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I can save an H.264 stream to an MKV file just fine until it reaches over 2GiB in size. As soon as it surpasses this, the fseeko function fails in gstfilesink.c:gst_file_sink_do_seek(). However, when removing the MKV muxer (matroskamux), filesink works without issue (tested up to file sizes around 12GB). This appears to be a problem with filesink when used with matroskamux, not an issue with matroskamux itself.

Environment is Windows 10 1903, MSYS2 using mingw-w64 Gstreamer 1.16.2 packages (/mingw64/bin/gst-launch-1.0.exe).

Pipeline to reproduce issue (bitrate is set so that it reaches 2GiB quickly; the error still occurs if omitted):

gst-launch-1.0.exe videotestsrc ! 'video/x-raw, width=1920, height=1080, framerate=30/1' ! x264enc bitrate=50000 ! matroskamux ! filesink location=test.mkv

The following pipelines execute without error (though the saved file might not be playable):

gst-launch-1.0.exe videotestsrc ! 'video/x-raw, width=1920, height=1080, framerate=30/1' ! x264enc bitrate=50000 ! filesink location=test.mkv

gst-launch-1.0.exe videotestsrc num-buffers=20000 ! 'video/x-raw, width=1920, height=1080, framerate=30/1' ! x264enc bitrate=50000 ! mp4mux ! filesink location=test.mp4

This appears to be a problem with Windows, as the bad pipeline gives no error when running on Linux (tested on Ubuntu 18.04 using GStreamer 1.14.5 and Ubuntu 20.04 using GStreamer 1.16.2 from apt packages). My guess is that fseek should be replaced with something else on Windows (_fseeki64?), though I have no experience using any of this.

Full log attached for reference (most of it is ignorablebad_mkv_pipeline.tar.gz), but problematic section is here:

... 
// Successful seek and write
0:02:38.703314800   836        25f1900 DEBUG               basesink gstbasesink.c:3354:gst_base_sink_event:<filesink0> received event 000000000348d850 segment event: 000000000348d850, time 99:99:99.999999999, seq-num 147, GstEventSegment, segment=(GstSegment)"GstSegment, flags=(GstSegmentFlags)GST_SEGMENT_FLAG_NONE, rate=(double)1, applied-rate=(double)1, format=(GstFormat)GST_FORMAT_BYTES, base=(guint64)0, offset=(guint64)0, start=(guint64)2108148716, stop=(guint64)18446744073709551615, time=(guint64)0, position=(guint64)0, duration=(guint64)18446744073709551615;";
0:02:38.703343800   836        25f1900 DEBUG               filesink gstfilesink.c:503:gst_file_sink_do_seek:<filesink0> Seeking to offset 2108148716 using fseeko
0:02:38.703373200   836        25f1900 DEBUG               basesink gstbasesink.c:3289:gst_base_sink_default_event:<filesink0> configured segment bytes segment start=2108148716, offset=0, stop=-1, rate=1.000000, applied_rate=1.000000, flags=0x00, time=0, base=0, position 0, duration -1
0:02:38.703409300   836        25f1900 DEBUG               GST_PADS gstpad.c:5781:gst_pad_send_event_unchecked:<filesink0:sink> sent event, ret ok
0:02:38.703428400   836        25f1900 LOG                GST_EVENT gstevent.c:222:_gst_event_free: freeing event 000000000348d7e0 type segment
0:02:38.703447500   836        25f1900 LOG                 GST_PADS gstpad.c:5224:store_sticky_event:<filesink0:sink> stored sticky event segment
0:02:38.703468800   836        25f1900 LOG                 GST_PADS gstpad.c:5415:gst_pad_push_event_unchecked:<matroskamux0:src> sent event 000000000348d850 (segment) to peerpad <filesink0:sink>, ret ok
0:02:38.703489300   836        25f1900 DEBUG               GST_PADS gstpad.c:3936:push_sticky:<matroskamux0:src> event segment marked received

...

// Next (failed) write
0:02:38.704550500   836        25f1900 DEBUG               basesink gstbasesink.c:3354:gst_base_sink_event:<filesink0> received event 000000000348d7e0 segment event: 000000000348d7e0, time 99:99:99.999999999, seq-num 148, GstEventSegment, segment=(GstSegment)"GstSegment, flags=(GstSegmentFlags)GST_SEGMENT_FLAG_NONE, rate=(double)1, applied-rate=(double)1, format=(GstFormat)GST_FORMAT_BYTES, base=(guint64)0, offset=(guint64)0, start=(guint64)2155122157, stop=(guint64)18446744073709551615, time=(guint64)0, position=(guint64)0, duration=(guint64)18446744073709551615;";
0:02:38.704578400   836        25f1900 DEBUG               filesink gstfilesink.c:503:gst_file_sink_do_seek:<filesink0> Seeking to offset 2155122157 using fseeko
0:02:38.704596900   836        25f1900 DEBUG               filesink gstfilesink.c:713:gst_file_sink_flush_buffer:<filesink0> Flushing out buffer of size 8
0:02:38.704616400   836        25f1900 DEBUG               filesink gstfilesink.c:655:gst_file_sink_render_buffers:<filesink0> writing 1 buffers (1 memories, 8 bytes) at position 2108148716
0:02:38.704636500   836        25f1900 LOG                  default gstelements_private.c:239:gst_writev_buffers:<filesink0> 1 buffers, 1 memories
0:02:38.704676800   836        25f1900 DEBUG               filesink gstfilesink.c:535:gst_file_sink_do_seek:<filesink0> Seeking failed: Invalid argument
0:02:38.704719100   836        25f1900 WARN                filesink gstfilesink.c:604:gst_file_sink_event:<filesink0> error: Error while seeking in file "test.mkv".
0:02:38.704744000   836        25f1900 WARN                filesink gstfilesink.c:604:gst_file_sink_event:<filesink0> error: system error: Invalid argument
0:02:38.704765400   836        25f1900 DEBUG            GST_MESSAGE gstelement.c:2110:gst_element_message_full_with_details:<filesink0> start
0:02:38.704791900   836        25f1900 INFO        GST_ERROR_SYSTEM gstelement.c:2140:gst_element_message_full_with_details:<filesink0> posting message: Error while seeking in file "test.mkv".

... // Pipeline closing and cleanup