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Date: Mon, 29 Dec 2014 08:13:14 +0100
From: Marco Atzeri <marco DOT atzeri AT gmail DOT com>
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To: cygwin AT cygwin DOT com
Subject: Re: bug: struct cmsghdr does not match system on cygwin64
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On 12/29/2014 6:03 AM, Kenton Varda wrote:
> Hello Cygwin,
>
> Cygwin defines 'struct cmsghdr' (see recvmsg(2)) as follows (cygwin/socket.h):
>
> struct cmsghdr
> {
>    socklen_t             cmsg_len;       /* Length of cmsghdr + data     */
>    int                   cmsg_level;     /* Protocol                     */
>    int                   cmsg_type;      /* Protocol type                */
> };
>
> Unfortunately, Winsock defines its equivalent type as follows (ws2def.h):
>
> typedef struct _WSACMSGHDR {
>      SIZE_T      cmsg_len;
>      INT         cmsg_level;
>      INT         cmsg_type;
>      /* followed by UCHAR cmsg_data[] */
> } WSACMSGHDR, *PWSACMSGHDR, FAR *LPWSACMSGHDR;
>
> As you can see, on a 64-bit build, the cmsg_len is 64 bits in the
> Winsock struct. However, Cygwin's socklen_t is always 32 bits,
> therefore cmsg_len is 32 bits in the Cygwin struct.

http://pubs.opengroup.org/onlinepubs/7908799/xns/syssocket.h.html


> Cygwin's recvmsg() calls WSARecvMsg() passing the control message
> buffer pointer verbatim, expecting the system to fill the buffer
> correctly. The system dutifully fills the buffer with control messages
> that are not legible using Cygwin's definition of the struct, leaving
> the calling application with apparent garbage data.
>
> sendmsg() likely has a similar problem, though I haven't checked.
>
> It seems like the best solution is to change the type of cmsg_len to
> size_t. While this would technically violate standards (I think?), it
> turns out the Linux headers define it this way, so presumably no one
> will mind.


https://cygwin.com/cygwin-ug-net/programming.html
http://technet.microsoft.com/en-us/library/bb496995.aspx

If I am not wrong, for 64bit,
on windows sizeof (SIZE_T) =4
while on cygwin sizeof(size_t)=8

> The other option is to perform some sort of translation of the control
> message buffer inside sendmsg() and recvmsg(), but that would
> obviously be painful and error-prone.
>
> AFAICT, this bug breaks all users of the IP_PKTINFO and IPV6_PKTINFO
> socket options on cygwin64.
>
> -Kenton
>

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