Something special (defining some preprocessor macro) has to be done for 32-bit POSIX-like operating systems to enable support for file sizes greater than can be represented by 31 bits. The exact preprocessor macro name that has to be defined varies from system to system. Boost Filesystem currently supplies the correct macro for Linux, but not some of the UNIX systems. Normally we would have a regression test that identifies the systems which fail. But we don't test large file support directly because to do so would be a burden on those who run the regression tests; such tests would chew up gigabytes of disk space and might also be very slow. It recently occurred to me that sizeof(stat.st_size) > 4 is a pretty good surrogate for an actual test on a large file. While in theory it could yield a false positive, in practice it should identify whether or not large file support has been correctly enabled. It should never yield a false negative. Accordingly, I've added a new regression test, large_file_support_test.cpp. Initially it should pass on Windows and Linux systems, but fail on any UNIX systems that need additional macros defined. We can then experiment with adding additional macro definitions to get the test to pass on any systems it is currently failing. --Beman