit type, @code{open} may not work
correctly with files 2 GiB and larger.  @xref{Large File Support}.
@item
This function does not fail when the file name argument ends in a slash
and (without the slash) names a nonexistent file or a file that is not a
directory, on some platforms:
macOS 14, FreeBSD 7.2, AIX 7.1, HP-UX 11.00, Solaris 9.
@item
@mindex nonblocking
This function does not support the @code{O_NONBLOCK} flag when it is defined
by the gnulib module @code{nonblocking} on some platforms:
mingw, MSVC 14.
@item
On Windows platforms (excluding Cygwin), this function does usually not
recognize the @file{/dev/null} filename.
@end itemize

Portability problems fixed by the Gnulib module @code{fchdir}:
@itemize
@item
On Windows platforms (excluding Cygwin), this function fails to open a
read-only descriptor for directories.
@end itemize

Portability problems not fixed by Gnulib:
@itemize
@item
The Gnulib replacement for @code{O_CLOEXEC} is not atomic, and so is
not safe in the presence of multiple threads or signal handlers.
@item
The Gnulib replacement for @code{O_SEARCH} may require read access
instead of search/execute access.
@item
On some platforms, @code{O_SEARCH} may reject directories
where you have search access even though @code{O_SEARCH} is not replaced:
macOS 12.6.
@item
When the file does not exist, POSIX says
the behavior of @code{O_CREAT | O_DIRECTORY} is unspecified.
Most platforms fail with @code{EINVAL} in this case.
However, in Linux kernels 6.3 and earlier, this function
creates a regular file and then either returns successfully
(Linux kernel 5.6 and earlier) or fails with @code{ENOTDIR}
(Linux kernels 5.7--6.3).
@item
@code{open ("symlink", O_CREAT ...)} fails when the argument points to a
nonexistent file in an existing directory on some platforms:
@c https://dev.haiku-os.org/ticket/18355
Haiku.
@item
@code{open ("symlink", O_NOFOLLOW ...)} fails with @code{errno} set to
@code{EMLINK} instead of the POSIX-required @code{ELOOP} on some
platforms:
FreeBSD 14.2.
@item
@code{open ("symlink", O_NOFOLLOW ...)} fails with @code{errno} set to
@code{EFTYPE} instead of the POSIX-required @code{ELOOP} on some
platforms:
NetBSD 10.0.
@item
On Windows, this function returns a file handle in @code{O_TEXT} mode by
default; this means that it translates @code{'\n'} to CR/LF by default.  Use the
@code{O_BINARY} flag if you need reliable binary I/O.
@end itemize
