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.ND "1 August 1978"
.RP
.TL
A Portable Fortran 77 Compiler
.AU
S. I. Feldman
.AU
P. J. Weinberger
.AI
.MH
.AB
.LP
The Fortran language has just been revised.
The new language, known as Fortran 77,
became an official American National Standard on April 3, 1978.
We report here on a compiler and run-time system for the new extended language.
This is believed to be the first complete Fortran 77 system to be implemented.
This compiler is designed to be portable,
to be correct and complete,
and to generate code compatible with calling sequences produced by C compilers.
In particular, this Fortran is quite usable on
.UX
systems.
In this paper, we describe the language compiled,
interfaces between procedures,
and file formats assumed by the I/O system.
An appendix describes the Fortran 77 language.
.AE
.CS 9 10 19 0 0 8
.NH 0
INTRODUCTION
.PP
The Fortran language has just been revised.
The new language, known as Fortran 77, became an official American National Standard [1] on April 3, 1978.
for the language, known as Fortran 77, is about to be published.
Fortran 77 supplants 1966 Standard Fortran [2].
We report here on a compiler and run-time system for the new extended language.
The compiler and computation library were written by SIF, the I/O system by PJW.
We believe ours to be the first complete Fortran 77 system to be implemented.
This compiler is designed to be portable to a number of different machines,
to be correct and complete,
and to generate code compatible with calling sequences produced
by compilers for the C language [3].
In particular,
it is in use on
.UX
systems.
Two families of C compilers are in use at Bell Laboratories,
those based on D. M. Ritchie's PDP-11 compiler[4]
and those based on S. C. Johnson's portable C compiler [5].
This Fortran compiler can drive the second passes of either family.
In this paper, we describe the language compiled,
interfaces between procedures,
and file formats assumed by the I/O system.
We will describe implementation details in companion papers.
.PP
.NH 2
Usage
.PP
At present, versions of the compiler run on and compile for the PDP-11,
the VAX-11/780,
and the Interdata 8/32
.UX
systems.
The command to run the compiler is
.DS C
f\|77  \fIflags  file . . .\fR
.DE
.B f\|77
is a general-purpose command for compiling and loading Fortran and Fortran-related files.
EFL [6] and Ratfor [7] source files will be preprocessed before being presented to the Fortran compiler.
C and assembler source files will be compiled by the appropriate programs.
Object files will be loaded.
(The
.B f\|77
and
.B cc
commands cause slightly different loading sequences to be generated,
since Fortran programs need a few extra libraries and a different startup routine
than do C programs.)
The following file name suffixes are understood:
.DS I
 .f	Fortran source file
 .e	EFL source file
 .r	Ratfor source file
 .c	C source file
 .s	Assembler source file
 .o	Object file
.DE
The following flags are understood:
.in +1i
.XX \(miS
Generate assembler output for each source file, but do not assemble it.
Assembler output for a source file
.B x.f,
.B x.e,
.B x.r,
or
.B x.c
is put on file
\fBx.s\fR.
.XX \(mic
Compile but do not load.
Output for
.B x.f,
.B x.e,
.B x.r,
.B x.c,
or
.B x.s
is put on file
.B x.o.
.XX \(mim
Apply the M4 macro preprocessor to each EFL or Ratfor source file before using the appropriate compiler.
.XX \(mif
Apply the EFL or Ratfor processor to all
relevant files, and leave the output from
.B x.e
or
.B x.r
on
.B x.f.
Do not compile the resulting Fortran program.
.XX \(mip
Generate code to produce usage profiles.
.XX "\(mio \fIf\fR"
Put executable module on file
.I f.
(Default is
\fBa.out\fR).
.XX \(miw
Suppress all warning messages.
.XX \(miw66
Suppress warnings about Fortran 66 features used.
.XX \(miO
Invoke the C object code optimizer.
.XX \(miC
Compile code the checks that subscripts are within array bounds.
.XX \(mionetrip
Compile code that performs every
.B do
loop at least once.
(see Section 2.10).
.XX \(miU
Do not convert upper case letters to lower case.
The default is to convert Fortran programs to lower case.
.XX \(miu
Make the default type of a variable
.B undefined.
(see Section 2.3).
.XX \(miI2
On machines which support short integers,
make the default integer constants and variables short.
(\fB\(miI4\fR is the standard value of this option).  (see Section 2.14).
All logical quantities will be short.
.XX \(miE
The remaining characters in the argument are used as an EFL flag argument.
.XX \(miR
The remaining characters in the argument are used as a Ratfor flag argument.
.XX \(miF
Ratfor and and EFL source programs are pre-processed into Fortran files,
but those files are not compiled or removed.
.in -1i
.LP
Other flags,
all library names (arguments beginning \fB\(mil\fR),
and any names not ending with one of the understood suffixes are passed to the loader.
