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:
- ACTION=character-expression
specifies the allowed data-transfer operations.
character-expression can be one of the
following arguments:
- 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:
- 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:
- 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:
- 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:
- 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:
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:
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=namespecifier 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”.
Printable version
