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During execution, your program may encounter errors or exception conditions. These conditions can result from errors that occur during I/O operations, from invalid input data, from argument errors in calls to the mathematical library, from arithmetic errors, or from system-detected errors.
The Compaq Fortran Run-Time Library (RTL) provides default processing for error conditions, generates appropriate messages, and takes action to recover from errors whenever possible. However, you can explicitly supplement or override default actions by using the following methods:
This chapter describes how the Compaq Fortran RTL processes errors. It also
provides information about using I/O specifiers for explicit error
processing and control, and shows how these methods affect the default
error processing of the Compaq Fortran RTL.
7.1 Compaq Fortran RTL Default Error Processing
The Compaq Fortran RTL contains condition handlers that process a number of errors that may occur during Compaq Fortran program execution. A default action is defined for each Fortran-specific error recognized by the Compaq Fortran RTL. The default actions described throughout this chapter occur unless overridden by explicit error-processing methods.
Unless you specify the /SYNCHRONOUS_EXCEPTIONS qualifier when you compile the program, error reporting of exceptions may be inexact; the exception may not be reported until a few instructions after the one that caused the exception. This makes continuation from an exception trap not feasible.
The way in which the Compaq Fortran RTL actually processes errors depends upon several factors:
The following FORTRAN command qualifiers are related to handling errors and exceptions:
The general form of Compaq Fortran run-time messages follows:
%FOR-severity-mnemonic, message-text |
The contents of the fields in run-time messages follow:
| % | The percent sign identifies the line as a message. |
| FOR | The facility code for Compaq Fortran 77 and Compaq Fortran. |
| severity | A single character that determines message severity. The types of run-time messages are: Fatal (F), Error (E), and Informational (I). |
| mnemonic | A 6- to 9-character name that uniquely identifies that message. |
| message_text | Explains the event or reason why the message appears. |
For example, the following message has a severity of Fatal, a mnemonic of ADJARRDIM, and message text of "adjustable array dimension error":
%FOR-F-ADJARRDIM, adjustable array dimension error |
In order of greatest to least severity, the classes of run-time diagnostic messages are as follows:
| Severity Code | Description |
|---|---|
| F |
Fatal (severe)
This must be corrected. The program cannot complete execution and is terminated when the error is encountered, unless for I/O statements the program uses the END, EOR, or ERR branch specifiers to transfer control, perhaps to a routine that uses the IOSTAT specifier (see Section 7.2.1 and Section 7.2.2). You can also continue from certain fatal-level messages by using a condition handler. |
| E |
Error
This should be corrected. The program may continue execution, but the output from this execution may be incorrect. |
| I |
Informational
This should be investigated. The program continues executing, but output from this execution may be incorrect. |
The severity depends on the source of the message. In some cases,
certain FORTRAN command qualifiers can change the severity level or
control whether messages are displayed (such as the /CHECK and
/IEEE_MODE keywords).
7.2 Handling Errors
Whenever possible, the Compaq Fortran RTL does certain error handling, such as generating appropriate messages and taking necessary action to recover from errors.
When no recovery method is specified for a statement and a fatal-level error occurs, a message appears and the program terminates. To prevent program termination, you must include either an appropriate I/O error-handling specifier (see Section 7.2) or a condition handler that performs an unwind (see Chapter 14). The I/O error-handling specifier or condition handler might also handle error-level messages.
You can explicitly supplement or override default actions by using the following Compaq Fortran methods:
When a fatal error occurs during program execution, the Compaq Fortran RTL default action is to print an error message and terminate the program. You can establish an OpenVMS condition handler that performs an unwind for certain fatal errors.
These error-processing methods are complementary; you can use all of them within the same program. However, before attempting to write a condition handler, you should be familiar with the OpenVMS condition-handling facility (CHF) and with the condition-handling description in Chapter 14.
Using the END, EOR, or ERR branch specifiers in I/O statements prevent signaling (display) of errors, including secondary return values for file system errors, such as RMS errors. Using these specifiers prevent the transfer of control to a condition handler.
There are certain file system errors where no handler (condition handler or vector exception handler) exists. To obtain the secondary file system errors in these cases, remove the END, EOR, and ERR specifiers, recompile, relink, and rerun the program.
