The FORTRAN default value for the maximum number of logical units that a program can have open at one time is 64. For current Solaris releases, this limit is 256. A FORTRAN program can increase this limit beyond 64 by calling the setrlim() function. See the man page setrlim(2). If you are running csh, you can also do this with the limit or unlimit command; see csh(1).
The standard logical units 0, 5, and 6 are preconnected as stderr, stdin, and stdout, respectively. These are not actual file names, and cannot be used for opening these units. INQUIRE does not return these names, and indicates that the above units are not named unless they have been opened to real files. However, these units can be redefined with an OPEN statement.
The names, stderr, stdin, and stdout, are meant to make error reporting more meaningful. To preserve error reporting, the system makes it an error to close logical unit 0, although it can be reopened to another file.
If you want to open a file with the default file name for any preconnected logical unit, remember to close the unit first. Redefining the standard units can impair normal console I/O. An alternative is to use shell redirection to externally redefine the above units.
To redefine default blank control or the format of the standard input or output files, use the OPEN statement, specifying the unit number and no file name, and use the options for the kind of blank control you want.