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File-based Input and Output

File-based Input and Output

Topics

• Unit Numbers

• Read Statement

• Write Statement

• Open Statement

• Close Statement

• Inquire Statement

• Direct Access Files

Unit Numbers

• Files are accessed by unit number.

• A program may connect a named file to a unit number or use a default value.

• Unit numbers must lie in the range 1...99

• Unit numbers may be specified as:
  • an integer constant e.g. 10

  • an integer expression e.g. NUNIT, NUINT+I

  • an asterisk * for the default value

  • the name of an internal file

• I/O statements permit the unit number to be specified as the first positional argument or using the keyword UNIT

Read Statement

General form

• There are two general forms:

READ(clist) [io_list]

• clist is defined as:

[FMT=] format

[,REC= record-number]

[,IOSTAT=ios]

[,ADVANCE=adv]

[,SIZE=integer_variable]

[,EOR=label]

[,END=label]

[,ERR=label]

• Unit number and format specifier must be supplied (in that order) but the other items are optional.

Examples

READ(5,*) line

READ(5,100) x, y, z

READ (UNIT=10,FMT=100, &

ERR=10,IOSTAT=ios)

• If an error occurs control is passed to the statement labelled 10
  • variable ios will contain an integer error number (0 otherwise).

Write Statement

General Form

WRITE(clist) [io_list]

• clist is defined as:

[FMT=] format-spec

[,REC= record-number]

[,IOSTAT=ios]

[ADVANCE=adv]

[,SIZE=integer-variable]

[,EOR=label]

[,ERR=label]

• unit number and format specifier must be supplied (in that order) but the other items are optional.

Examples

WRITE(6,*) i,j

WRITE(6,100) i

WRITE(6,*,ERR=10) line

WRITE(UNIT=file1,FMT=100,&

REC=recordnumber, ERR=10) &

newline

Open Statement

• Connect a unit number to a named file.

• Specify characteristics of a file which differ from the defaults.

• Open statements often contain system specific elements such as file names and as such should be easily identifiable in a program.

• General form:

• A unit number must be supplied

olist

• The options list, olist, may be empty or a combination of the following options

where filename is a valid filename

STATUS=st

st may be 'OLD', 'NEW', 'REPLACE', 'SCRATCH' or 'UNKNOWN'

ERR=label

Go to label if an error occurs.

IOSTAT=ios

ios is an integer variable which is set to zero if the statement is executed successfully or to an implementation dependent constant otherwise.

FORM=fm

fm may be 'FORMATTED' or 'UNFORMATTED', the default is 'FORMATTED' for sequential files and 'UNFORMATTED' for direct access files.

ACCESS=acc

acc may be 'SEQUENTIAL' or 'DIRECT'

RECL=rl

rl is the maximum record length (positive integer) for a direct access file. For formatted files this is the number of characters and for unformatted it is usually the number of bytes or words (system dependent).

BLANK=bl

bl is either 'NULL' or 'ZERO'

POSITION=pos

pos may be 'ASIS', 'REWIND' or 'APPEND'. Defaults to ASIS.

PAD=pad

pad may be 'YES' or 'NO'

DELIM=del

del may be 'APOSTROPHE' or 'QUOTE' or 'NONE'. Defaults to 'NONE'.

ACTION=act

act may be 'READ', 'WRITE' or 'READWRITE'. Default is processor dependent.

Examples

OPEN (UNIT=11,FILE='fibonacci.out',&

STATUS='NEW',ERR=10)

.......

10 WRITE(6,*) 'Error opening file:&

&fibonacci.out.'

OPEN (UNIT=12, STATUS='OLD',&

FILE='student.records',&

ACCESS='DIRECT',RECL=200,&

FORM='FORMATTED',&

ERR=20, IOSTAT=IOS)

........

20 IF (ERR .GE. 0) THEN

WRITE(6,*) 'Error opening file:&

&student.records.'

WRITE(6,*) 'IOS = ',IOS

ENDIF

STOP

Close Statement

• Orderly disconnection of a file from a unit

• General form:

[,ERR=label] [,STATUS=st])

• STATUS may be given the values 'KEEP' or 'DELETE' but 'KEEP' may not be used for a file opened as 'SCRATCH'

• Examples:

CLOSE(UNIT=nunit, ERR=10)

CLOSE(10,STATUS='DELETE',ERR=10)

Inquire Statement

• Obtain the status of a file or a unit number

• General form:

where inquiry-list must contain either

FILE=fname

or

UNIT=unum

plus any of the following

Inquiry list

OPENED=lod !true or false

NUMBER=unum !unit number

NAME=fnm !filename

ACCESS=acc !'DIRECT' or

!'SEQUENTIAL'

SEQUENTIAL=seq !'YES or 'NO'

DIRECT=dir !'YES' or 'NO'

FORMATTED=fmt !'YES' or 'NO'

UNFORMATTED=unfmt !'YES' or 'NO'

FORM=frm !'FORMATTED' or

!'UNFORMATTED'

NEXTREC=recn !number of next record

RECL=recl !record length

Direct Access Files

• Records in a direct access file may be randomly accessed by record number

• Created or connected using the ACCESS and RECL keyword on an OPEN statement

• Access using the REC keyword to READ and WRITE statements

• For example:

RECL=200, ACCESS='DIRECT')

• The following commands would read and write to record 20 of the file staff.records

staffrecord

WRITE(UNIT=10,FMT=100,REC=20) &

staffrecord

• Error conditions such as attempting to read a record beyond the end of the file should be handled with appropriate use of the ERR and IOSTAT keywords