You do not need to remove the ERR or IOSTAT specifiers if you use a
vectored exception handler (established using SYS$SETEXV), which will
receive control instead of the ERR and IOSTAT specifiers. The ERRSNS
subroutine allows you to obtain secondary return values for file system
errors (see the Compaq Fortran Language Reference Manual).
7.2.1 Using the ERR, EOR, and END Branch Specifiers
When an error, end-of-record, or end-of-file condition occurs during program execution, the Compaq Fortran RTL default action is to display a message and terminate execution of the program.
You can use the ERR, EOR, and END specifiers in I/O statements to override this default by transferring control to a specified point in the program. To override this default action, there are three branch specifiers you can use in I/O statements to transfer control to a specified point in the program:
If an END, EOR, or ERR branch specifier is present, and the corresponding condition occurs, no message is displayed and execution continues at the statement designated in the appropriate specifier.
For example, consider the following READ statement:
READ (8, 50, END=400) X,Y,Z |
If an end-of-file condition occurs during execution of this statement, the contents of variables X, Y, and Z become undefined, and control is transferred to the statement at label 400. You can also add an ERR specifier to transfer control if an error condition occurs. Note that an end-of-file or end-of-record condition does not cause an ERR specifier branch to be taken.
When using nonadvancing I/O, use the EOR specifier to handle the end-of-record condition. For example:
150 FORMAT (F10.2, F10.2, I6)
READ (UNIT=20, FMT=150, SIZE=X, ADVANCE='NO', EOR=700) A, F, I
|
You can also specify ERR as a keyword to such I/O statements as OPEN, CLOSE, or INQUIRE statement. For example:
OPEN (UNIT=10, FILE='FILNAM', STATUS='OLD', ERR=999) |
If an error is detected during execution of this OPEN statement, control transfers to statement 999.
You can use the IOSTAT specifier to continue program execution after an I/O error and to return information about I/O operations. It can supplement or replace the END, EOR, and ERR transfers. Execution of an I/O statement containing the IOSTAT specifier suppresses printing of an error message and causes the specified integer variable, array element, or scalar field reference to be defined as one of the following:
Following execution of the I/O statement and assignment of an IOSTAT value, control transfers to the END, EOR, or ERR statement label, if any. If there is no control transfer, normal execution continues.
Your program can include the $FORIOSDEF library module from the FORSYSDEF library (automatically searched during compilation) to obtain symbolic definitions for the values of IOSTAT.
The values of the IOSTAT symbols from the $FORIOSDEF library module are not the same as the values of the Fortran condition symbols from the $FORDEF library module.
The symbolic names in the $FORIOSDEF library module have a form similar to the Fortran condition symbols:
| Fortran Condition Symbol ($FORDEF) | IOSTAT Symbolic Name ($FORIOSDEF) |
|---|---|
| FOR$_mnemonic | FOR$IOS_mnemonic |
Example 7-1 uses the ERR and IOSTAT specifiers to handle an OPEN statement error (in the FILE specifier). Condition symbols are included from the $FORIOSDEF library module (in FORSYSDEF).
| Example 7-1 Handling OPEN Statement File Name Errors |
|---|
CHARACTER(LEN=40) :: FILNM ! Typed file specification
INCLUDE '($FORIOSDEF)' ! Include condition symbol definitions
DO I=1,4 ! Allow four tries
FILNM = ''
WRITE (6,*) 'Type file name '
READ (5,*) FILNM
OPEN (UNIT=1, FILE=FILNM, STATUS='OLD', IOSTAT=IERR, ERR=100)
WRITE (6,*) 'Opening file: ', FILNM
.
. ! Process records
.
