Problem with scatter/gather IO

Problem with scatter/gather IO

[Randi Botse]

Hi,

I will read/write file with readv() and writev(), I have problem to
use that since the struct iovec will be a variable-length array, if i
have code.

struct foo {
    unsigned char data[100];
    unsigned char another_data[28];
};
unsigned len;
....

/* calculate len */
/* use len as array's member size */
struct iovec iov[len];
printf("len is: %i\n", len);
readv(fd, iov, len);

I got: BAD ADDRESS, and if I remove the printf() then I will get
INVALID ARGUMENT. What the possible problem with that?

Thanks,

Re: Problem with scatter/gather IO

[Leon Romanovsky]

On Wed, Jul 13, 2011 at 14:54, Randi Botse <nightdecoder@gmail.com> wrote:
>
> Hi,
>
> I will read/write file with readv() and writev(), I have problem to
> use that since the struct iovec will be a variable-length array, if i
> have code.
>
> struct foo {
>    unsigned char data[100];
>    unsigned char another_data[28];
> };
> unsigned len;
> ....
>
> /* calculate len */
> /* use len as array's member size */
> struct iovec iov[len];
> printf("len is: %i\n", len);
> readv(fd, iov, len);
>
> I got: BAD ADDRESS, and if I remove the printf() then I will get
> INVALID ARGUMENT. What the possible problem with that?
>
> Thanks,
> --

Did you try "man readv" ?
You didn't initialized iovec struct.
http://pubs.opengroup.org/onlinepubs/009695399/functions/readv.html

--
Leon Romanovsky | Independent Linux Consultant
        www.leon.nu | leon@leon.nu
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Fwd: Problem with scatter/gather IO

[Randi Botse]

Ahh,, you are right Leon, each iov member need to be initialized
first, my bad. Everything is OK now. I also use sysconf(_SC_IOV_MAX)
to determine maximum iovs count.

Thanks,


On Wed, Jul 13, 2011 at 7:16 PM, Leon Romanovsky <leon@leon.nu> wrote:
> On Wed, Jul 13, 2011 at 14:54, Randi Botse <nightdecoder@gmail.com> wrote:
>>
>> Hi,
>>
>> I will read/write file with readv() and writev(), I have problem to
>> use that since the struct iovec will be a variable-length array, if i
>> have code.
>>
>> struct foo {
>>    unsigned char data[100];
>>    unsigned char another_data[28];
>> };
>> unsigned len;
>> ....
>>
>> /* calculate len */
>> /* use len as array's member size */
>> struct iovec iov[len];
>> printf("len is: %i\n", len);
>> readv(fd, iov, len);
>>
>> I got: BAD ADDRESS, and if I remove the printf() then I will get
>> INVALID ARGUMENT. What the possible problem with that?
>>
>> Thanks,
>> --
>
> Did you try "man readv" ?
> You didn't initialized iovec struct.
> http://pubs.opengroup.org/onlinepubs/009695399/functions/readv.html
>
> --
> Leon Romanovsky | Independent Linux Consultant
>         www.leon.nu | leon@leon.nu
>
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