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Chapter 2

Fortran Input/Output

This chapter discusses the input/output features provided by Sun Fortran compilers.

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.

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. See the Sun FORTRAN 77 Language Reference Manual for a full discussion of the OPEN statement.

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. (See man page entries for getarg(3F) and getenv(3F) for details; these and other useful library routines are also described in the Fortran Library Reference).

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
      READ *, F 
      FN = FULLNAME( F ) 
      PRINT *, 'PATH IS: ',FN
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 ) 
&                         NAME(2:LNBLNK(NAME)) 
            CALL GETCWD( PREFIX ) 
&                         '/' // NAME(:LNBLNK(NAME)) 

Compiling and running GetFilNam.f results in:

demo% pwd
demo% f77 -silent -o getfil GetFilNam.f
demo% getfil

Opening Files Without a Name

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

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, unless (with f77) STATUS='KEEP' is specified in the CLOSE statement.

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.

Preconnected 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:

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.

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
      WRITE( IU, '(I4)' ) IU 

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

demo% f77 -silent -o testunit TestUnit.f
demo% testunit
demo% cat fort.25

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)
         WRITE(51,*) 'Writing to file: ', outfile
demo% f77 -silent -o tstarg testarg.f
demo% tstarg AnyFileName
demo% cat AnyFileName
 Writing to file: AnyFileName

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)
         WRITE(51,*) 'Writing to file: ', outfile
demo% f77 -silent -o tstenv testenv.f
demo% setenv OUTFILE EnvFileName
demo% tstenv
demo% cat EnvFileName
 Writing to file: EnvFileName

When using getarg or getenv, care should be taken regarding leading or trailing blanks. (FORTRAN 77 programs can use the library function LNBLNK; Fortran 95 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 f95.)

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 
&                   /.TRUE.,.FALSE.,.FALSE., 'TST',.FALSE. / 
      READ(1, *) I, B, N 
      WRITE(2, *) I, B, N 

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

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/<release>/src/ioinit.f, where <release> varies for each software release. (Contact your system adminstrator for details.)

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:

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.

f77: VAX / VMS Logical File Names

If you are porting from VMS FORTRAN to FORTRAN 77, the VMS-style logical file names in the INCLUDE statement are mapped to UNIX path names. The environment variable LOGICALNAMEMAPPING defines the mapping between the logical names and the UNIX path name. If the environment variable LOGICALNAMEMAPPING is set and the -vax, -xl or -xld compiler options are used, the compiler interprets VMS logical file names on the INCLUDE statement.

The compiler sets the environment variable to a string with the following syntax:

      "lname1=path1; lname2=path2; ... " 

Each lname is a logical name, and each path is the path name of a directory (without a trailing /). All blanks are ignored when parsing this string. Any trailing /list or /nolist is stripped from the file name in the INCLUDE statement. Logical names in a file name are delimited by the first colon in the VMS file name. The compiler converts file names of the form:




Uppercase and lowercase are significant in logical names. If a logical name is encountered on the INCLUDE statement that was not specified by LOGICALNAMEMAPPING, the file name is used unchanged.

Direct I/O

Direct or random I/O allows you to access a file directly by record number. Record numbers are assigned when a record is written. Unlike sequential I/O, direct I/O records can be read and written in any order. However, in a direct access file, all records must be the same fixed length. Direct access files are declared with the ACCESS='DIRECT' specifier on the OPEN statement for the file.

A logical record in a direct access file is a string of bytes of a length specified by the OPEN statement's RECL= specifier. READ and WRITE statements must not specify logical records larger than the defined record size. (Record sizes are specified in bytes.) Shorter records are allowed. Unformatted, direct writes leave the unfilled part of the record undefined. Formatted, direct writes cause the unfilled record to be padded with blanks.

Direct access READ and WRITE statements have an extra argument, REC=n, to specify the record number to be read or written.

