Non buffered writes with POSIX are broken

[isosc]

Failure to specify a large enough buffer when writing with the POSIX method results in junk data being written to the file. A warning is printed to stdout, but the code does not abort.

[isosc]

This appears to have been fixed at some point. Current behavior is to write vars that fit in the buffer, then print error message. No incorrect data is present in the output file.