Passing File Names to Programs (Fortran Programming Guide)

Fortran Programming Guide

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.

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% f77 -silent -o tstarg testarg.f
demo% tstarg AnyFileName
demo% cat AnyFileName
 Writing to file: AnyFileName
demo%

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% f77 -silent -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 77 programs can use the library function LNBLNK; Fortran 90 programs can use the intrinsic function TRIM.) 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.

`f77`: Logical Unit Preattachment Using `IOINIT`

The library routine IOINIT can also be used with f77 to attach logical units to specific files at runtime. IOINIT looks in the environment for names of a user-specified form and then opens the corresponding logical unit for sequential formatted I/O. Names must be of the general form PREFIXnn, where the particular PREFIX is specified in the call to IOINIT, and nn is the logical unit to be opened. Unit numbers less than 10 must include the leading 0. See the Sun Fortran Library Reference, and the IOINIT(3F) man page. (The IOINIT facility is not implemented for f90.)

Example: Associate physical files test.inp and test.out in the current directory to logical units 1 and 2:

First, set the environment variables.

With ksh or sh:

demo$ TST01=ini1.inp
demo$ TST02=ini1.out
demo$ export TST01 TST02

With csh:

demo% setenv TST01 ini1.inp
demo% setenv TST02 ini1.out

demo% cat ini1.f 
      CHARACTER PRFX*8 
      LOGICAL CCTL, BZRO, APND, VRBOSE 
      DATA CCTL, BZRO, APND, PRFX, VRBOSE 
&                   /.TRUE.,.FALSE.,.FALSE., 'TST',.FALSE. / 
      CALL IOINIT( CCTL, BZRO, APND, PRFX, VRBOSE ) 
      READ(1, *) I, B, N 
      WRITE(2, *) I, B, N 
      END 
demo%

The program ini1.f reads 1 and writes 2:

With environment variables and ioinit, ini1.f reads ini1.inp and writes to ini1.out:

demo% cat ini1.inp
 12 3.14159012 6
demo% f77 -silent -o tstinit ini1.f
demo% tstinit
demo% cat ini1.out
  12    3.14159  6
demo%

IOINIT is adequate for most programs as written. However, it is written in Fortran specifically to serve as an example for similar user-supplied routines. Retrieve a copy from the following file, a part of the FORTRAN 77 package installation: /opt/SUNWspro/SC5.0/src/ioinit.f

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>&1on the command line), read or write to any other named file.

This is shown in the following table:

Table 2-1 csh/sh/ksh Redirection and Piping on the Command Line


Action	Using C Shell	Using Bourne or Korn Shell
Standard input --read from mydata	`myprog < mydata`	`myprog < mydata`
Standard output --write (overwrite) myoutput	`myprog > myoutput`	`myprog > myoutput`
Standard output -- write/append to myoutput	`myprog >> myoutput`	`myprog >> myoutput`
Redirect standard error to a file	`myprog >& errorfile`	`myprog 2> errorfile`
Pipe standard output to input of another program	`myprog1 \| myprog2`	`myprog1 \| myprog2`
Pipe standard error and output to another program	`myprog1 \|& myprog2`	`myprog1 2>&1 \| myprog2`

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