.NH 2
Documentation Conventions
.PP
In running text, we write Fortran keywords and other literal strings in boldface lower case.
Examples will be presented in lightface lower case.
Names representing a class of values will be printed in italics.
.NH 2
Implementation Strategy
.PP
The compiler and library are written entirely in C.
The compiler  generates C compiler intermediate code.
Since there are C compilers running on a variety of machines,
relatively small changes will make this Fortran compiler generate code for any of them.
Furthermore, this approach guarantees that the resulting programs are compatible with C usage.
The runtime computational library is complete.
The mathematical functions are computed to at least 63 bit precision.
The runtime I/O library makes use of D. M. Ritchie's Standard C I/O package [8]
for transferring data.
With the few exceptions described below, only documented calls are used,
so it should be relatively easy to modify to run on other operating
systems.
.NH 1
LANGUAGE EXTENSIONS
.PP
Fortran 77 includes almost all of Fortran 66 as a subset.
We describe the differences briefly in the Appendix.
The most important additions are a character string data type,
file-oriented input/output statements, and random access I/O.
Also, the language has been cleaned up considerably.
.PP
In addition to implementing the language specified in the new Standard,
our compiler implements a few extensions described in this section.
Most are useful additions to the language.
The remainder are extensions
to make it easier to communicate with C procedures
or to permit compilation of
old (1966 Standard) programs.
.NH 2
Double Complex Data Type
.IP
The new type
.B "double complex"
is defined.
Each datum is represented by a pair of double precision real variables.
A double complex version of every
.B complex
built-in function is provided.
The specific function names begin with \fBz\fR instead of \fBc\fR.
.NH 2
Internal Files
.IP
The Fortran 77 standard introduces ``internal files'' (memory arrays), but
restricts their use to formatted sequential I/O statements.
Our I/O system also permits internal files to be used
in direct and unformatted reads and writes.
.NH 2
Implicit Undefined statement
.IP
Fortran 66 has a fixed rule that the type of a variable that does not appear in a type statement
is
.B integer
if its first letter is
\fBi, j, k, l, m\fR or \fBn\fR,
and
.B real
otherwise.
Fortran 77 has an
.B implicit
statement for overriding this rule.
As an aid to good programming practice, we permit an additional type,
.B undefined.
The statement
.DS
implicit undefined(a-z)
.DE
turns off the automatic data typing mechanism,
and the compiler will issue a diagnostic for each variable that is used but does
not appear in a type statement.
Specifying the
.B \(miu
compiler flag is equivalent to beginning each procedure with this statement.
.NH 2
Recursion
.IP
Procedures may call themselves, directly or through a chain of other procedures.
.NH 2
Automatic Storage
.IP
Two new keywords are recognized,
.B static
and
.B automatic.
These keywords may appear as ``types'' in type statements and in
.B implicit
statements.
Local variables are static by default;
there is exactly one copy of the datum, and its value is retained between calls.
There is one copy of each variable declared
.B automatic
for each invocation of the procedure.
Automatic variables may not appear in
.B equivalence,
.B data,
or
.B save
statements.
.NH 2
Source Input Format
.IP
The Standard expects input to the compiler to be in 72 column format:
except in comment lines,
the first five characters are the statement number, the next is the continuation character,
and the next sixty-six are the body of the line.
(If there are fewer than seventy-two characters on a line, the compiler pads it with blanks;
characters after the seventy-second are ignored).
.IP
In order to make it easier to type Fortran programs,
our compiler also accepts input in variable length lines.
An ampersand (``&'') in the first position of a line indicates a continuation
line; the remaining characters form the body of the line.
A tab character in one of the first six positions of a line signals the
end of the statement number and continuation part of the line;
the remaining characters form the body of the line.
A tab elsewhere on the line is treated as another kind of blank by the
compiler.
.IP
In the Standard, there are only 26 letters \(em Fortran is a one-case language.
Consistent with ordinary
.UX
system usage, our compiler expects lower case input.
By default, the compiler converts all upper case characters to lower case except those inside character constants.
However, if the
.B \(miU
compiler flag is specified, upper case letters are not transformed.
In this mode, it is possible to specify external names with upper case letters in them,
and to have distinct variables differing only in case.
Regardless of the setting of the flag,
keywords will only be recognized in lower case.
.NH 2
Include Statement
.IP
The statement
.DS
include \(fmstuff\|\(fm
.DE
is replaced by the contents of the file
.B stuff.
\fBinclude\fRs may be nested to a reasonable depth, currently ten.
.NH 2
Binary Initialization Constants
.IP
A
.B logical,
.B real,
or
.B integer
variable may be initialized in a
.B data
statement
by a binary constant, denoted by a letter followed by a quoted string.
