fwrite() is the only interface allowed for writing to a file opened for record I/O. Only one record is written at a time. If you attempt to write more new data than a full record can hold or you try to update a record with more data than it currently has, z/OS® XL C/C++ truncates your output at the record boundary. When z/OS XL C/C++ performs a truncation, it sets errno and raises SIGIOERR, if SIGIOERR is not set to SIG_IGN.
When you update a record, you can update less than the full record. The remaining data that you do not update is left untouched in the file.
When you are writing new records to a fixed-record I/O file, if you try to write a short record, z/OS XL C/C++ pads the record with nulls out to LRECL.
At the completion of an fwrite(), the file position is at the start of the next record. For new data, the block is flushed out to the system as soon as it is full.