Connects file to a unit.
Syntax
 |
|
- io-specifier-list
is a list of the following comma-separated I/O specifiers:
- [UNIT=]unit
specifies the unit
to connect to an external file. unit must be an integer expression that evaluates to a
number greater than 0. If the optional keyword UNIT= is omitted, unit must be the first item in io-specifier-list.
- ACCESS=character-expression
specifies the method
of file access. character-expression can be one of the following arguments:
Table 10-33 Title not available (OPEN)
| 'DIRECT' | Open
file for direct access. |
| 'SEQUENTIAL' | Open file for sequential access (default). |
' POSITION=
APPEND' | To open a file for append (to position the
file just before the end-of-file record) |
- ACTION=character-expression
specifies the allowed
data-transfer operations. character-expression can be one of the following arguments:
Table 10-34 Title not available (OPEN)
| 'READ' | Do
not allow WRITE and ENDFILE statements. |
| 'WRITE' | Do not allow READ statements. |
| 'READWRITE' | Allow any data transfer statement (default). |
- BLANK=character-expression
specifies treatment
of blanks within numeric data on input. This specifier is applicable
to formatted input only. character-expression can be one of the following arguments:
Table 10-35 Title not available (OPEN)
| 'NULL' | Ignore
blanks (default). |
| 'ZERO' | Substitute zeroes for blanks. |
- DELIM=character-expression
specifies the delimiter
to use (if any) when delimiting character constants in list-directed
and namelist-directed formatting. This specifier is applicable to formatted
output only. character-expression can be one of the following arguments:
Table 10-36 Title not available (OPEN)
| 'APOSTROPHE' | Use
the apostrophe to delimit character constants in list-directed and namelist-directed
formatting. |
| 'QUOTE' | Use double-quotation marks to delimit
character constants in list-directed and namelist-directed formatting. |
| 'NONE' | Use no delimiter to delimit character constants
in list-directed and namelist-directed formatting (default). |
- ERR=stmt-label
specifies the label
of the executable statement to which control passes if an error
occurs during statement execution.
- FILE=character-expression
specifies the name
of the file to be connected to unit. character-expression can also be the ASCII representation of a device file.
If this specifier does not appear in the OPEN statement, a temporary scratch file is created.
- FORM=character-expression
specifies whether
the file is connected for formatted or unformatted I/O. character-expression can be one of the following arguments:
Table 10-37 Title not available (OPEN)
| 'FORMATTED' | Specify
formatted I/O. If the file is to be opened for sequential access,
this is the default. |
| 'UNFORMATTED' | Specify unformatted I/O. If the file is to
be opened for direct access, this is the default. |
- IOSTAT=integer-variable
returns the I/O
status after the statement executes. If the statement successfully
executes, integer-variable is set to zero. If an error occurs, it is set to
a positive integer that indicates which error occurred.
- PAD=character-expression
specifies whether
or not to pad the input record with blanks if the record contains
fewer characters than required by the format specification. This specifier
is applicable to formatted input only. character-expression can be one of the following arguments:
Table 10-38 Title not available (OPEN)
| 'YES' | Pad
input records with blanks (if necessary) to fill it out to length
required by format specification (default). |
| 'NO' | Do not pad input record with blanks if it is
not as long as record specified by format specification. |
- POSITION=character-expression
specifies the position
of an existing file to be opened for sequential access. character-expression can be one of the following arguments:
Table 10-39 Title not available (OPEN)
| 'ASIS' | Leave
file position unchanged (default). |
| 'REWIND' | Position the file at its start. |
| 'APPEND' | Position the file just before the end-of-file
record. |
If the file to be opened does not exist, this specifier is
ignored. New files are always positioned at their start.
- RECL=integer-expression
specifies the length
of each record in a file to be opened for direct access. The length
is measured in characters (bytes). This specifier must be present when
a file is opened for direct access and is ignored if file is opened
for sequential access.
