In Intel Fortran, a logical unit is a channel through which data transfer occurs between the program and a device or file. You identify each logical unit with a logical unit number, which can be any nonnegative integer from 0 to a maximum value of 2,147,483,647 (2**31-1). For example:
READ (2,100) I,X,Y
This READ statement specifies that data is to be entered from the device or file corresponding to logical unit 2, in the format specified by the FORMAT statement labeled 100. When opening a file, use the UNIT specifier to indicate the unit number.
Fortran programs are inherently device-independent. The association between the logical unit number and the physical file can occur at run-time. Instead of changing the logical unit numbers specified in the source program, you can change this association at run time to match the needs of the program and the available resources. For example, before running the program, a script file can set the appropriate environment variable or allow the terminal user to type a directory path, file name, or both.
Use the same logical unit number specified in the OPEN statement for other I/O statements to be applied to the opened file, such as READ and WRITE.
The OPEN statement connects a unit number with an external file and allows you to explicitly specify file attributes and run-time options using OPEN statement specifiers (all files except internal files are called external files).
Certain unit numbers are preconnected to standard devices. Unit number 5 is associated with stdin, unit 6 with stdout, and unit 0 with stderr. At run time, if units 5 and 6 are specified by a record I/O statement (such as READ or WRITE) without having been explicitly opened by an OPEN statement, Intel Fortran implicitly opens units 5, 6, and 0 and associates them with their respective operating system standard I/O files (if the corresponding FORTn environment variable is not set).