hi erik, i've got another strange issue with libnsdfile (1.0.10beta6 and earlier). it seems to be related to SFM_RDWR mode again. if i open an existing (but empty) file in RDWR mode, then switch to UPDATE_HEADER_AUTO mode, then write to the file, then try to seek to the end of the file in order to write again: then the seek command will fail or misbehave. when seeking to file-end, SEEK SET, it will fail with "Internal psf_fseek() failed.", if seeking to 0, SEEK_END, it will just seek to zero regardless of the number of frames in the file. all this proceeds if not switching to UPDATE_HEADER_AUTO. though not included in the test case, i got the same behavior when using WRITE_HEADER_NOW after each write instead of UPDATE_HEADER_AUTO with my "real-world" code. this behavior seems to be independent from the sndfile format, but just tested with WAV/16bit, AIFF/16bit, AIFF/float. attached the "small test case", hope erik will like it again... bests, martin