.ND .TL Update to the f77 I/O Library January 1980 .AU David L. Wasley .AI University of California Berkeley, Calif. 94720 .PP The fortran-77 I/O library, libI77.a, has been extensively updated. All known bugs have been fixed, and I/O error reporting has been improved. Several non-standard extensions to FORTRAN I/O have been added. .PP Some general concepts regarding f77 I/O deserve clarification. There are three forms of I/O: formatted, unformatted, and list-directed. The last is related to formatted but does not obey all the rules for formatted I/O. There are two modes of access to external and internal files: direct and sequential. The definition of a logical record depends upon the combination of I/O form and mode specified by the fortran I/O statement. .PP A logical record in direct access external files is a string of bytes of a length specified when the file is opened. Read and write statements must not specify logical records longer than the original record size definition. Shorter logical records are allowed. Unformatted direct writes leave the unfilled part of the record undefined. Formatted direct writes cause the unfilled record to be padded with blanks. .PP Logical records in sequentially accessed external files may be of arbitrary and variable length. Logical record length for unformatted sequential files is determined by the size of items in the iolist. For formatted write statements, logical record length is determined by the format statement interacting with the iolist at execution time. Formatted sequential access causes one or more logical records ending with 'newline' characters to be read or written. .PP Logical record length for list-directed I/O is relatively meaningless. On output, the record length is dependent on the magnitude of the data items. On input, the record length is determined by the data types and the file contents. .PP The logical record length for "internal" files is the length of the character variable or array element. Thus a simple character variable is a single logical record. A character variable array is similar to a fixed length direct access file, and obeys the same rules. Unformatted I/O is not allowed on "internal" files. .PP Note that each execution of a fortran unformatted I/O statement causes a single logical record to be read or written. Each execution of a fortran formatted I/O statement causes one or more logical records to be read or written. .PP Any error detected during I/O processing will cause the program to abort unless alternate action has been provided for specifically in the program. Any I/O statement may include an err= clause (and iostat= clause) to specify an alternate branch to be taken on errors (and return the specific error code). Read or write statements may include end= to branch on end-of-file. File position and the value of I/O list items is undefined following an error. I. Implementation details. .PP The maximum number of logical units that a program may have open at one time has been set to correspond with the UNIX system limit, currently 20. However, the I/O library uses UNIX file access for internal purposes. Therefore fatal errors are possible if the maximum number of files are open. Specifically, 'close' or 'endfile' on an old file, and "'inquire' by file" may fail. .PP Vertical format control is implemented. The logical unit must be opened for sequential access and "form = 'print'" (see below). Control codes '0' and '1' are replaced in the output file with '\\n' and '\\f' respectively. The control character '+' isn't implemented and, like any other character in the first position of a record written to a "print" file, is dropped. No vertical format control is recognized for direct formatted output or list directed output. .PP Default logical units 0, 5, and 6 can be re-defined with an 'open' statement. To preserve error reporting, it is an error to close logical unit 0. If you want to open the default filename for any preconnected logical unit, remember to 'close' the unit first. Redefining the standard units may impair normal console I/O. An alternative is to use shell re-direction to externally re-define the above units. To re-define default blank control or format of the standard input or output files, use the 'open' statement specifying the unit number and no filename (see below). .PP An 'open' statement need not specify a filename. If it refers to a logical unit that is already open, the "blank= " and "form= " specifiers may be redefined without affecting the current file position. Otherwise, if "status='scratch'" is specified, a temporary file with a name of the form 'tmp.FXXXXXX' will be opened, and, by default, will be deleted when closed or during termination of program execution. Any other "status= " specifier without an associated filename results in opening a file named 'fort.N' where N is the specified logical unit number. It is an error to try to open an existing file with "status='new'". It is an error to try to open a nonexistent file with "status='old'". By default "status='unknown'" will be assumed, and a file will be created if necessary. Existing files are never truncated on opening but are positioned at the end-of-file. .PP