i91@nikhefh.UUCP (Fons Rademakers) (06/03/87)
Does somebody know what according to the FORTRAN 77 standard
should happen in the following substring expression:
CHARACTER*10 CH, STR
STR = 'ABCDEFGHIJ'
I = 6
NC = 0
CH = 'FOO'//STR(I:I+NC-1)//'BAR'
My results are 1) VAX/VMS FORTRAN: CH = 'FOOBAR'
2) Gould UTX32 f77: CH = 'FOOBAR'
3) Apollo FTN : CH = 'FOOFGHIJ..' CRASH!!!
(the var CH gets scrambled which
results, a few statements later,
in a segmentation fault)
Is Apollo's FTN compiler allowed to crash (and are the other compilers
too permissive) or is it an FTN compiler bug?
-- Fons Rademakerssteved@sun.UUCP (06/03/87)
In article <286@nikhefh.UUCP> i91@nikhefh.UUCP (Fons Rademakers) writes: >Does somebody know what according to the FORTRAN 77 standard >should happen in the following substring expression: > > CHARACTER*10 CH, STR > > STR = 'ABCDEFGHIJ' > I = 6 > NC = 0 > CH = 'FOO'//STR(I:I+NC-1)//'BAR' > The results of this program fragment are undefined. The FORTRAN 77 standard states (section 5.7.1) that in a substring reference of the form: V ( e1 : e2) "The values of e1 and e2 *must* be such that: 1 <= e1 <= e2 <= len where len is the length of the character variable or array element" So a program with a substring like this is does not conform to the standard. The standard does not prohibit a FORTRAN implementation from accepting this as an extension nor does it require that it signals an error either. -- --------------------------- Steve Dever steved@Sun.COM or Sun Microsystems sun!steved
BEEBE%SCIENCE@utah-cs.UUCP (06/03/87)
>> CHARACTER*10 CH, STR >> >> STR = 'ABCDEFGHIJ' >> I = 6 >> NC = 0 >> CH = 'FOO'//STR(I:I+NC-1)//'BAR' >> >> My results are 1) VAX/VMS FORTRAN: CH = 'FOOBAR' >> 2) Gould UTX32 f77: CH = 'FOOBAR' >> 3) Apollo FTN : CH = 'FOOFGHIJ..' CRASH!!! Section 5.7.1 of ANSI X3.9-1978 FORTRAN 77 discusses substring expressions: CHARACTER*n S S(e1:e2) It is a REQUIREMENT that 1 <= e1 <= e2 <= n; in your example, you have e2 = e1-1, which is illegal. Presumably a run-time system should detect this error. I tried such an example on the DEC-20 (TOPS-20), and it gives a run-time error "?Illegal length for character expression". -------
ark@alice.UUCP (06/04/87)
In article <286@nikhefh.UUCP>, i91@nikhefh.UUCP writes: > Does somebody know what according to the FORTRAN 77 standard > should happen in the following substring expression: > > CHARACTER*10 CH, STR > > STR = 'ABCDEFGHIJ' > I = 6 > NC = 0 > CH = 'FOO'//STR(I:I+NC-1)//'BAR' This is not a valid Fortran 77 program. On page 5-9 of the Fortran 77 specification, the form of a substring expression is given as v ( e1 : e2 ) where v is a string variable and e1 and e2 are optional integer expressions. e1 gives the index of the first character of the substring and e2 gives the index of the last character. Where len is the length of v, e1 and e2 must be such that 1 <= e1 <= e2 <= len If e1 is omitted, 1 is assumed. If e2 is omitted, len is assumed. In the program fragment given as an example, I is 6 and I+NC-1 is 5. Thus the inequality is not satisfied and the program is invalid. The Fortran 77 language definition says elsewhere that it (the language definition) is restrictive on programs and permissive on implementations. That is: the definition says what is a valid program. A valid implementation is one that does the right thing when given a valid program as input. A fortran 77 implementation is thus permitted to do whatever it pleases with an invalid program, including crash or deliver garbage results.
bill@westpt.UUCP (06/04/87)
In article <286@nikhefh.UUCP>, i91@nikhefh.UUCP (Fons Rademakers) writes: > > My results are 1) VAX/VMS FORTRAN: CH = 'FOOBAR' > 2) Gould UTX32 f77: CH = 'FOOBAR' 2a) Pr1me PRIMOS : CH = 'FOOBAR' > 3) Apollo FTN : CH = 'FOOFGHIJ..' CRASH!!! > (the var CH gets scrambled which > results, a few statements later, > in a segmentation fault) > I can't wait to see which is right and which is wrong. Aren't standards wonderful. :-) bill gunshannon UUCP: {philabs,phri}!westpt!bill PHONE: (914)446-7747 US SNAIL: Martin Marietta Data Systems RADIO: KB3YV USMA, Bldg 600, Room 26 AX.25 KB3YV @ WA2RKN West Point, NY 10996
danny@masscomp.UUCP (06/04/87)
In article <286@nikhefh.UUCP> i91@nikhefh.UUCP (Fons Rademakers) writes: >Does somebody know what according to the FORTRAN 77 standard >should happen in the following substring expression: > > CHARACTER*10 CH, STR > > STR = 'ABCDEFGHIJ' > I = 6 > NC = 0 > CH = 'FOO'//STR(I:I+NC-1)//'BAR' Some compilers have extended to allow "null substrings"; they are sometimes useful. Often, a null substring is denoted by STR(start:finish) where the value of the finish expression is start-1. Another implementation might say that if finish<start then the length of the substring is 0. But according to the standard, the end value for the substring must be less than the start value. Therefore, the program is not standard conforming. What processors do with non-standard-conforming programs is up to the processor, so the Apollo processor, although it produced a segmentation fault at runtime, could be called standard- conforming (assuming it did everything else according to the standard ;-)
ark@alice.UUCP (06/06/87)
In article <844@westpt.usma.edu>, bill@westpt.UUCP writes: [description of a statement that does different things on different machines] > I can't wait to see which is right and which is wrong. > Aren't standards wonderful. :-) When you define a language standard, you can do one of several things. The particular thing Fortran 77 does is sometimes described by the phrase "permissive on implementations, restrictive on users." In English, this means that the Fortran 77 standard defines what is a valid Fortran 77 program, and then goes on to say that a valid implementation is one that does the right thing when given a valid program as input. That means that the standard says nothing at all about what the implementation should do when given an invalid program. If you want your program to work on a variety of implementations, it's up to you to ensure it is valid.