Example: Direct access, unformatted:

      OPEN( 2, FILE='data.db', ACCESS='DIRECT', RECL=200,
&             FORM='UNFORMATTED', ERR=90 ) 
      READ( 2, REC=13, ERR=30 ) X, Y 

This program opens a file for direct access, unformatted I/O, with a fixed record length of 200 bytes, then reads the thirteenth record into X and Y.

Example: Direct access, formatted:

      OPEN( 2, FILE='inven.db', ACCESS='DIRECT', RECL=200, 
&             FORM='FORMATTED', ERR=90 )
      READ( 2, FMT='(I10,F10.3)', REC=13, ERR=30 ) X, Y

This program opens a file for direct access, formatted I/O, with a fixed record length of 200 bytes. It then reads the thirteenth record and converts it with the format (I10,F10.3).

For formatted files, the size of the record written is determined by the FORMAT statement. In the preceding example, the FORMAT statement defines a record of 20 characters or bytes. More than one record can be written by a single formatted write if the amount of data on the list is larger than the record size specified in the FORMAT statement. In such a case, each subsequent record is given successive record numbers.

Example: Direct access, formatted, multiple record write:

      WRITE(21,'(10F10.3)',REC=11) (X(J),J=1,100) 

The write to direct access unit 21 creates 10 records of 10 elements each (since the format specifies 10 elements per record) these records are numbered 11 through 20.

Binary I/O

Sun Workshop Fortran 95 and Fortran 77 extend the OPEN statement to allow declaration of a "binary" I/O file.

Opening a file with FORM='BINARY' has roughly the same effect as FORM='UNFORMATTED', except that no record lengths are embedded in the file. Without this data, there is no way to tell where one record begins, or ends. Thus, it is impossible to BACKSPACE a FORM='BINARY' file, because there is no way of telling where to backspace to. A READ on a 'BINARY' file will read as much data as needed to fill the variables on the input list.

Internal Files

An internal file is an object of type CHARACTER such as a variable, substring, array, element of an array, or field of a structured record. Internal file READ can be from a constant character string. I/O on internal files simulates formatted READ and WRITE statements by transferring and converting data from one character object to another data object. No file I/O is performed.

When using internal files:

Example: Sequential formatted read from an internal file (one record only):

demo% cat intern1.f
      CHARACTER X*80 
      READ( *, '(A)' ) X 
      READ( X, '(I3,I4)' ) N1, N2 ! This codeline reads the internal file X
      WRITE( *, * )  N1, N2
demo% f77 -silent -o tstintern intern1.f
demo% tstintern
 12 99
  12  99

Example: Sequential formatted read from an internal file (three records):

demo% cat intern2.f 
      CHARACTER  LINE(4)*16   ! This is our "internal file"
*                      12341234
      DATA  LINE(1) / ' 81  81 ' /
      DATA  LINE(2) / ' 82  82 ' /
      DATA  LINE(3) / ' 83  83 ' /
      DATA  LINE(4) / ' 84  84 ' /
      READ( LINE,'(2I4)') I,J,K,L,M,N 
      PRINT *, I, J, K, L, M, N
demo% f77 -silent intern2.f 
demo% a.out 
   81  81  82  82  83  83 

Example: Direct access read from an internal file (one record) (f77 only):

demo% cat intern3.f 
      CHARACTER LINE(4)*16   ! This is our "internal file" 
*                      12341234
      DATA  LINE(1) / ' 81  81 ' /
      DATA  LINE(2) / ' 82  82 ' /
      DATA  LINE(3) / ' 83  83 ' /
      DATA  LINE(4) / ' 84  84 ' /
      READ ( LINE, FMT=20, REC=3 ) M, N   
20       FORMAT( I4, I4 ) 
      PRINT *, M, N 
demo% f77 -silent intern3.f 
demo% a.out 
   83  83 

f77: Tape I/O

Most typical Fortran I/O is done to disk files. However, by associating a logical unit number to a physically mounted tape drive via the OPEN statement, it is possible to do I/O directly to tape.

It could be more efficient to use the TOPEN() routines rather than Fortran I/O statements to do I/O on magnetic tape.

