On 07/01/2013 08:41 PM, fangdong wrote:
> On 06/29/2013 05:11 AM, Jiaxing Wang wrote:
>> After pread(), file->f_pos and m->read_pos get different,
>> and lseek() to m->read_pos did not update file->f_pos, then
>> a subsequent read may read from a wrong position, the following
>> program shows the problem:
>>
>> char str1[32] = { 0 };
>> char str2[32] = { 0 };
>> int poffset = 10;
>> int count = 20;
>>
>> /*open any seq file*/
>> int fd = open("/proc/modules", O_RDONLY);
>>
>> pread(fd, str1, count, poffset);
>> printf("pread:%s\n", str1);
>>
>> /*seek to where m->read_pos is*/
>> lseek(fd, poffset+count, SEEK_SET);
>>
>> /*supposed to read from poffset+count, but this read from position 0*/
>> read(fd, str2, count);
>> printf("read:%s\n", str2);
>>
>> Signed-off-by: Jiaxing Wang <hello.wjx@xxxxxxxxx>
>> ---
>> fs/seq_file.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/seq_file.c b/fs/seq_file.c
>> index 774c1eb..4b22b26 100644
>> --- a/fs/seq_file.c
>> +++ b/fs/seq_file.c
>> @@ -328,7 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence)
>> m->read_pos = offset;
>> retval = file->f_pos = offset;
>> }
>> - }
>> + } else
>> + file->f_pos = offset;
>> }
>> file->f_version = m->version;
>> mutex_unlock(&m->lock);
>>
> This does not appear to be a problem, in linux man page, the behaver seems clearly defined:
>
> DESCRIPTION
> pread() reads up to count bytes from file descriptor fd at offset off-set (from the start of the file) into the buffer starting at buf. The file offset is not changed.
>
> pwrite() writes up to count bytes from the buffer starting at buf to the file descriptor fd at offset offset. The file offset is not changed.
>
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There's no problem that pread() don't change file->f_pos, but I think lseek() should have changed it.
Any comments from other people, and Alexander?
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