Hi,
I have run into an issue where our code tries to read very large files
(>2^31 bytes in size) and is effectively running into what looks like an
integer overflow issue. What happens is that the int64_t members of stat_t
("size") and also the return value of llseek are implicitly down-cast into
signed ints. Here is what we do to mount our file system (slightly
simplified for brevity):
var node = Module.FS.createFile('/', emscriptenPath, null, true,
true);
node.node_ops = {
getattr: function(ganode) {
return {
dev: 1,
ino: ganode.id,
mode: ganode.mode,
nlink: 1,
uid: 0,
gid: 0,
rdev: ganode.rdev,
size: size, // <-- this is a file size > 2^31
atime: new Date(ganode.timestamp),
mtime: new Date(ganode.timestamp),
ctime: new Date(ganode.timestamp),
blksize: 4096,
blocks: Math.ceil(size / 4096)
};
}
};
node.stream_ops = {
llseek: function(stream, offset, whence) {
switch (whence) {
case 0: // SEEK_SET
stream.position = offset;
break;
case 1: // SEEK_CUR
stream.positon += offset;
break;
case 2: // SEEK_END
stream.position = size + offset;
break;
default:
throw new Module.FS.ErrnoError(22); // EINVAL
}
return stream.position; // <-- can be > 2^31
},
read: function(stream, buffer, heapOffset, numberOfBytes,
fileOffset) {
// ...
}
};
I suspect that the issue arises from the fact that int64_t has no native
counterpart in JS and is, hence, downcast in the interface between the
asm.js and the file system code. Is there a quick fix to address this
issue? I tried -s PRECISE_I64_MATH=2, but to no avail. Also, I am not
entirely sure where exactly the precision is lost. I guess, it happens in
the __syscallXY functions for fstat, lseek (and probably also for the
arguments passed into read).
One idea I had was to patch the syscalls in a way that I render the int64_t
values as strings on the heap and pass back the pointer to that string
inside the stat_t structure and the return value of llseek. These strings
would then have to be parsed back into int64_t values inside the syscalls.
Not exactly elegant, but it might work. Or is there a generic solution?
Thanks heaps in advance for any suggestions...
Soeren
--
You received this message because you are subscribed to the Google Groups
"emscripten-discuss" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
For more options, visit https://groups.google.com/d/optout.
