11.  Input/Output

11.1.  Format Variables

• A format may be the value of a character expression (constant or otherwise), or be stored in a character array, as in

  write(6, '(i5)') x
  

11.2.  END=, ERR=, and IOSTAT= Clauses

• A read or write statement may contain end=, err=, and iostat= clauses, as in

  write(6, 101, err=20, iostat=a(4))
  read(5, 101, err=20, end=30, iostat=x)
  

  Here 5 and 6 are the units on which the I/O is done, 101 is the statement number of the associated format, 20 and 30 are statement numbers, and a and x are integers. If an error occurs during I/O, control returns to the program at statement 20. If the end of the file is reached, control returns to the program at statement 30. In any case, the variable referred to in the iostat= clause is given a value when the I/O statement finishes. (Yes, the value is assigned to the name on the right side of the equal sign.) This value is zero if all went well, negative for end of file, and some positive value for errors.

11.3.  Formatted I/O

11.3.1.  Character Constants

• Character constants in formats are copied literally to the output. Character constants cannot be read into.

  write(6,'(i2,'' isn''''t '',i1)') 7, 4
  

  produces

  7 isn't 4
  

  Here the format is the character constant

  (i2,' isn''t ',i1)
  

  and the character constant

  isn't
  

  is copied into the output.

11.3.2.  Positional Editing Codes

• t, tl, tr, and x codes control where the next character is in the record. trn or nx specifies that the next character is to the right of the current position. tln specifies that the next character is to the left of the current position, allowing parts of the record to be reconsidered. tn says that the next character is to be character number in the record. (See section 3.4 in the main text.)

11.3.3.  Colon

• A colon in the format terminates the I/O operation if there are no more data items in the I/O list, otherwise it has no effect. In the fragment

  x='("hello", :, " there", i4)'
  write(6, x) 12
  write(6, x)
  

  the first write statement prints hello there 12, while the second only prints hello.

11.3.4.  Optional Plus Signs

• According to the Standard, each implementation has the option of putting plus signs in front of non-negative numeric output. The sp format code may be used to make the optional plus signs actually appear for all subsequent items while the format is active. The ss format code guarantees that the I/O system will not insert the optional plus signs, and the s format code restores the default behavior of the I/O system. (Since we never put out optional plus signs, ss and s codes have the same effect in our implementation.)

11.3.5.  Blanks on Input

• Blanks in numeric input fields, other than leading blanks will be ignored following a bn code in a format statement, and will be treated as zeros following a bz code in a format statement. The default for a unit may be changed by using the open statement. (Blanks are ignored by default.)

11.3.6.  Unrepresentable Values

• The Standard requires that if a numeric item cannot be represented in the form required by a format code, the output field must be filled with asterisks. (We think this should have been an option.)

11.3.7.  Iw.m

• There is a new integer output code, iw.m. It is the same as iw, except that there will be at least digits in the output field, including, if necessary, leading zeros. The case i is special, in that if the value being printed is 0, the output field is entirely blank. iw.1 is the same as iw.

11.3.8.  Floating Point

• On input, exponents may start with the letter E, D, e, or d. All have the same meaning. On output we always use e. The e and d format codes also have identical meanings. A leading zero before the decimal point in e output without a scale factor is optional with the implementation. (We do not print it.) There is a gw.d format code which is the same as ew.d and fw.d on input, but which chooses f or e formats for output depending. on the size of the number and of .

11.3.9.  ``A'' Format Code

• A codes are used for character values. aw use a field width of , while a plain a uses the length of the character item.

11.4.  Standard Units

• There are default formatted input and output units. The statement

  read 10, a, b
  

  reads from the standard unit using format statement 10. The default unit may be explicitly specified by an asterisk, as in

  read(*, 10) a,b
  

  Similarly, the standard output units is specified by a print statement or an asterisk unit:

  print 10
  write(*, 10)
  

11.5.  List-Directed Formatting

• List-directed I/O is a kind of free form input for sequential I/O. It is invoked by using an asterisk as the format identifier, as in

  read(6, *) a,b,c
  

• On input, values are separated by strings of blanks and possibly a comma. Values, except for character strings, cannot contain blanks. End of record counts as a blank, except in character strings, where it is ignored. Complex constants are given as two real constants separated by a comma and enclosed in parentheses. A null input field, such as between two consecutive commas, means the corresponding variable in the I/O list is not changed. Values may be preceded by repetition counts, as in

  4*(3.,2.) 2*, 4*'hello'
  

  which stands for 4 complex constants, 2 null values, and 4 string constants.
• For output, suitable formats are chosen for each item. The values of character strings are printed; they are not enclosed in quotes, so they cannot be read back using list-directed input.

11.6.  Direct I/O

• A file connected for direct access consists of a set of equal-sized records each of which is uniquely identified by a positive integer. The records may be written or read in any order, using direct access I/O statements.
• Direct access read and write statements have an extra argument, rec=, which gives the record number to be read or written.

  read(2, rec=13, err=20) (a(i), i=1, 203)
  

  reads the thirteenth record into the array a.
• The size of the records must be given by an open statement (see below). Direct access files may be connected for either formatted or unformatted I/O.

11.7.  Internal Files

• Internal files are character string objects, such as variables or substrings, or arrays of type character. In the former cases there is only a single record in the file, in the latter case each array element is a record. The Standard includes only sequential formatted I/O on internal files. (I/O is not a very precise term to use here, but internal files are dealt with using read and write). There is no list-directed I/O on internal files. Internal files are used by giving the name of the character object in place of the unit number, as in

  character*80 x
  read(5,"(a)") x
  read(x,"(i3,i4)") n1,n2
  

  which reads a card image into x and then reads two integers from the front of it. A sequential read or write always starts at the beginning of an internal file.
• (We also support a compatible extension, direct I/O on internal files. This is like direct I/O on external files, except that the number of records in the file cannot be changed.)