Using TOPEN Routines

With the nonstandard tape I/O package (see topen(3F)) you can transfer blocks between the tape drive and buffers declared as Fortran character variables. You can then use internal I/O to fill and empty these buffers. This facility does not integrate with the rest of Fortran I/O and even has its own set of tape logical units. Refer to the man pages for complete information.

Fortran Formatted I/O for Tape

The Fortran I/O statements provide facilities for transparent access to formatted, sequential files on magnetic tape. There is no limit on formatted record size, and records may span tape blocks.

Fortran Unformatted I/O for Tape

Using the Fortran I/O statements to connect a magnetic tape for unformatted access is less satisfactory. The implementation of unformatted records implies that the size of a record (plus eight characters of overhead) cannot be bigger than the buffer size.

As long as this restriction is complied with, the I/O system does not write records that span physical tape blocks, writing short blocks when necessary. This representation of unformatted records is preserved (even though it is inappropriate for tapes) so that files can be freely copied between disk and tapes.

Since the block-spanning restriction does not apply to tape reads, files can be copied from tape to disk without any special considerations.

Tape File Representation

A Fortran data file is represented on tape by a sequence of data records followed by an endfile record. The data is grouped into blocks, with maximum block size determined when the file is opened. The records are represented in the same way as records in disk files: formatted records are followed by newlines; unformatted records are preceded and followed by character counts. In general, there is no relation between Fortran records and tape blocks; that is, records can span blocks, which can contain parts of several records.

The only exception is that Fortran does not write an unformatted record that spans blocks; thus, the size of the largest unformatted record is eight characters less than the block size.

The dd Conversion Utility

An end-of-file record in Fortran maps directly into a tape mark. In this respect, Fortran files are the same as tape system files. But since the representation of Fortran files on tape is the same as that used in the rest of UNIX, naive Fortran programs cannot read 80-column card images on tape. If you have an existing Fortran program and an existing data tape to read with it, translate the tape using the dd(1) utility, which adds newlines and strips trailing blanks.

Example: Convert a tape on mt0 and pipe that to the executable ftnprg:

demo% dd if=/dev/rmt0 ibs=20b cbs=80 conv=unblock | ftnprg 

The getc Library Routine

As an alternative to dd, you can call the getc(3F) library routine to read characters from the tape. You can then combine the characters into a character variable and use internal I/O to transfer formatted data. See also TOPEN(3F).


The end-of-file condition is reached when an end-of-file record is encountered during execution of a READ statement. The standard states that the file is positioned after the end-of-file record. In real life, this means that the tape read head is poised at the beginning of the next file on the tape. Although it seems as if you could read the next file on the tape, this is not strictly true, and is not covered by the ANSI FORTRAN 77 Language Standard.

The standard also says that a BACKSPACE or REWIND statement can be used to reposition the file. Consequently, after reaching end-of-file, you can backspace over the end-of-file record and further manipulate the file--for example, writing more records at the end, rewinding the file, and rereading or rewriting it.

Multifile Tapes

The name used to open the tape file determines certain characteristics of the connection, such as the recording density and whether the tape is automatically rewound when opened and closed.

To access a file on a tape with multiple files, first use the mt(1) utility to position the tape to the needed file. Then open the file as a no-rewind magnetic tape such as
/dev/nrmt0. Referencing the tape with this name prevents it from being repositioned when it is closed. By reading the file until end-of-file and then reopening it, a program can access the next file on the tape. Any program subsequently referencing the same tape can access it where it was last left, preferably at the beginning of a file, or past the end-of-file record.

However, if your program terminates prematurely, it may leave the tape positioned anywhere. Use the SunOSTM operating system command mt(1) to reposition the tape appropriately.

Fortran 95 I/O Considerations

Sun WorkShop 6 Fortran 95 and Fortran 77 are I/O compatible. Executables containing intermixed f77 and f95 compilations can do I/O to the same unit from both the f77 and f95 parts of the program.

However, Fortran 95 provides some additional features:

Sun Microsystems, Inc.
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