You can use the environment variable mechanism of the operating system and shells to associate logical units with external files. For example, setting the environment variable FORT6 to a file lets you redirect stdout to the specified file (see Table 7-4).

Compaq Fortran attempts to use certain environment variables in the absence of a file name.

When using scratch files, you can use the TMPDIR environment variable to specify where the scratch file gets created (see Section 7.4.2).

Setting and Unsetting Environment Variables

Before program execution, you can use shell commands to specify a value for an environment variable. This specified value might be a directory and/or file name of an external file you want to associate with a preconnected unit or a specific unit number.

With the C Shell, use the setenv command to set an environment variable:

% setenv FORT8 /usr/users/smith/test.dat

To remove the association of an environment variable and an external file with the C shell, use the unsetenv command.

% unsetenv FORT8

With the Bourne shell (sh) and Korn shell (ksh) and bash shell (bash) (L*X ONLY), use the export command and assignment command to set the environment variable:

$ export FORT8 $ FORT8=/usr/users/smith/test.dat

To remove the association of an environment variable and an external file with the Bourne shell or Korn shell or bash shell (L*X ONLY), use the unset command:

$ unset FORT8

Implied Compaq Fortran Logical Unit Numbers

The ACCEPT, PRINT, and TYPE statements, and the use of an asterisk (*) in place of a unit number in READ and WRITE statements, do not include an explicit logical unit number. Each of these Fortran 95/90 statements uses an implicit internal logical unit number and environment variable. Each environment variable is in turn associated by default with one of the Fortran 95/90 file names that are associated with standard I/O files. Table 7-7 shows these relationships.

Table 7-7 Implicit Compaq Fortran Logical Units

Compaq Fortran Statement	Environment Variable When -vms Specified	Environment Variable When -vms Omitted	Standard I/O File Name
READ (*,f) iolist	FOR_READ	FORT5	stdin
READ f,iolist	FOR_READ	FORT5	stdin
ACCEPT f,iolist	FOR_ACCEPT	FORT5	stdin
WRITE (*,f) iolist	FOR_PRINT	FORT6	stdout
PRINT f,iolist	FOR_PRINT	FORT6	stdout
TYPE f,iolist	FOR_TYPE	FORT6	stdout

You can change the file associated with these Compaq Fortran environment variables, as you would any other environment variable, by means of the environment variable assignment command. For example, with the C shell:

% setenv FOR_READ /usr/users/smith/test.dat

After executing the preceding command, the environment variable for the READ statement using an asterisk refers to file test.dat in directory /usr/users/smith .

For More Information:

• On a list of Compaq Fortran I/O statements, see Table 7-1.
• On the shell commands you can use to set or unset environment variables, see Appendix B.
• On record I/O, see Section 7.6.
• On Compaq Fortran I/O statements and specifier values, including defaults, see Table 7-1.
• On statement syntax, see the Compaq Fortran Language Reference Manual.
• On the ERR and IOSTAT specifiers, see Chapter 8.
• On closing files, see Section 7.5.3.

The INQUIRE statement returns information about a file and has three forms:

• Inquiry by unit
• Inquiry by file name
• Inquiry by output item list

An inquiry by unit is usually done for an opened (connected) file. An inquiry by unit causes the Compaq Fortran RTL to check whether the specified unit is connected or not. One of the following occurs:

• If the unit is connected:
  • The EXIST and OPENED specifier variables indicate a true value
  • The pathname and file name are returned in the NAME specifier variable (if the file is named)
  • Other information requested on the previously connected file is returned
  • Default values are usually returned for the INQUIRE specifiers also associated with the OPEN statement (see Table 7-5)
  • The RECL value unit for connected formatted files is always 1-byte units. For unformatted files, the RECL unit is 4-byte units, unless you specify the -assume byterecl option to request 1-byte units (see Section 3.6).
• If the unit is not connected:
  • The OPENED specifier indicates a false value
  • The unit NUMBER specifier variable is returned as a value of --1
  • Any other information returned will be undefined or default values for the various specifiers

For example, the following INQUIRE statement shows whether unit 3 has a file connected (OPENED specifier) in logical variable I_OPENED, the name (case sensitive) in character variable I_NAME, and whether the file is opened for READ, WRITE, or READWRITE access in character variable I_ACTION:

INQUIRE (3, OPENED=I_OPENED, NAME=I_NAME, ACTION=I_ACTION)

7.5.2.2 Inquiry by File Name

An inquiry by name causes the Compaq Fortran RTL to scan its list of open files for a matching file name. One of the following occurs:

