I’m writing a tool which receives data streamed into stdin by my Google Chrome
extension (using their new Native Messaging API). I think they use stdin for
such interprocess communication in order to make it cross-platform.
Anyhow, I was surprised to find that the following code, which repeatedly
invokes -waitForDataInBackgroundAndNotify and -availableData on +[NSFileHandle
fileHandleWithStandardInput], delivers no more than 4096 bytes on each
iteration. It just stops at 4096, and the next -availableData starts with the
next byte.
Although it was fun to write the code to buffer and stitch the message fields
and length fields back together, I’d like to know what is causing these 4K
breaks. The documentation for -availableData says that it will return “the
data currently available through the receiver, up to the the maximum size that
can be represented by an NSData object.” I think that must be more than 4K.
One possibility is that Chrome must be spitting it out in 4K bursts with little
delays in between. I tried to prove that by replacing the NSFileHandle stuff
with repeated calls to getchar(), and changing my extension to output a string
of 50,000 characters. But when I graphed the receive times of these
characters, it was a nice continuous, quite straight line.
Jerry
int main(int argc, const char * argv[]) {
@autoreleasepool {
/* -[NSFileManager waitForDataInBackgroundAndNotify] doc say I need
an "active run loop". I don't know what they mean by "active". */
NSDate* verySoon = [NSDate dateWithTimeIntervalSinceNow:0.01] ;
[[NSRunLoop mainRunLoop] runUntilDate:verySoon] ;
NSFileHandle* input = [NSFileHandle fileHandleWithStandardInput] ;
[input waitForDataInBackgroundAndNotify] ;
NSNotificationCenter* dc = [NSNotificationCenter defaultCenter] ;
[dc addObserverForName:NSFileHandleDataAvailableNotification
object:input
queue:nil
usingBlock:^(NSNotification* note) {
NSFileHandle* input ;
input = [NSFileHandle fileHandleWithStandardInput] ;
NSData* data = [input availableData] ;
if ([data length] > 0) {
[[Chromessenger sharedMessenger]
gulpStdinData:data] ;
[input waitForDataInBackgroundAndNotify] ;
}
else {
// Must have received end-of-file (EOF)
exit(0) ;
}
}] ;
[[NSRunLoop mainRunLoop] run] ;
}
// Will never execute
return 0 ;
}
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