Sequentially accessed external files are truncated to the current file position on 'close', 'backspace', or 'rewind' only if the last access to the file was a write. .PP Upper as well as lower case characters are recognized in format statements and all alphabetic arguments to the I/O library routines. This has always been true for statements that are part of the source code, but not for format statements or character arguments from a file. .PP If the external representation of a datum is too large for the field width specified, the specified field is filled with asterisks (*). On 'Ew.dEe' output, the e field will be filled with asterisks if the exponent representation is too large. (This will only happen if e==0) .PP List-directed output of complex values now includes an appropriate comma. List-directed output now distinguishes between real*4 and real*8 values and formats them differently. Output of a character string that includes '\\n' now works correctly. .PP If I/O errors are not trapped by the user's program an appropriate error message will be written to 'stderr' before aborting. An error number will be printed in [ ] along with a brief error message showing the logical unit and I/O state. Error numbers < 100 refer to UNIX errors, and are described in the introduction to chapter 2 of the UNIX Programmer's Manual. Error numbers >= 100 come from the I/O library, and are described further in the appendix to this writeup. For internal I/O, part of the string will be printed with '|' at the current position in the string. For external I/O, part of the current record will be displayed if the error was caused during reading from a file that can backspace. .PP Direct access list-directed I/O is not allowed. Unformatted internal I/O is not allowed. Both the above will be caught by the compiler. All other flavors of I/O are allowed, although some are not part of the ANSI standard. .PP The standard units, 0, 5, and 6, are now named internally 'stderr', 'stdin', and 'stdout' respectively. These are not actual filenames and can not be used for opening these units. \'inquire' will not return these names and will indicate that the above units are not named unless they have been opened to real files. The names are meant to make error reporting more meaningful. .PP On output, a real value that is truly zero will display as '0.' to distinguish it from a very small non-zero value. This occurs in 'F', 'E', 'D', and 'G' format conversions. .PP Non-destructive tabbing is implemented for both internal and external formatted I/O. Tabbing left or right on output does not affect previously written portions of a record. Tabbing right on output causes unwritten portions of a record to be filled with blanks. Tabbing left or right off the end of a logical record is an error. The format specifier 'T' must be followed by a positive non-zero number. If it is not, it will have a different meaning (See below). Note that spacing with 'X' always writes blanks in the output record. II. Non-"ANSI Standard" Extensions .PP B is an acceptable edit control specifier. It causes return to the default mode of blank interpretation (NULL) and is identical to BN. This is consistent with S which returns to default sign control. .PP P by itself is equivalent to 0P. It resets the scale factor to the default value, 0. .PP The form of the 'Ew.dEe' format specifier has been extended to 'D' also. The form 'Ew.d.e' is allowed but is not standard. The 'e' field specifies the minimum number of digits or spaces in the exponent field on output. If the value of the exponent is too large, the exponent notation 'e' or 'd' will be dropped from the output to allow one more character position. If this is still not adequate, the 'e' field will be filled with asterisks (*). The default value for 'e' is 2. .PP An additional form of tab control specification has been added. The ANSI standard forms 'TRn', 'TLn', and 'Tn' are supported where n is a positive non-zero number. If 'T' or 'nT' is specified, tabbing will be to the next (or n-th) 8-column tab stop. Thus columns of alphanumerics can be lined up without counting. (See above for a description of the tabbing implementation.) .PP A format control specifier has been added to suppress the newline at the end of the last record of a formatted sequential write. The specifier is a dollar sign ($). It is constrained by the same rules as the colon (:). It is used typically for console prompts. For example: .DS write (*, "('enter value for x: ',$)") read (*,*) x .DE .PP Radices other than 10 can be specified for formatted integer I/O conversion. The specifier is patterned after P, the pre-scale factor for floating point conversion. It remains in effect until another radix is specified or format interpretation is complete. The specifier is defined as [n]R where 2 <= n <= 36. If n is omitted, the default decimal radix is restored. .PP In conjunction with the above, a sign control specifier has been added to cause integer values to be interpreted as unsigned during output conversion. The specifier is SU and remains in effect until another sign control specifier is encountered, or format interpretation is complete. Radix and 'unsigned' specifiers could