• If a match occurs:
  • The EXIST and OPENED specifier variables indicate a true value.
  • The pathname and file name are returned in the NAME specifier variable.
  • The UNIT number is returned in the NUMBER specifier variable.
  • Other information requested on the previously connected file is returned.
  • Default values are usually returned for the INQUIRE specifiers also associated with the OPEN statement (see Table 7-5).
  • The RECL value unit for connected formatted files is always 1-byte units. For unformatted files, the RECL unit is 4-byte units, unless you specify the -assume byterecl option to request 1-byte units (see Section 3.6).
• If no match occurs:
  • The OPENED specifier variable indicates a false value.
  • The unit NUMBER specifier variable is returned as a value of --1.
  • The EXIST specifier variable indicates (true or false) whether the named file exists on the device or not.
  • If the file does exist, the NAME specifier variable contains the pathname and file name.
  • Any other information returned will be default values for the various specifiers, based on any information specified when calling INQUIRE.

The following INQUIRE statement returns whether the file named log_file is a file connected in logical variable I_OPEN, whether the file exists in logical variable I_EXIST, and the unit number in integer variable I_NUMBER.

INQUIRE (FILE='log_file', OPENED=I_OPEN, EXIST=I_EXIST, NUMBER=I_NUMBER)

7.5.2.3 Inquiry by Output Item List

Unlike inquiry by unit or inquiry by name, inquiry by output item list does not attempt to access any external file. It returns the length of a record for a list of variables that would be used for unformatted WRITE, READ, and REWRITE statements (REWRITE is a Compaq Fortran extension).

The following INQUIRE statement returns the maximum record length of the variable list in variable I_RECLENGTH. This variable is then used to specify the RECL value in the OPEN statement:

INQUIRE (IOLENGTH=I_RECLENGTH) A, B, H OPEN (FILE='test.dat', FORM='UNFORMATTED', RECL=I_RECLENGTH, UNIT=9)

For an unformatted file, the RECL value is returned using 4-byte units, unless you specify the -assume byterecl option to request 1-byte units.

For More Information:

• On the INQUIRE statement and its specifiers, see the Compaq Fortran Language Reference Manual.
• On record I/O, see Section 7.6.
• On OPEN statement specifiers, see Table 7-5.
• On the -assume byterecl option, see Section 3.6.

Usually, any external file opened should be closed by the same program before it completes. The CLOSE statement disconnects the unit and its external file. You must specify the unit number (UNIT specifier) to be closed.

You can also specify:

• Whether the file should be deleted or kept (STATUS specifier)
• Error handling information (ERR and IOSTAT specifiers)

To delete a file when closing it:

• In the OPEN statement, specify the ACTION keyword (such as ACTION='READ'). Avoid using the READONLY keyword, because a file opened using the READONLY keyword cannot be deleted when it is closed.
• In the CLOSE statement, specify the keyword STATUS='DELETE'.

If you opened an external file and did an inquire by unit, but do not like the default value for the ACCESS specifier, you can close the file and then reopen it, explicitly specifying the ACCESS desired.

There usually is no need to close preconnected units. Internal files are neither opened nor closed.

For More Information:

• On a list of Compaq Fortran I/O statements, see Table 7-1.
• On opening files using a C routine (USEROPEN) and then using Compaq Fortran I/O statements, see Section 7.7.
• On OPEN statement specifiers, see Section 7.5.1.2.
• On statement syntax and specifier values, see the Compaq Fortran Language Reference Manual.

After you open a file or use a preconnected file, you can use the following statements:

• READ, WRITE and PRINT to perform record I/O.
• BACKSPACE, ENDFILE, REWIND to set record position within the file.
• ACCEPT, DELETE, REWRITE, TYPE, DEFINE FILE, and FIND to perform various operations. These statements are Compaq extensions.

These statements are described in Section 7.2 and the Compaq Fortran Language Reference Manual.

The record I/O statement must use the appropriate record I/O form (formatted, list-directed, namelist, or unformatted), as described in Section 7.3.

7.6.1 Record I/O Statement Specifiers

You can use the following specifiers with the READ and WRITE record I/O statements:

• UNIT specifies the unit number to or from which input or output will occur.
• END specifies a label to branch to if an error occurs; only applies to input statements like READ.
• ERR specifies a label to branch to if an error occurs.
• IOSTAT specifies an integer variable to contain the IOSTAT number if an error occurs.
• FMT specifies a label of a FORMAT statement.
• NML specifies a label of a NAMELIST statement.
• REC specifies a record number for direct access.

When using nonadvancing I/O, use the ADVANCE, EOR, and SIZE specifiers, as described in Section 7.6.4.

When using the REWRITE statement (a Compaq Fortran extension), you can use the UNIT, FMT, ERR, and IOSTAT specifiers.

