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 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 the 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 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 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