be used to format a hexadecimal dump, as follows: .DS 2000 format( SU, 16R, 8I10.8) .DE Note: Unsigned integer values greater than (2**30 - 1), i.e. any signed negative value, can not be read by FORTRAN input routines. All internal values will be output correctly. .PP The ANSI standard is ambiguous regarding the definition of a "print" file. Since UNIX has no default "print" file, an additional 'form' specifier is now recognized in the 'open' statement. Specifying "form='print'" implies 'formatted' and enables vertical format control for that logical unit (see above). Vertical format control is interpreted only on sequential formatted writes to a "print" file. .PP The 'inquire' statement will return 'print' in the 'FORM=' string variable for logical units opened as "print" files. It will return -1 for the unit number of an unconnected file. .PP If a logical unit is already open, an 'open' statement including the 'form=' option or the 'blank=' option will do nothing but re-define those options. This instance of the 'open' statement need not include the filename, and must not include a filename if 'unit=' refers to the standard input or outputs. Therefore, to re-define the standard output as a "print" file, use: .DS open (unit=6, form='print') .DE .PP In a 'close' statement, "status='keep'" may be specified for temporary files. This is the default for all other files. Remember to get the file's real name, using 'inquire', if you want to re-open it later. .PP List directed read has been modified to allow input of a string not enclosed in quotes. The string must not start with a digit, and can not contain a separator (, or /) or blank (space or tab). A newline will terminate the string unless escaped with \\. Any string not meeting the above restrictions must be enclosed in quotes (" or '). .PP Internal list-directed I/O has been implemented. During internal list reads, bytes are consummed until the iolist is satisfied, or the 'end-of-file' is reached. During internal list writes, records are filled until the iolist is satisfied. The length of an internal array element should be at least 20 bytes to avoid logical record overflow when writing double precision values. Internal list read was implemented to make command line decoding easier. Internal list write should be avoided. .bp .ce 2 Appendix A I/O Library Error Messages .PP The following error messages are generated by the I/O library. The error numbers are returned in the "iostat=" variable if the "err=" return is taken. Error numbers < 100 are generated by UNIX. See the UNIX Programmers Manual, introduction to chapter 2. .DS /* 100 */ "error in format" See error message output for the location of the error in the format. Can be caused by more than 10 levels of nested (), or an extremely long format statement. /* 101 */ "illegal unit number" It is illegal to close logical unit 0. Negative unit numbers are not allowed. The upper limit is system dependent. /* 102 */ "formatted io not allowed" The logical unit was opened for unformatted I/O. /* 103 */ "unformatted io not allowed" The logical unit was opened for formatted I/O. /* 104 */ "direct io not allowed" The logical unit was opened for sequential access, or the logical record length was specified as 0. /* 105 */ "sequential io not allowed" The logical unit was opened for direct access I/O. /* 106 */ "can't backspace file" The file associated with the logical unit can't seek. May be a device or a pipe. /* 107 */ "off beginning of record" The format specified a left tab off the beginning of the record. /* 108 */ "can't stat file" The system can't return status information about the file. Perhaps the directory is unreadable. .DE .DS /* 109 */ "no * after repeat count" Repeat counts in list-directed I/O must be followed by an * with no blank spaces. /* 110 */ "off end of record" A formatted write tried to go beyond the logical end-of-record. An unformatted read or write will also cause this. /* 111 */ "truncation failed" The truncation of external sequential files on 'close', 'backspace', or 'rewind' tries to do a copy. It failed. Perhaps the temp file couldn't be created. /* 112 */ "incomprehensible list input" List input has to be just right. /* 113 */ "out of free space" The library dynamically creates buffers for internal use. You ran out of memory for this. Your program is too big! /* 114 */ "unit not connected" The logical unit was not open. /* 115 */ "read unexpected character" Certain format conversions can't tolerate non-numeric data. Logical data must be T or F. /* 116 */ "blank logical input field" /* 117 */ "'new' file exists" You tried to open an existing file with "status='new'". /* 118 */ "can't find 'old' file" You tried to open a non-existent file with "status='old'". /* 119 */ "unknown system error" Shouldn't happen, but ..... (Send me a documented example.) /* 120 */ "requires seek ability" Direct access requires seek ability. Sequential unformatted I/O requires seek ability on the file due to the special data structure required. Tabbing left also requires seek ability. /* 121 */ "illegal argument" Certain arguments to 'open', etc. will be checked for legitimacy. Often only non- default forms are looked for. .DE