2.1 Accessing Files From Within Fortran Programs

Data is transferred between the program and devices or files through a Fortran logical unit. Logical units are identified in an I/O statement by a logical unit number, a nonnegative integer from 0 to the maximum 4-byte integer value (2,147,483,647).

The character * can appear as a logical unit identifier. The asterisk stands for standard input file when it appears in a READ statement; it stands for standard output file when it appears in a WRITE or PRINT statement.

A Fortran logical unit can be associated with a specific, named file through the OPEN statement. Also, certain preconnected units are automatically associated with specific files at the start of program execution.

2.1.1 Accessing Named Files

The OPEN statement’s FILE= specifier establishes the association of a logical unit to a named, physical file at runtime. This file can be pre-existing or created by the program.

The FILE= specifier on an OPEN statement may specify a simple file name (FILE=’myfile.out’) or a file name preceded by an absolute or relative directory path (FILE=’../Amber/Qproj/myfile.out’). Also, the specifier may be a character constant, variable, or character expression.

Library routines can be used to bring command-line arguments and environment variables into the program as character variables for use as file names in OPEN statements.

The following example (GetFilNam.f) shows one way to construct an absolute path file name from a typed-in name. The program uses the library routines GETENV, LNBLNK, and GETCWD to return the value of the $HOME environment variable, find the last non-blank in the string, and determine the current working directory:

      CHARACTER F*128, FN*128, FULLNAME*128
      PRINT*, ’ENTER FILE NAME:’
      READ *, F
      FN = FULLNAME( F )
      PRINT *, ’PATH IS: ’,FN
      END

      CHARACTER*128 FUNCTION FULLNAME( NAME )
      CHARACTER NAME*(*), PREFIX*128
C          This assumes C shell.
C           Leave absolute path names unchanged.
C           If name starts with ’~/’, replace tilde with home
C           directory; otherwise prefix relative path name with
C           path to current directory.
      IF ( NAME(1:1) .EQ. ’/’ ) THEN
            FULLNAME = NAME
      ELSE IF ( NAME(1:2) .EQ. ’~/’ ) THEN
            CALL GETENV( ’HOME’, PREFIX )
            FULLNAME = PREFIX(:LNBLNK(PREFIX)) //
     1                     NAME(2:LNBLNK(NAME))
      ELSE
            CALL GETCWD( PREFIX )
            FULLNAME = PREFIX(:LNBLNK(PREFIX)) //
     1                     ’/’ // NAME(:LNBLNK(NAME))
      ENDIF
      RETURN
      END

Compiling and running GetFilNam.f results in:

demo% pwd
/home/users/auser/subdir
demo% f95 -o getfil GetFilNam.f
demo% getfil
 ENTER FILE NAME:
getfil
 PATH IS: /home/users/auser/subdir/atest.f

demo%

These routines are further described in 2.1.4 Passing File Names to Programs. See man page entries for getarg(3F), getcwd(3F), and getenv(3F) for details; these and other useful library routines are also described in the Fortran Library Reference.

2.1.2 Opening Files Without a Name

The OPEN statement need not specify a name; the runtime system supplies a file name according to several conventions.

2.1.2.1 Opened as Scratch

Specifying STATUS=’SCRATCH’ in the OPEN statement opens a file with a name of the form tmp.FAAAxnnnnn, where nnnnn is replaced by the current process ID, AAA is a string of three characters, and x is a letter; the AAA and x make the file name unique. This file is deleted upon termination of the program or execution of a CLOSE statement. When compiling in FORTRAN 77 compatibility mode (-f77), you can specify STATUS=’KEEP’ in the CLOSE statement to preserve the scratch file. (This is a non-standard extension.)

2.1.2.2 Already Open

If the file has already been opened by the program, you can use a subsequent OPEN statement to change some of the file’s characteristics; for example, BLANK and FORM. In this case, you would specify only the file’s logical unit number and the parameters to change.

2.1.2.3 Preconnected or Implicitly Named Units

Three unit numbers are automatically associated with specific standard I/O files at the start of program execution. These preconnected units are standard input, standard output, and standard error:

• Standard input is logical unit 5

• Standard output is logical unit 6

• Standard error is logical unit 0

Typically, standard input receives input from the workstation keyboard; standard output and standard error display output on the workstation screen.

In all other cases where a logical unit number but no FILE= name is specified on an OPEN statement, a file is opened with a name of the form fort.n, where n is the logical unit number.

2.1.3 Opening Files Without an OPEN Statement

Use of the OPEN statement is optional in those cases where default conventions can be assumed. If the first operation on a logical unit is an I/O statement other than OPEN or INQUIRE, the file fort.n is referenced, where n is the logical unit number (except for 0, 5, and 6, which have special meaning).

These files need not exist before program execution. If the first operation on the file is not an OPEN or INQUIRE statement, they are created.

Example: The WRITE in the following code creates the file fort.25 if it is the first input/output operation on that unit:

demo% cat TestUnit.f
      IU=25
      WRITE( IU, ’(I4)’ ) IU
      END
demo%

The preceding program opens the file fort.25 and writes a single formatted record onto that file:

demo% f95 -o testunit TestUnit.f
demo% testunit
demo% cat fort.25
  25
demo%

2.1.4 Passing File Names to Programs

The file system does not have any automatic facility to associate a logical unit number in a Fortran program with a physical file.

However, there are several satisfactory ways to communicate file names to a Fortran program.

2.1.4.1 Via Runtime Arguments and GETARG

The library routine getarg(3F) can be used to read the command-line arguments at runtime into a character variable. The argument is interpreted as a file name and used in the OPEN statement FILE= specifier:

demo% cat testarg.f
         CHARACTER outfile*40
C  Get first arg as output file name for unit 51
         CALL getarg(1,outfile)
         OPEN(51,FILE=outfile)
         WRITE(51,*) ’Writing to file: ’, outfile
         END
demo% f95 -o tstarg testarg.f
demo% tstarg AnyFileName
demo% cat AnyFileName
 Writing to file: AnyFileName
demo%

2.1.4.2 Via Environment Variables and GETENV

Similarly, the library routine getenv(3F) can be used to read the value of any environment variable at runtime into a character variable that in turn is interpreted as a file name:

demo% cat testenv.f
         CHARACTER outfile*40
C  Get $OUTFILE as output file name for unit 51
         CALL getenv(’OUTFILE’,outfile)
         OPEN(51,FILE=outfile)
         WRITE(51,*) ’Writing to file: ’, outfile
         END
demo% f95 -o tstenv testenv.f
demo% setenv OUTFILE EnvFileName
demo% tstenv
demo% cat EnvFileName
 Writing to file: EnvFileName
demo%

When using getarg or getenv, care should be taken regarding leading or trailing blanks. (Fortran 95 programs can use the intrinsic function TRIM, or the older FORTRAN 77 library routine LNBLNK()) Additional flexibility to accept relative path names can be programmed along the lines of the FULLNAME function in the example at the beginning of this chapter.

2.1.4.3 Command-Line I/O Redirection and Piping

Another way to associate a physical file with a program’s logical unit number is by redirecting or piping the preconnected standard I/O files. Redirection or piping occurs on the runtime execution command.

In this way, a program that reads standard input (unit 5) and writes to standard output (unit 6) or standard error (unit 0) can, by redirection (using <, >, >>, >&, |, |&, 2>, 2>&1 on the command line), read or write to any other named file.

This is shown in the following table:

See the csh, ksh, and sh man pages for details on redirection and piping on the command line.