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Re: [libvirt] [PATCH] safe{read, write}: Document usage with nonblocking FD



On Thu, Jan 17, 2013 at 08:55:14AM +0100, Michal Privoznik wrote:
> Currently, whenever somebody calls saferead() on nonblocking FD
> (safewrite() is totally interchangeable for purpose of this message)
> he might get wrong return value. For instance, in the first iteration
> some data is read. The number of bytes read is stored into local
> variable 'nread'. However, in next iterations we can get -1 from
> read() with errno == EAGAIN, in which case the -1 is returned despite
> fact some data has already been read. So the caller gets confused.
> Bare read() should be used for nonblocking FD.
> ---
>  src/util/virutil.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/src/util/virutil.c b/src/util/virutil.c
> index b36e9d3..bc970a4 100644
> --- a/src/util/virutil.c
> +++ b/src/util/virutil.c
> @@ -98,7 +98,9 @@ verify(sizeof(gid_t) <= sizeof(unsigned int) &&
>  
>  #define VIR_FROM_THIS VIR_FROM_NONE
>  
> -/* Like read(), but restarts after EINTR */
> +/* Like read(), but restarts after EINTR.
> + * Doesn't play nicely with nonblocking FD and EAGAIN,
> + * in which case you want to use bare read() */
>  ssize_t
>  saferead(int fd, void *buf, size_t count)
>  {
> @@ -118,7 +120,9 @@ saferead(int fd, void *buf, size_t count)
>      return nread;
>  }
>  
> -/* Like write(), but restarts after EINTR */
> +/* Like write(), but restarts after EINTR.
> + * Doesn't play nicely with nonblocking FD and EAGAIN,
> + * in which case you want to use bare write() */
>  ssize_t
>  safewrite(int fd, const void *buf, size_t count)
>  {

ACK

Or even use virSocket() if the FD is related to a socket rather than a
plain file or pipe.

Daniel
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