SylixOS write 0字节问题
在移植中间件过程中，在SylixOS调用write函数写入0字节的数据到文件中时，会导致对应的中间件测试用例失败，失败的原因是文件系统中的write函数在Linux系统和SylixOS有区别，两种实现的差别如下。
2 write函数的实现机制2.1 SylixOS实现机制在SylixOS下通过write 函数写数据到普通文件中时，第一步会判断写入的数据是否为0，如果是0直接返回失败，具体实现如程序清单 2.1所示。
程序清单 2.1 SylixOS文件系统写函数
static ssize_t __ramFsPWrite (PLW_FD_ENTRY pfdentry,
PCHAR pcBuffer,
size_t stNBytes,
off_t oftPos)
{
PLW_FD_NODE pfdnode = (PLW_FD_NODE)pfdentry->FDENTRY_pfdnode;
PRAM_NODE pramn = (PRAM_NODE)pfdnode->FDNODE_pvFile;
ssize_t sstWriteNum = PX_ERROR;
if (!pcBuffer || !stNBytes || (oftPos < 0)) {
_ErrorHandle(EINVAL);
return (PX_ERROR);
}
if (pramn == LW_NULL) {
_ErrorHandle(EISDIR);
return (PX_ERROR);
}
if (__RAMFS_FILE_LOCK(pramn) != ERROR_NONE) {
_ErrorHandle(ENXIO);
return (PX_ERROR);
}
if (S_ISDIR(pramn->RAMN_mode)) {
__RAMFS_FILE_UNLOCK(pramn);
_ErrorHandle(EISDIR);
return (PX_ERROR);
}
sstWriteNum = __ram_write(pramn, pcBuffer, stNBytes, (size_t)oftPos);
if (sstWriteNum > 0) {
pfdnode->FDNODE_oftSize = (off_t)pramn->RAMN_stSize;
}
__RAMFS_FILE_UNLOCK(pramn);
return (sstWriteNum);
}
2.2 Linux实现机制在Linux下通过write 函数写数据到普通文件中时，第一步会判断写入的数据是否为0，如果是0直接返回0，具体实现如程序清单 2.2所示。
程序清单 2.2 Linux 文件系统写函数
ssize_t __generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
unsigned long nr_segs, loff_t *ppos)
{
struct file *file = iocb->ki_filp;
struct address_space * mapping = file->f_mapping;
size_t ocount; /* original count */
size_t count; /* after file limit checks */
struct inode *inode = mapping->host;
loff_t pos;
ssize_t written;
ssize_t err;
ocount = 0;
err = generic_segment_checks(iov, &nr_segs, &ocount, VERIFY_READ);
if (err)
return err;
count = ocount;
pos = *ppos;
/* We can write back this queue in page reclaim */
current->backing_dev_info = mapping->backing_dev_info;
written = 0;
err = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
if (err)
goto out;
if (count == 0)
goto out;
err = file_remove_suid(file);
if (err)
goto out;
err = file_update_time(file);
if (err)
goto out;
/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
if (unlikely(file->f_flags & O_DIRECT)) {
loff_t endbyte;
ssize_t written_buffered;
written = generic_file_direct_write(iocb, iov, &nr_segs, pos,
ppos, count, ocount);
if (written < 0 || written == count)
goto out;
/*
* direct-io write to a hole: fall through to buffered I/O
* for completing the rest of the request.
*/
pos += written;
count -= written;
written_buffered = generic_file_buffered_write(iocb, iov,
nr_segs, pos, ppos, count,
written);
/*
* If generic_file_buffered_write() retuned a synchronous error
* then we want to return the number of bytes which were
* direct-written, or the error code if that was zero. Note
* that this differs from normal direct-io semantics, which
* will return -EFOO even if some bytes were written.
*/
if (written_buffered < 0) {
err = written_buffered;
goto out;
}
/*
* We need to ensure that the page cache pages are written to
* disk and invalidated to preserve the expected O_DIRECT
* semantics.
*/
endbyte = pos + written_buffered - written - 1;
err = filemap_write_and_wait_range(file->f_mapping, pos, endbyte);
if (err == 0) {
written = written_buffered;
invalidate_mapping_pages(mapping,
pos >> PAGE_CACHE_SHIFT,
endbyte >> PAGE_CACHE_SHIFT);
} else {
/*
* We don't know how much we wrote, so just return
* the number of bytes which were direct-written
*/
}
} else {
written = generic_file_buffered_write(iocb, iov, nr_segs,
pos, ppos, count, written);
}
out:
current->backing_dev_info = NULL;
return written ? written : err;
}
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