CLOSE (UNIT=1)
STOP
100 IF (IERR .EQ. FOR$IOS_FILNOTFOU) THEN
WRITE (6,*) 'File: ', FILNM, ' does not exist '
ELSE IF (IERR .EQ. FOR$IOS_FILNAMSPE) THEN
WRITE (6,*) 'File: ', FILNM, ' was bad, enter new file name'
ELSE
PRINT *, 'Unrecoverable error, code =', IERR
STOP
END IF
END DO
! After four attempts or a Ctrl/Z on the READ statement, allow program restart
WRITE (6,*) 'File not found. Type DIRECTORY to find file and run again'
END PROGRAM
|
Table 7-1 lists the Fortran-specific errors processed by the Compaq Fortran RTL. For each error, the table shows the Fortran-specific message mnemonic (follows either FOR$_ or FOR$IOS_ for condition symbols), the Fortran-specific message number, the severity (fatal, error, or informational), the message text. For more detailed descriptions of errors processed by the Compaq Fortran RTL, see Table B-1.
| Mnemonic | Number1 | Severity | Message Text |
|---|---|---|---|
| NOTFORSPE 2 | 1 | F | not a Fortran-specific error |
| BUG_CHECK | 8 | F | internal consistency check failure |
| SYNERRNAM | 17 | F | syntax error in NAMELIST input |
| TOOMANVAL | 18 | F | too many values for NAMELIST variable |
| INVREFVAR | 19 | F | invalid reference to variable in NAMELIST input |
| REWERR | 20 | F | REWIND error |
| DUPFILSPE | 21 | F | duplicate file specifications |
| INPRECTOO | 22 | F | input record too long |
| BACERR | 23 | F | BACKSPACE error |
| ENDDURREA | 24 | F | end-of-file during read |
| RECNUMOUT | 25 | F | record number outside range |
| OPEDEFREQ | 26 | F | OPEN or DEFINE FILE required |
| TOOMANREC | 27 | F | too many records in I/O statement |
| CLOERR | 28 | F | CLOSE error |
| FILNOTFOU | 29 | F | file not found |
| OPEFAI | 30 | F | open failure |
| MIXFILACC | 31 | F | mixed file access modes |
| INVLOGUNI | 32 | F | invalid logical unit number |
| ENDFILERR | 33 | F | ENDFILE error |
| UNIALROPE | 34 | F | unit already open |
| SEGRECFOR | 35 | F | segmented record format error |
| ATTACCNON | 36 | F | attempt to access non-existent record |
| INCRECLEN | 37 | F | inconsistent record length |
| ERRDURWRI | 38 | F | error during write |
| ERRDURREA | 39 | F | error during read |
| RECIO_OPE | 40 | F | recursive I/O operation |
| INSVIRMEM | 41 | F | insufficient virtual memory |
| NO_SUCDEV | 42 | F | no such device |
| FILNAMSPE | 43 | F | file name specification error |
| INCRECTYP | 44 | F | inconsistent record type |
| KEYVALERR | 45 | F | keyword value error in OPEN statement |
| INCOPECLO | 46 | F | inconsistent OPEN/CLOSE parameters |
| WRIREAFIL | 47 | F | write to READONLY file |
| INVARGFOR | 48 | F | invalid argument to Fortran Run-Time Library |
| INVKEYSPE | 49 | F | invalid key specification |
| INCKEYCHG | 50 | F | inconsistent key change or duplicate key |
| INCFILORG | 51 | F | inconsistent file organization |
| SPERECLOC | 52 | F | specified record locked |
| NO_CURREC | 53 | F | no current record |
| REWRITERR | 54 | F | REWRITE error |
| DELERR | 55 | F | DELETE error |
| UNLERR | 56 | F | UNLOCK error |
| FINERR | 57 | F | FIND error |
| FMYSYN | 58 | I | format syntax error at or near xxx |
| LISIO_SYN 3 | 59 | F | list-directed I/O syntax error |
| INFFORLOO | 60 | F | infinite format loop |
| FORVARMIS 3 | 61 | F or I 4 | format/variable-type mismatch |
| SYNERRFOR | 62 | F | syntax error in format |
| OUTCONERR 3,4 | 63 | E or I 4 | output conversion error |
| INPCONERR 3 | 64 | F | input conversion error |
| FLTINV | 65 | E | floating invalid |
| OUTSTAOVE | 66 | F | output statement overflows record |
| INPSTAREQ | 67 | F | input statement requires too much data |
| VFEVALERR 3 | 68 | F | variable format expression value error |
| INTOVF | 70 | F | integer overflow |
| INTDIV | 71 | F | integer divide by zero |
| FLTOVF | 72 | E | floating overflow |