For More Information

• On specifier syntax and complete information, see the Compaq Fortran Language Reference Manual.
• On available record types, see Section 7.4.3 and Section 7.8.
• On the error-related record I/O specifiers ERR, END, and IOSTAT, see Chapter 8.
• On the ADVANCE, EOR, and SIZE specifiers, see Section 7.6.4.
• Record positioning, see Section 7.6.3.
• Record I/O transfer, see Section 7.6.5.
• Record advancement, see Section 7.6.4.

Record access refers to how records will be read from or written to a file, regardless of its organization. Record access is specified each time you open a file; it can be different each time. The type of record access permitted is determined by the combination of file organization and record type.

For instance, you can:

• Add records to a sequential file with ORGANIZATION= ' SEQUENTIAL ' and POSITION= ' APPEND ' (or use the Compaq extension ACCESS= ' APPEND ' ).
• Add records sequentially by using multiple WRITE statements, close the file, and then open it again with ORGANIZATION= ' SEQUENTIAL ' and ACCESS= ' SEQUENTIAL ' (or ACCESS= ' DIRECT ' if the sequential file has fixed-length records).

Sequential access transfers records sequentially to or from files or I/O devices such as terminals. You can use sequential I/O with any type of supported file organization and record type.

If you select sequential access mode for files with sequential or relative organization, records are written to or read from the file starting at the beginning of the file and continuing through it, one record after another. A particular record can be retrieved only after all of the records preceding it have been read; new records can be written only at the end of the file.

7.6.2.2 Direct Access

Direct access transfers records selected by record number to and from either sequential files stored on disk with a fixed-length record type or relative organization files.

If you select direct access mode, you can determine the order in which records are read or written. Each READ or WRITE statement must include the relative record number, indicating the record to be read or written.

You can directly access a sequential disk file only if it contains fixed-length records. Because direct access uses cell numbers to find records, you can enter successive READ or WRITE statements requesting records that either precede or follow previously requested records. For example, the first of the following statements reads record 24; the second reads record 10:

READ (12,REC=24) I READ (12,REC=10) J

7.6.2.3 Limitations of Record Access by File Organization and Record Type

You can use both access modes on sequential and relative files. However, direct access to a sequential organization file can only be done if the file resides on disk and contains fixed-length records.

Table 7-8 summarizes the types of access permitted for the various combinations of file organizations and record types.

Table 7-8 Allowed Record Access for File Organizations and Record Types

Organization	Record Type	Sequential Access	Direct Access
Sequential file	Fixed Variable Segmented Stream Stream_CR Stream_LF	Yes Yes Yes Yes Yes Yes	Yes 1 No No No No No
Relative file 1	Fixed	Yes	Yes

Depending on the value specified by the ACTION (or READONLY) specifier in the OPEN statement, the file will be opened by your program for reading, writing, or both reading and writing records. This simply checks that the program itself executes the type of statements intended.

For performance reasons, record-locking and shared-file checking are not supported by the current version of Compaq Fortran on Compaq Tru64 UNIX and Linux systems. When you open the file, access is always granted, regardless of whether:

• The OPEN statement SHARED specifier was specified
• Other processes have already opened the file

Similarly, the UNLOCK statement is ignored using the current version of Compaq Fortran.

You might open a file for writing records (or reading and writing records) and know another process might simultaneously have the file open and be writing records. In this case, you need to coordinate access times among those processes to handle the possibility of simultaneous WRITE and REWRITE statements on the same record positions.

7.6.3 Specifying the Initial Record Position

When you open a disk file, you can use the OPEN statement's POSITION specifier to request one of the following initial record positions within the file:

• The initial position before the first record (POSITION= ' REWIND ' ). A sequential access READ or WRITE statement will read or write the first record in the file.
• A point beyond the last record in the file (POSITION= ' APPEND ' ), just before the end-of-file record, if one exists. For a new file, this is the initial position before the first record (same as ' REWIND ' ). You might specify ' APPEND ' before you write records to an existing sequential file using sequential access.
• The current position (ASIS). This is usually used only to maintain the current record position when reconnecting a file. The second OPEN specifies the same unit number and specifies the same file name (or omits it), which leaves the file open, retaining the current record position.
  However, if the second OPEN specifies a different file name for the same unit number, the file will be closed and then opened, causing a loss of current record position.

The following I/O statements allow you to change the current record position:

• REWIND sets the record position to the initial position before the first record. A sequential access READ or WRITE statement would read or write the first record in the file.
• BACKSPACE sets the record position to the previous record in a file. Using sequential access, if you wrote record 5, issued a BACKSPACE to that unit, and then read from that unit, you would read record 5.
• ENDFILE writes an end-of-file marker. This is typically done after writing records using sequential access just before you close the file.

