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Asked: 2026-05-24T13:49:48+00:00

I’ve encountered an issue when sending large segments of data through a TCP socket,

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I’ve encountered an issue when sending large segments of data through a TCP socket, having spend about 3 days trying to pick apart the issue and failing I decided it was best to turn here for help / advice.

My Project
I’ve written a basic HTTP server which (slightly irrelevant) can run lua scripts to output pages. This all works perfectly fine under Windows (32 bit).

The Problem

When sending medium/large files (anything from roughly 8000 bytes and above appears to have issues) over the TCP socket on Ubuntu Linux(64bit), they appear to cut out at different lengths (the result displayed in the browser is a value between 8000 and 10200 bytes. When I check the return value of the send function it’s exactly 9926 bytes every time the send ends. No error.

Smaller files send absolutely fine, and there are no issues under windows. Going on this information I thought it could be a buffer size issues, so I did 

cat /proc/sys/net/ipv4/tcp_mem

which outputted 188416 192512 196608

those numbers are far above 9926 so I assume that isn’t the problem.

I’m using CSimpleSockets as a socket library, I haven’t had any issues before. In case the issue is inside of this library the code I dug around for what the send function used under unix is:

#define SEND(a,b,c,d)          send(a, (const int8 *)b, c, d)

send(socket, buffer, bytestosend, 0);

buffer gets cast from a const char * to const unsigned char* to const int8 * before getting passed to the OS to be sent.

OK, I think that covers everything I checked. If you need any more information or I’ve missed anything glaringly obvious I’ll do my best to provide. Thanks for your help!

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1 Answer

  1. Editorial Team

    Your problem is that send does not guarantee to send the amount of data passed to it.

    It has internal buffers that can fill, socket parameters that affect buffers, etc. You need to note how many bytes were sent, wait for a few milliseconds (for the send to move data over the wire and empty the buffer), then send the remaining data. There is no automatic way to do this and you’ll need to write a bit of logic which advances your buffer by the amount of bytes that were actually sent.

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