If the letter is \fBb\fR, the string is binary, and only zeroes and ones are permitted.
If the letter is \fBo\fR, the string is octal, with digits \fB0\(mi7\fR.
If the letter is \fBz\fR or \fBx\fR, the string is hexadecimal, with digits \fB0\(mi9\fR, \fBa\(mif\fR.
Thus, the statements
.DS
integer a(3)
data a / b\(fm1010\(fm, o\(fm12\(fm, z\(fma\(fm /
.DE
initialize all three elements of
.B a
to ten.
.NH 2
Character Strings
.IP
For compatibility with C usage, the following backslash escapes are recognized:
.DS
\en	newline
\et	tab
\eb	backspace
\ef	form feed
\e0	null
\e\(fm	apostrophe (does not terminate a string)
\e"	quotation mark (does not terminate a string)
\e\e	\e
\e\fIx\fR	\fIx\fR,  where \fIx\fR is any other character
.DE
Fortran 77 only has one quoting character, the apostrophe.
Our compiler and I/O system recognize
both the apostrophe ( \(fm ) and the double-quote ( " ).
If a string begins with one variety of quote mark, the other may be embedded within it
without using the repeated quote or backslash escapes.
.IP
Every unequivalenced scalar local character variable and every character string constant is aligned
on an
.B integer
word boundary.
Each character string constant appearing outside a
.B data
statement is followed by a
null character to ease communication with C routines.
.NH 2
Hollerith
.IP
Fortran 77 does not have the old Hollerith (\fIn\|\fBh\fR)
notation,
though the new Standard recommends implementing the old Hollerith feature
in order to improve compatibility with old programs.
In our compiler, Hollerith data may be used in place of character string constants,
and may also be used to initialize non-character variables in
.B data
statements.
.NH 2
Equivalence Statements
.IP
As a very special and peculiar case,
Fortran 66 permits an element of a multiply-dimensioned array to be represented by
a singly-subscripted reference in
.B equivalence
statements.
Fortran 77 does not permit this usage, since
subscript lower bounds may now be different from 1.
Our compiler permits single subscripts in
.B equivalence
statements,
under the interpretation that all missing subscripts are equal to 1.
A warning message is printed for each such incomplete subscript.
.NH 2
One-Trip DO Loops
.IP
The Fortran 77 Standard requires that the range of a
.B do
loop not be performed
if the initial value is already past the limit value,
as in
.DS
do 10 i = 2, 1
.DE
The 1966 Standard stated that the effect of such a statement was undefined,
but it was common practice that the range of a
.B do
loop would be performed
at least once.
In order to accommodate old programs, though they were in violation of the 1966 Standard,
the
.B \(mionetrip
compiler flag causes non-standard loops to be generated.
.NH 2
Commas in Formatted Input
.IP
The I/O system attempts to be more lenient than the
Standard when it seems worthwhile.
When doing a formatted read of non-character variables,
commas may be used as value separators in the input record,
overriding the field lengths given in the format statement.
Thus,
the format
.DS
(i10, f20.10, i4)
.DE
will read the record
.DS
\(mi345,.05e\(mi3,12
.DE
correctly.
.NH 2
Short Integers
.IP
On machines that support halfword integers,
the compiler accepts declarations of type
.B integer\(**2.
(Ordinary integers follow the Fortran rules about occupying the same
space as a REAL variable; they are assumed to be of C type
.B "long int" ;
halfword integers are of C type
.B "short int" .)
An expression involving only objects of type
.B integer\(**2
is of that type.
Generic functions return short or long integers depending on the actual types of their arguments.
If a procedure is compiled using the
.B \(miI2
flag, all small integer constants will be
of type
.B integer\(**2.
If the precision of an integer-valued intrinsic function is not determined by the generic function rules,
one will be chosen that returns the prevailing length
(\fBinteger\(**2\fR when the \fB\(miI2\fR command flag is in effect).
When the
.B \(miI2
option is in effect, all quantities of type
.B logical
will be short.
Note that these short integer and logical quantities do not obey the standard rules for storage association.
.NH 2
Additional Intrinsic Functions
.IP
This compiler supports all of the intrinsic functions specified in the Fortran 77 Standard.
In addition, there are functions for performing bitwise Boolean operations
(
.B or,
.B and,
.B xor,
and
.B not)
and for accessing the
.UX
command arguments
(
.B getarg
and
.B iargc
).
.NH 1
VIOLATIONS OF THE STANDARD
.PP
We know only thre ways in which our Fortran system violates the new standard:
.NH 2
Double Precision Alignment
.IP
The Fortran standards (both 1966 and 1977)
permit
.B common
or
.B equivalence
statements to force a double precision quantity onto an odd word boundary,