Which access options are available for other file users is defined by the type of OPEN call used when the program is designed. Consequently it should be obvious that processing modes exceeding the requirements of the intended application should not be chosen. A person who only wishes to read and does not open the file with INPUT prevents other users from gaining read access. In the case of files which are of general interest, the effort of making a decision concerning reading/updating at the time of the OPEN command is worthwhile.
Opening a file is a complicated process. Because of the overhead involved, it should be done as rarely as possible. Even the requirement that a file should be locked for as short a time as possible is subordinate to this requirement. If necessary, one must resort to the access method PAM or ISAM using SHARUPD mode.
If it can be accepted that a file is not 100% up-to-date, then it is worth installing a copy for read accesses. This should be write-protected in order to prevent users from opening the copy file for writing purposes.
This solution is more efficient because using SHARUPD mode causes an increase in the access overhead. If the probability of a collision is small, then the additional effort involved here is not worthwhile.