Hi All,
I know the topic of "multiple [protobuff] messages in a single file" has
been covered a bunch, but I have a slightly different question.
Most of the answers to "multiple messages in a single file" has been, use a
CodedInputStream to do the reading and writing and to write the size of the
message before the message in the file. While this works fine, my use case
is different as I don't want to always read every single message in a file
upon file read. So, to set up this use case, instead of writing the sizes
of the messages before each message in a file, I write a "header" message
at the top of the file and then individual messages after that. My header
is as follows
<code>
option optimize_for=LITE_RUNTIME;
package MyPackage;
message ListHeader {
message Entry
{
optional string name = 1;
optional uint32 byte_offset_from_header = 2;
optional uint32 size_bytes = 3;
}
repeated Entry entry = 1;
}
</code>
Conceptually, at application start, I read this header for a given file and
store it somewhere. After that, when a particular entry is needed from a
file (referenced by name), I want to be able to open the file, jump to a
given entry (via the entry byte offset into the file), read the message out
and continue on my merry way. The problem is, none of the messages I read
are "valid" (contain correct data). They parse ok, but are corrupt. The
header message parses fine and contains proper data.
I'm using the LITE_RUNTIME optimization, so I made subclasses of
ZeroCopyInputStream which take in an std::ifstream. When I want to read one
of the "entry" messages, I use seekg on the ifstream created from the file,
then I created a ZeroCopyInputStream from that stream (via my own class),
and then I created a CodedInputStream. I set a limit on the
CodedInputStream to be the size of the entry from the header and then parse
via parseFromCodedInputStream. Is this a valid workflow (using seekg on the
stream which I then made a ZeroCopyInputStream and CodedInputStream from)?
If not, how can I get the functionality I want?
I do calculate the byte offsets that I seekg to to be the entry's
"byte_offset_from_header" + the coded input stream used to parse the
header's CurrentPosition which I believe should yield the total byte offset
from beginning of the file.
-Jonathan
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