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1.1       root        1: .TI F77/IO_INTRO "Sep. 4, 1985"
                      2: F77 I/O - Introduction
                      3: 
                      4: By default, Fortran units 5, and 6 are connected to standard input and
                      5: standard output and unit 0 is connected to standard error.
                      6: 
                      7: A 'read' with no unit number reads from unit 5; a 'print' writes
                      8: to unit 6.  Thus, the default is that
                      9: 
                     10: .nf
                     11:        read 8010, ...
                     12:        read (5,8010) ...
                     13:        read (*,8010) ...
                     14: .fi
                     15: 
                     16: read from standard input (the terminal),
                     17: 
                     18: .nf
                     19:        print 8020, ...
                     20:        write (6,8020) ...
                     21:        write (*,8020) ...
                     22: .fi
                     23: 
                     24: write to standard output (the terminal) and
                     25: 
                     26:        write (0,8020) ...
                     27: 
                     28: writes to standard error (the terminal).
                     29: 
                     30: To enter an end-of-file from a terminal, type control-D (type the letter "d"
                     31: while holding down the key labeled "control").
                     32: 
                     33: You can avoid the use of format statements by using list directed I/O.
                     34: For example, the statements
                     35: 
                     36: .nf
                     37:        read *,  i,j,x,y
                     38:        print *, i,j,x,y
                     39: .fi
                     40: 
                     41: read i, j, x, and y from the terminal and write them back to it.
                     42: 
                     43: To read and write from disk files, it is simplest to write your program
                     44: as if you are reading and writing on the terminal.  Then use shell
                     45: I/O redirection:
                     46: 
                     47:        a.out < infile
                     48: 
                     49: reads from the file 'infile' instead of the terminal and
                     50: 
                     51:        a.out > outfile
                     52: 
                     53: writes to the file 'outfile' instead of the terminal.  These may be
                     54: combined as in:
                     55: 
                     56:        a.out < infile > outfile
                     57: 
                     58: F77 allows unit numbers to be between 0 and 99.  If you use a unit number
                     59: N other than 0, 5, or 6, then the default is for it to reference a file
                     60: named 'fort.N'.
                     61: 
                     62: Disk files may be explicitly opened for reading and writing with the 'open'
                     63: statement:
                     64: 
                     65:        open( N, file='filename')
                     66: 
                     67: where N is any of 0 ... 99 .
                     68: 
                     69: Appropriately named environment variables override default file names
                     70: or file names in 'open' statements.
                     71: The corresponding environment variable name is the same
                     72: as the file name with periods deleted.
                     73: For example, a program containing:
                     74: .nf
                     75: 
                     76:       open(32,file="data.d")
                     77:       read(32,100) vec
                     78:       write(44) vec
                     79: 
                     80: .fi
                     81: normally will read from file 'data.d' and write to
                     82: file 'fort.44' in the current directory.
                     83: If the environment variables 'datad' and 'fort44' are set:
                     84: .nf
                     85: 
                     86: % setenv datad mydata
                     87: % setenv fort44 myout
                     88: 
                     89: .nf
                     90: in the C shell or:
                     91: .nf
                     92: 
                     93: $ datad=mydata
                     94: $ fort44=myout
                     95: $ export datad fort44
                     96: 
                     97: .fi
                     98: in the Bourne shell, then the program will read from 'mydata' and
                     99: write to 'myout'.
                    100: If the file name in the open statement is a path name including
                    101: slashes, then only the tail (the part after the last slash) is used
                    102: in looking for an environment variable.
                    103: 
                    104: Carriage control is not normally recognized by the f77 I/O library,
                    105: see "help f77 carriage_cc" to see how to use carriage control.
                    106: 
                    107: To find out if a logical unit is connected to a terminal, use the 
                    108: logical function 'isatty()',  see "man 3f ttynam" for details.
                    109: 
                    110: For an example of random access I/O, see "help f77 io_random".
                    111: For a list of errors from the f77 I/O library, see "help f77 io_err_msgs".

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