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Date:	Sun, 12 Mar 2023 11:35:26 +0100
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Subject:	Re: 3.4.6-1 shm_open always returns -1, errno EINVAL
To:	Matthew Rickard <rickard DOT matt AT gmail DOT com>, cygwin list <cygwin AT cygwin DOT com>
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From:	Csaba Raduly via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Csaba Raduly <rcsaba AT gmail DOT com>
Sender:	"Cygwin" <cygwin-bounces+archive-cygwin=delorie DOT com AT cygwin DOT com>

Hi Matthew,

On Sun, 12 Mar 2023 at 07:44, Matthew Rickard  wrote:
>
> Hi all,
>
> Cygwin 3.4.6-1 shm_open seems to reject all calls, returning  the value
> -1 and setting errno to 22 EINVAL.
>
> For example, this program:
>
> #include <sys/mman.h>
> #include <fcntl.h>
> #include <errno.h>
> #include <stdio.h>
>
> int main() {
>    int res = shm_open("123", O_CREAT | O_RDWR, 0666);
>    int error = errno;
>    printf("res=%d errno=%d\n", res, errno);
>    if (error == EINVAL)
>      printf("That's EINVAL\n");
>    return 0;
> }
>
> Says:
>
> $ gcc -Og -o tiny tiny.c; ./tiny
> res=-1 errno=22
> That's EINVAL
>

https://man7.org/linux/man-pages/man3/shm_open.3.html  says:

For portable use, a shared memory object should be identified by
       a name of the form /somename; that is, a null-terminated string
       of up to NAME_MAX (i.e., 255) characters consisting of an initial
       slash, followed by one or more characters, none of which are
       slashes.

Changing the shm_open call to

shm_open("/123", O_CREAT | O_RDWR, 0666);

returns 3 and sets errno to zero.

Csaba
-- 
You can get very substantial performance improvements
by not doing the right thing. - Scott Meyers, An Effective C++11/14 Sampler
So if you're looking for a completely portable, 100% standards-conformant way
to get the wrong information: this is what you want. - Scott Meyers (C++TDaWYK)

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