| FLTDIV | 73 | E | floating divide by zero |
| FLTUND | 74 | E | floating underflow |
| SUBRNG | 77 | F | subscript out of range |
| WRONUMARG | 80 | F | wrong number of arguments |
| INVARGMAT | 81 | F | invalid argument to math library |
| UNDEXP 5 | 82 | F | undefined exponentiation |
| LOGZERNEG 5 | 83 | F | logarithm of zero or negative value |
| SQUROONEG 5 | 84 | F | square root of negative value |
| SIGLOSMAT 5 | 87 | F | significance lost in math library |
| FLOOVEMAT 5 | 88 | F | floating overflow in math library |
| FLOUNDMAT | 89 | E | floating underflow in math library |
| ADJARRDIM 6 | 93 | F | adjustable array dimension error |
| INVMATKEY | 94 | F | invalid key match specifier for key direction |
| FLOCONFAI | 95 | E | floating point conversion failed |
| FLTINE | 140 | E | floating inexact |
| ROPRAND | 144 | F | reserved operand |
| ASSERTERR | 145 | F | assertion error |
| NULPTRERR | 146 | F | null pointer error |
| STKOVF | 147 | F | stack overflow |
| STRLENERR | 148 | F | string length error |
| SUBSTRERR | 149 | F | substring error |
| RANGEERR | 150 | F | range error |
| INVREALLOC | 151 | F | allocatable array is already allocated |
| RESACQFAI | 152 | F | unresolved contention for Compaq Fortran RTL global resource |
| INVDEALLOC | 153 | F | allocatable array is not allocated |
| INVDEALLOC2 | 173 | F | A pointer passed to DEALLOCATE points to an array that cannot be deallocated |
| SHORTDATEARG | 175 | F | DATE argument to DATE_AND_TIME is too short (LEN=n), required LEN=8 |
| SHORTTIMEARG | 176 | F | TIME argument to DATE_AND_TIME is too short (LEN=n), required LEN=10 |
| SHORTZONEARG | 177 | F | ZONE argument to DATE_AND_TIME is too short (LEN=n), required LEN=5 |
| DIV | 178 | F | divide by zero |
| ARRSIZEOVF | 179 | F | cannot allocate array --- overflow on array size calculation |
| UNFIO_FMT | 256 | F | unformatted I/O to unit open for formatted transfers |
| FMTIO_UNF | 257 | F | formatted I/O to unit open for unformatted transfers |
| DIRIO_KEY | 258 | F | direct-access I/O to unit open for keyed access |
| SEQIO_DIR | 259 | F | sequential-access I/O to unit open for direct access |
| KEYIO_DIR | 260 | F | keyed-access I/O to unit open for direct access |
| OPERREQDIS | 264 | F | operation requires file to be on disk or tape |
| OPEREQSEQ | 265 | F | operation requires sequential file organization and access |
| ENDRECDUR | 268 | F | end of record during read |
| FLOINEEXC | 296 | I | nn floating inexact traps |
| FLOINCEXC | 297 | I | nn floating invalid traps |
| FLOOVREXC | 298 | I | nn floating overflow traps |
| FLODIV0EXC | 299 | I | nn divide-by-zero traps |
| FLOUNDEXC | 300 | I | nn floating underflow traps |
The message mnemonic shown in the first column is part of the condition status code symbols signaled by the Compaq Fortran RTL I/O support routines. You can define these symbolic values in your program by including the library module $FORDEF or $FORIOSDEF from the system-supplied default library FORSYSDEF.TLB:
If you will be using the IOSTAT specifier for error handling, you should include the $FORIOSDEF library module (instead of $FORDEF) from the FORSYSDEF.TLB library (see Section 7.2.2).
The standard Compaq Fortran error numbers that are generally compatible with other versions of Compaq Fortran are shown in the second column. Most of these error values are returned to IOSTAT variables when an I/O error is detected.
The codes in the third column indicate the severity of the error conditions (see Section 7.1.2).
For more detailed descriptions of errors processed by the Compaq Fortran RTL, see Table B-1 or type the following DCL command to obtain a list of mnemonics (such as ADJARRDIM):
$ HELP FORTRAN ERROR RUN_TIME |
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