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Date: Thu, 8 Oct 2020 18:23:34 +0200
From: Markus Wichmann <>
To: John Scott <>
Subject: Re: fputs/fputc doesn't set errno on failure

On Thu, Oct 08, 2020 at 10:55:47AM -0400, John Scott wrote:
> Hi,
> I'm not subscribed, please keep me CC'd.

Here's hoping I chose the correct option this time. I went with "group
reply" but directed the message to you, which should CC the list.

> In this corner case, errno is unset despite fputs giving an error. I'm on Debian testing using musl-gcc 1.2.1.
> #define _POSIX_C_SOURCE 200809L
> #include <assert.h>
> #include <errno.h>
> #include <stdio.h>
> int main(void) {
> 	FILE *f = fopen("/", "r");
> 	if(f == NULL) {
> 		perror("fopen()");
> 	}
> 	if(fputs("Hello world\n", f) == EOF) {
> 		assert(errno);
> 		perror("fputs()");
> 	}
> }
> With glibc this prints EBADF for fputs.

Interesting. The error occurs because f is only open for reading, so
__towrite() delivers an error without setting errno. Though my POSIX
manpage does not say which error to return for this. EBADF speaks about
the FD underlying the stream, and in this case that one happens to be
the case, but the error occurs solely because F_NOWR is set in the file.
The FD does not come into play. On the other hand, using fdopen() such
that the mode given to open() and the string given to fdopen() are
discordant is undefined behavior. So we might as well assume that F_NOWR
means O_RDONLY. Besides, no other documented error on that page really
fits this case.

I suggest adding error codes to the error paths in __towrite() and
__toread() (which has the same problem).

> The wiki page about writing tests [1], which I thought this would be a good candidate for, has the 404 link

It would appear Szabolcs' repo is broken. I cannot access .../git/, but
accessing .../ yields a landing page that references the non-working
pages given here. I managed to find a mirror here:

But that one also reference Szabolcs' repo as the original.


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