[Pvfs-users] pvfs_read beyond EOF

[Wei-keng Liao]

Rob,

My question was sent to pvfs-user mailing list and was intended for PVFS 1.
I did not test PVFS 2 for this case, but am interested in how PVFS 2 would 
behave.

I am using PVFS 1.6.3.

Wei-keng


On Mon, 23 Jan 2006, Rob Ross wrote:

> Hi Wei-keng,
>
> PVFS2 isn't doesn't hold to the POSIX standard for all cases.  In the past we 
> have cleared the entire read buffer in order to solve the problem of sparse 
> file data access (servers with no data don't return anything).  In the 
> general case (noncontiguous buffers and I/O regions that might be read in any 
> order), this is a hard problem to solve. That's why you're seeing the 
> behavior that you are seeing.
>
> However, I think that this behavior has changed recently and is likely closer 
> to what you are expecting.  Sam, what's the latest behavior in this case? 
> Wei-keng, what version of PVFS2 are you using?
>
> For my own edification, I found a copy of the POSIX spec here:
>  http://www.opengroup.org/onlinepubs/009695399/toc.htm
>
> Here's the closest text that I found describing EOF behavior in that spec:
>
>  No data transfer shall occur past the current end-of-file. If the
>  starting position is at or after the end-of-file, 0 shall be returned.
>  If the file refers to a device special file, the result of subsequent
>  read() requests is implementation-defined.
>
> That's ambiguous.  "data transfer" is vague.  They probably intended your 
> interpretation, but they should have specified the condition of the buffer 
> explicitly, perhaps in the rationale section.  I wouldn't count on that 
> behavior on other systems either.
>
> Anyway, Sam will fill us in on the current semantic.  Given how much trouble 
> it has been to get to where we are, I honestly doubt we'll adjust that code 
> again in the near future.
>
> Regards,
>
> Rob
>
> Wei-keng Liao wrote:
>> 
>> What will happen if a read (pvfs_read) reads beyond the current end of file 
>> (EOF)? According to POSIX, read() should stop at EOF and return the number 
>> of bytes read to the buffer. The buffer beyond EOF should not be touched. 
>> (Please correct me if this is wrong.)
>> 
>> But, I found the pvfs_read will set the buffer contents to null for the 
>> beyond part. Here is the example code I tested: 1) create a new file, 2) 
>> initialize read buffer, 3) read 10 bytes, 4) print out buffer. (I am using 
>> PVFS 1.6.3) Is this the way PVFS behaves for reading beyond EOF?
>> 
>> Wei-keng Liao
>> ---------------------------------------------------------------------
>> 
>> 
>> #include <string.h>
>> #include <sys/types.h>
>> #include <sys/stat.h>
>> #include <fcntl.h>
>> #include <pvfs.h>
>> 
>> #define PVFS_FILE_NAME "/pvfs/wkliao/testfile.dat"
>> #define UNIX_FILE_NAME "testfile.dat"
>> 
>> #define TEST_FILE(filename,ofunc,cfunc,rfunc)    {               \
>>     fd = ofunc(filename, flag, 0600, 0, 0);                      \
>>                                                                  \
>>     readsize = rfunc(fd, buf, 10);                               \
>>     cfunc(fd);                                                   \
>>                                                                  \
>>     printf("readsize for %s = %d bytes\n", filename, readsize);  \
>>     printf("buf[*]= ");                                          \
>>     for (i=0; i<10; i++) printf(" '%c'", buf[i]);                \
>>     printf("\n");                                                \
>> }
>> 
>> int main(int argc, char **argv) {
>>     int  i, readsize, flag;
>>     int  fd;
>>     char buf[10];
>> 
>>     flag = O_TRUNC | O_CREAT | O_RDWR;
>> 
>>     /* test pvfs file -------------------------------------------*/
>>     for (i=0; i<10; i++) buf[i] = '1';
>>     TEST_FILE(PVFS_FILE_NAME, pvfs_open, pvfs_close, pvfs_read);
>> 
>>     /* test unix file -------------------------------------------*/
>>     for (i=0; i<10; i++) buf[i] = '1';
>>     TEST_FILE(UNIX_FILE_NAME, open, close, read);
>> 
>>     return 1;
>> }
>