3.4.6-1 shm_open always returns -1, errno EINVAL

3.4.6-1 shm_open always returns -1, errno EINVAL

[Matthew Rickard]

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

Not sure if relevant, but I am currently running cygserver, with a 
default config.

Re: 3.4.6-1 shm_open always returns -1, errno EINVAL

[Csaba Raduly]

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)

Re: 3.4.6-1 shm_open always returns -1, errno EINVAL

[Matthew Rickard]

That's perfect, thank you!

On 12/03/2023 9:35 pm, Csaba Raduly wrote:
> Hi Matthew,
...
> 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

RE: [EXTERNAL] Re: 3.4.6-1 shm_open always returns -1, errno EINVAL

[Lavrentiev, Anton (NIH/NLM/NCBI) [C]]

> > returns 3 and sets errno to zero.

Note that "setting errno to zero" is not guaranteed in case of a successful completion of a library function or a system call.
Generally, "errno" reflects an error that occurred last when such a call failed (so in other words, in case of a successful
completion, "errno" can still carry over an error from a previously failed call).  In this particular use case, it looks like
"errno" was clear from the get go because nothing was called prior to "shm_open()", and so it remained so when the call returned (successfully).

HTH,

Anton Lavrentiev
Contractor NIH/NLM/NCBI