- STATUS=character-expression
specifies the state
of the file when it is opened. character-expression can be one of the following arguments:
Table 10-40 Title not available (OPEN)
| 'OLD' | Open
an existing file. FILE= must also be specified and the named file must
exist. |
| 'NEW' | Create a new file. FILE= must also be specified and the named file must
not exist. |
| 'UNKNOWN' | If the file named in FILE= exists, open it with the status of OLD; if it does not exist, open it with the status
of NEW. This is the default status. |
| 'REPLACE' | If the file does not exist, create it
with a status of OLD; if it does exist, delete it and open it with a
status of NEW. If STATUS='REPLACE' is specified, FILE= must also be specified. |
| 'SCRATCH' | Create a scratch file. FILE= specifier must not be specified. For information about scratch files,
see “Scratch
files”. |
Description
The OPEN statement connects a unit to a file so that data
can be read from or written to that file. Once a file is connected
to a unit, the unit can be referenced by any program unit in the program.
I/O specifiers do not have to appear in any specific order
in the OPEN statement. However, if the optional keyword UNIT= is omitted, unit must be the first item in the list.
Only one unit can be connected to a file at a time. That is,
the same file cannot be connected to two different units. Attempting
to open a file that is connected to a different unit will produce undefined
results.
However,
multiple OPENs can be performed on the same unit. In other words,
if a unit is connected to a file that exists, it is permissible
to execute another OPEN statement for the same unit. If FILE= specifies a different file, the previously opened
file is automatically closed before the second file is connected
to the unit. If FILE= specifies the same file, the file remains connected
in the same position; the values of the BLANK=, DELIM=, PAD=, ERR=, and IOSTAT= specifiers can be changed, but attempts to change
the values of any of the other specifiers will be ignored.
Examples
The following examples illustrate different uses of the OPEN statement.
Opening a file for sequential access
The following OPEN statement connects the existing file inv to unit 10 and opens it (by default) for sequential
access. Only READ statements are permitted to perform data transfers. If
an error occurs, control passes to the executable statement labeled 100 and the error code is placed in the variable ios:
OPEN(10, FILE='inv', ERR=100, I0STAT=ios, &
ACTION='READ', STATUS='OLD') |
Opening a file for direct access
The following OPEN statement opens the file whose name is contained
in the variable next1, connecting it to unit 4 as a formatted, direct-access
file with a record length of 50 characters:
OPEN(ACCESS=”DIRECT”, UNIT=4, RECL=50, &
FORM=”FORMATTED”, FILE=next1) |
Opening a device for I/O transfers
The next example connects the system device /dev/console to unit 6; all data transfers that specify unit
6 will go to this device:
OPEN(6,FILE='/DEV/CONSOLE') |
Opening a scratch file
The following two OPEN statements produce the same results: open a scratch
file that is connected to unit 19 (if the FILE=name specifier had appeared in the first statement, the named
file would have been opened instead):
OPEN (UNIT=19)
OPEN (UNIT=19, STATUS=”SCRATCH”) |
I/O specifiers in an OPEN statement
Because the I/O specifiers that can be used in an OPEN statement do not have to appear in any specific
order, the following three OPEN statements are all equivalent:
OPEN(UNIT=3, STATUS='NEW', FILE='OUT.DAT')
OPEN(3, STATUS='NEW', FILE='OUT.DAT')
OPEN(STATUS='NEW', FILE='OUT.DAT', UNIT=3) |
Note, however, that in the second OPEN statement the number 3 must appear first because
of the omission of the optional keyword UNIT=. Thus, the following OPEN statement is illegal:
OPEN(STATUS='NEW', 3, FILE='OUT.DAT') ! illegal |
Related statements
CLOSE, INQUIRE, READ, and WRITE
Related concepts
For information about I/O concepts and examples of programs
that perform I/O, see Chapter 8 “I/O
and file handling”. For information about I/O formatting, see Chapter 9 “I/O
formatting”.
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