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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