The READONLY keyword specifies that an existing file can be read or found, but it cannot be written to. It takes the following form:
READONLY
The Fortran I/O system's default file access privileges are read-write, which can cause run-time I/O errors if the file protection does not permit write access.
The READONLY keyword has no effect on the protection specified for a file. Its main purpose is to allow a file to be read simultaneously by two or more programs. For example, if you wish to open a file for the purpose of reading the file but want to allow others to read the same file while you have it open, specify the READONLY keyword.
For details on file sharing, see your user manual.