11.8.  OPEN, CLOSE, and INQUIRE Statements

• These statements are used to connect and disconnect units and files, and to gather information about units and files.

11.8.1.  OPEN

• The open statement is used to connect a file with a unit, or to alter some properties of the connection. The following is a minimal example.

  open(1, file='fort.junk')
  

  open takes a variety of arguments with meanings described below.

\(IP .nr IP +1

unit= a small non-negative integer which is the unit to which the file is to be connected. We allow, at the time of this writing, 0 through 9. If this parameter is the first one in the open statement, the unit= can be omitted. \(IP .nr IP +1

iostat= is the same as in read or write. \(IP .nr IP +1

err= is the same as in read or write. \(IP .nr IP +1

file= a character expression, which when stripped of trailing blanks, is the name of the file to be connected to the unit. The filename should not be given if the status=scratch. \(IP .nr IP +1

status= one of old, new, scratch, or unknown. If this parameter is not given, unknown is assumed. If scratch is given, a temporary file will be created. Temporary files are destroyed at the end of execution. If new is given, the file will be created if it doesn't exist, or truncated if it does. The meaning of unknown is processor dependent; our system treats it as synonymous with old. \(IP .nr IP +1

access= sequential or direct, depending on whether the file is to be opened for sequential or direct I/O. \(IP .nr IP +1

form= formatted or unformatted. \(IP .nr IP +1

recl= a positive integer specifying the record length of the direct access file being opened. We measure all record lengths in bytes. On UNIXsystems a record length of 1 has the special meaning explained in section 5.1 of the text. \(IP .nr IP +1

blank= null or zero. This parameter has meaning only for formatted I/O. The default value is null. zero means that blanks, other than leading blanks, in numeric input fields are to be treated as zeros.

• Opening a new file on a unit which is already connected has the effect of first closing the old file.

11.8.2.  CLOSE

• close severs the connection between a unit and a file. The unit number must be given. The optional parameters are iostat= and err= with their usual meanings, and status= either keep or delete. Scratch files cannot be kept, otherwise keep is the default. delete means the file will be removed. A simple example is

  close(3, err=17)
  

11.8.3.  INQUIRE

• The inquire statement gives information about a unit (``inquire by unit'') or a file (``inquire by file''). Simple examples are:

  inquire(unit=3, namexx)
  inquire(file='junk', number=n, exist=l)
  

\(IP .nr IP +1

file= a character variable specifies the file the inquire is about. Trailing blanks in the file name are ignored. \(IP .nr IP +1

unit= an integer variable specifies the unit the inquire is about. Exactly one of file= or unit= must be used. \(IP .nr IP +1

iostat=, err= are as before. \(IP .nr IP +1

exist= a logical variable. The logical variable is set to .true. if the file or unit exists and is set to .false. otherwise. \(IP .nr IP +1

opened= a logical variable. The logical variable is set to .true. if the file is connected to a unit or if the unit is connected to a file, and it is set to .false. otherwise. \(IP .nr IP +1

number= an integer variable to which is assigned the number of the unit connected to the file, if any. \(IP .nr IP +1

named= a logical variable to which is assigned .true. if the file has a name, or .false. otherwise. \(IP .nr IP +1

name= a character variable to which is assigned the name of the file (inquire by file) or the name of the file connected to the unit (inquire by unit). The name will be the full name of the file. \(IP .nr IP +1

access= a character variable to which will be assigned the value 'sequential' if the connection is for sequential I/O, 'direct' if the connection is for direct I/O. The value becomes undefined if there is no connection. \(IP .nr IP +1

sequential= a character variable to which is assigned the value 'yes' if the file could be connected for sequential I/O, 'no' if the file could not be connected for sequential I/O, and 'unknown' if we can't tell. \(IP .nr IP +1

direct= a character variable to which is assigned the value 'yes' if the file could be connected for direct I/O, if the file could not be connected for direct I/O, and 'unknown' if we can't tell. \(IP .nr IP +1

form= a character variable to which is assigned the value 'formatted' if the file is connected for formatted I/O, or 'unformatted' if the file is connected for unformatted I/O. \(IP .nr IP +1

formatted= a character variable to which is assigned the value 'yes' if the file could be connected for formatted I/O, 'no' if the file could not be connected for formatted I/O, and 'unknown' if we can't tell. \(IP .nr IP +1

unformatted= a character variable to which is assigned the value 'yes' if the file could be connected for unformatted I/O, 'no' if the file could not be connected for unformatted I/O, and 'unknown' if we can't tell. \(IP .nr IP +1

recl= an integer variable to which is assigned the record length of the records in the file if the file is connected for direct access. \(IP .nr IP +1

nextrec= an integer variable to which is assigned one more than the number of the the last record read from a file connected for direct access. \(IP .nr IP +1

blank= a character variable to which is assigned the value 'null' if null blank control is in effect for the file connected for formatted I/O, 'zero' if blanks are being converted to zeros and the file is connected for formatted I/O.

      The gentle reader will remember that the people who wrote the standard probably weren't thinking of his needs. Here is an example. The declarations are omitted.

open(1, file="/dev/console")

      In the UNIXsystem environment, the only way to discover what permissions you have for a file is to open it and try to read and write it. The err= parameter will return system error numbers. The inquire statement does not give a way of determining permissions.