Unless you use nonadvancing I/O (see Section 7.6.4), reading and writing records usually advances the current record position by one record. As discussed in Section 7.6.5, more than one record might be transferred using a single record I/O statement.

7.6.4 Advancing and Nonadvancing Record I/O

After you open a file, if you omit the ADVANCE specifier (or specify ADVANCE= ' YES ' ) in READ and WRITE statements, advancing I/O (normal FORTRAN-77 I/O) will be used for record access. When using advancing I/O:

• Record I/O statements transfer one entire record (or multiple records).
• Record I/O statements advance the current record position to a position before the next record.

You can request nonadvancing I/O for the file by specifying the ADVANCE= ' NO ' specifier in a READ and WRITE statement. You can use nonadvancing I/O only for sequential access to external files using formatted I/O (not list-directed or namelist).

When you use nonadvancing I/O, the current record position does not change, and part of record might be transferred, unlike advancing I/O where one entire record or records are always transferred.

You can alternate between advancing and nonadvancing I/O by specifying different values for the ADVANCE specifier ( ' YES ' and ' NO ' ) in the READ and WRITE record I/O statements.

When reading records with either advancing or nonadvancing I/O, you can use the END branch specifier to branch to a specified label when the end of the file is read.

Because nonadvancing I/O might not read an entire record, it also supports an EOR branch specifier to branch to a specified label when the end of the record is read. If you omit the EOR and the IOSTAT specifiers when using nonadvancing I/O, an error results when the end-of-record is read.

When using nonadvancing input, you can use the SIZE specifier to return the number of characters read. For example, in the following READ statement, SIZE=X (where variable X is an integer) returns the number of characters read in X and an end-of-record condition causes a branch to label 700:

150 FORMAT (F10.2, F10.2, I6) READ (UNIT=20, FMT=150, SIZE=X, ADVANCE='NO', EOR=700) A, F, I

7.6.5 Record Transfer

I/O statements transfer all data as records. The amount of data that a record can contain depends on the following circumstances:

• With formatted I/O (except for fixed-length records), the number of items in the I/O statement and its associated format specifier jointly determine the amount of data to be transferred.
• With namelist and list-directed output, the items listed in the NAMELIST statement or I/O statement list (in conjunction with the NAMELIST or list-directed formatting rules) determine the amount of data to be transferred.
• With unformatted I/O (except for fixed-length records), the I/O statement alone specifies the amount of data to be transferred.
• When you specify fixed-length records (RECORDTYPE= ' FIXED ' ), all records are the same size. If the size of an I/O record being written is less than the record length (RECL), extra bytes are added (padding).

Typically, the data transferred by an I/O statement is read from or written to a single record. It is possible, however, for a single I/O statement to transfer data from or to more than one record, depending on the form of I/O used.

7.6.5.1 Input Record Transfer

When using advancing I/O, if an input statement specifies fewer data fields (less data) than the record contains, the remaining fields are ignored.

If an input statement specifies more data fields than the record contains, one of the following occurs:

• For formatted input using advancing I/O, if the file was opened with PAD= ' YES ' , additional fields are read as spaces. If the file is opened with PAD= ' NO ' , an error occurs (the input statement should not specify more data fields than the record contains).
  For formatted input using nonadvancing I/O (ADVANCE= ' NO ' ), an end-of-record (EOR) condition is returned. If the file was opened with PAD= ' YES ' , additional fields are read as spaces.
• For list-directed input, another record is read.
• For namelist input, another record is read.
• For unformatted input, an error occurs.

If an output statement specifies fewer data fields than the record contains (less data than required to fill a record), the following occurs:

• With fixed-length records (RECORDTYPE= ' FIXED ' ), all records are the same size. If the size of an I/O record being written is less than the record length (RECL), extra bytes are added (padding) in the form of spaces (for a formatted record) or zeros (for an unformatted record).
• With other record types, the fields present are written and those omitted are not written (might result in a short record).

If the output statement specifies more data than the record can contain, an error occurs, as follows:

• With formatted or unformatted output using fixed-length records
  If the items in the output statement and its associated format specifier result in a number of bytes that exceed the maximum record length (RECL), an error occurs.
• With formatted or unformatted output not using fixed-length records
  If the items in the output statement and its associated format specifier result in a number of bytes that exceed the maximum record length (RECL), the Compaq Fortran RTL attempts to increase the RECL value and write the longer record. To obtain the RECL value, use an INQUIRE statement.
• For list-directed output and namelist output, if the data specified exceeds the maximum record length (RECL), another record is written.

For More Information:

• On Compaq Fortran I/O statements, see Table 7-1.
• On record I/O specifiers, see Section 7.6.1.
• On statement syntax and specifier values, see the Compaq Fortran Language Reference Manual.
• On improving Compaq Fortran I/O performance, see Section 5.5.