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Date: Thu, 13 Sep 2018 14:25:15 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] improve error handling of ttyname_r and isatty
On Thu, Sep 13, 2018 at 09:27:55AM -0700, Benjamin Peterson wrote:
> POSIX allows ttyname(_r) and isatty to return EBADF if passed file descriptor is
> invalid.
> ---
> src/unistd/isatty.c | 6 +++++-
> src/unistd/ttyname_r.c | 5 ++++-
> 2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/src/unistd/isatty.c b/src/unistd/isatty.c
> index c8badaf5..3d5213c6 100644
> --- a/src/unistd/isatty.c
> +++ b/src/unistd/isatty.c
> @@ -1,9 +1,13 @@
> #include <unistd.h>
> +#include <errno.h>
> #include <sys/ioctl.h>
> #include "syscall.h"
>
> int isatty(int fd)
> {
> struct winsize wsz;
> - return !__syscall(SYS_ioctl, fd, TIOCGWINSZ, &wsz);
> + unsigned long r = __syscall_ret(__syscall(SYS_ioctl, fd, TIOCGWINSZ, &wsz));
> + if (r == 0) return 1;
> + if (errno != EBADF) errno = ENOTTY;
> + return 0;
> }
__syscall_ret(__syscall(...)) is just an ugly way of writing
syscall(). :-)
But the whole reason __syscall_ret exists is for the sake of setting
errno via a tailcall after the __syscall returns. When you have to
patch up the value, rather than shuffling it in and out of errno and
breaking the tailcall, a cleaner method is something like:
int r = __syscall(...);
if (r && r!=-EBADF) r=-ENOTTY;
return __syscall_ret(r);
> diff --git a/src/unistd/ttyname_r.c b/src/unistd/ttyname_r.c
> index 33aa4ae1..3f836a6a 100644
> --- a/src/unistd/ttyname_r.c
> +++ b/src/unistd/ttyname_r.c
> @@ -10,7 +10,10 @@ int ttyname_r(int fd, char *name, size_t size)
> char procname[sizeof "/proc/self/fd/" + 3*sizeof(int) + 2];
> ssize_t l;
>
> - if (!isatty(fd)) return ENOTTY;
> + if (!isatty(fd)) {
> + if (errno != EBADF) errno = ENOTTY;
> + return errno;
> + }
>
> __procfdname(procname, fd);
> l = readlink(procname, name, size);
> --
> 2.17.1
With isatty setting errno reasonably, there's no need for errno fixup
here. Just::
if (!isatty(fd)) return errno;
suffices.
Rich
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