g77.info: What is GNU Fortran?

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What is GNU Fortran?

   GNU Fortran, or `g77', is designed initially as a free replacement
for, or alternative to, the UNIX `f77' command.  (Similarly, `gcc' is
designed as a replacement for the UNIX `cc' command.)
   `g77' also is designed to fit in well with the other fine GNU
compilers and tools.
   Sometimes these design goals conflict--in such cases, resolution
often is made in favor of fitting in well with Project GNU.  These
cases are usually identified in the appropriate sections of this manual.
   As compilers, `g77', `gcc', and `f77' share the following
characteristics:
   * They read a user's program, stored in a file and containing
     instructions written in the appropriate language (Fortran, C, and
     so on).  This file contains "source code".
   * They translate the user's program into instructions a computer can
     carry out more quickly than it takes to translate the instructions
     in the first place.  These instructions are called "machine
     code"--code designed to be efficiently translated and processed by
     a machine such as a computer.  Humans usually aren't as good
     writing machine code as they are at writing Fortran or C, because
     it is easy to make tiny mistakes writing machine code.  When
     writing Fortran or C, it is easy to make big mistakes.
   * They provide information in the generated machine code that can
     make it easier to find bugs in the program (using a debugging
     tool, called a "debugger", such as `gdb').
   * They locate and gather machine code already generated to perform
     actions requested by statements in the user's program.  This
     machine code is organized into "libraries" and is located and
     gathered during the "link" phase of the compilation process.
     (Linking often is thought of as a separate step, because it can be
     directly invoked via the `ld' command.  However, the `g77' and
     `gcc' commands, as with most compiler commands, automatically
     perform the linking step by calling on `ld' directly, unless asked
     to not do so by the user.)
   * They attempt to diagnose cases where the user's program contains
     incorrect usages of the language.  The "diagnostics" produced by
     the compiler indicate the problem and the location in the user's
     source file where the problem was first noticed.  The user can use
     this information to locate and fix the problem.  (Sometimes an
     incorrect usage of the language leads to a situation where the
     compiler can no longer make any sense of what follows--while a
     human might be able to--and thus ends up complaining about many
     "problems" it encounters that, in fact, stem from just one
     problem, usually the first one reported.)
   * They attempt to diagnose cases where the user's program contains a
     correct usage of the language, but instructs the computer to do
     something questionable.  These diagnostics often are in the form
     of "warnings", instead of the "errors" that indicate incorrect
     usage of the language.
   How these actions are performed is generally under the control of
the user.  Using command-line options, the user can specify how
persnickety the compiler is to be regarding the program (whether to
diagnose questionable usage of the language), how much time to spend
making the generated machine code run faster, and so on.
   `g77' consists of several components:
   * A modified version of the `gcc' command, which also might be
     installed as the system's `cc' command.  (In many cases, `cc'
     refers to the system's "native" C compiler, which might be a
     non-GNU compiler, or an older version of `gcc' considered more
     stable or that is used to build the operating system kernel.)
   * The `g77' command itself, which also might be installed as the
     system's `f77' command.
   * The `libg2c' run-time library.  This library contains the machine
     code needed to support capabilities of the Fortran language that
     are not directly provided by the machine code generated by the
     `g77' compilation phase.
     `libg2c' is just the unique name `g77' gives to its version of
     `libf2c' to distinguish it from any copy of `libf2c' installed
     from `f2c' (or versions of `g77' that built `libf2c' under that
     same name) on the system.
     The maintainer of `libf2c' currently is <dmg@bell-labs.com>.
   * The compiler itself, internally named `f771'.
     Note that `f771' does not generate machine code directly--it
     generates "assembly code" that is a more readable form of machine
     code, leaving the conversion to actual machine code to an
     "assembler", usually named `as'.
   `gcc' is often thought of as "the C compiler" only, but it does more
than that.  Based on command-line options and the names given for files
on the command line, `gcc' determines which actions to perform,
including preprocessing, compiling (in a variety of possible
languages), assembling, and linking.
   For example, the command `gcc foo.c' "drives" the file `foo.c'
through the preprocessor `cpp', then the C compiler (internally named
`cc1'), then the assembler (usually `as'), then the linker (`ld'),
producing an executable program named `a.out' (on UNIX systems).
   As another example, the command `gcc foo.cc' would do much the same
as `gcc foo.c', but instead of using the C compiler named `cc1', `gcc'
would use the C++ compiler (named `cc1plus').
   In a GNU Fortran installation, `gcc' recognizes Fortran source files
by name just like it does C and C++ source files.  It knows to use the
Fortran compiler named `f771', instead of `cc1' or `cc1plus', to
compile Fortran files.
   Non-Fortran-related operation of `gcc' is generally unaffected by
installing the GNU Fortran version of `gcc'.  However, without the
installed version of `gcc' being the GNU Fortran version, `gcc' will
not be able to compile and link Fortran programs--and since `g77' uses
`gcc' to do most of the actual work, neither will `g77'!
   The `g77' command is essentially just a front-end for the `gcc'
command.  Fortran users will normally use `g77' instead of `gcc',
because `g77' knows how to specify the libraries needed to link with
Fortran programs (`libg2c' and `lm').  `g77' can still compile and link
programs and source files written in other languages, just like `gcc'.
   The command `g77 -v' is a quick way to display lots of version
information for the various programs used to compile a typical
preprocessed Fortran source file--this produces much more output than
`gcc -v' currently does.  (If it produces an error message near the end
of the output--diagnostics from the linker, usually `ld'--you might
have an out-of-date `libf2c' that improperly handles complex
arithmetic.)  In the output of this command, the line beginning `GNU
Fortran Front End' identifies the version number of GNU Fortran;
immediately preceding that line is a line identifying the version of
`gcc' with which that version of `g77' was built.
   The `libf2c' library is distributed with GNU Fortran for the
convenience of its users, but is not part of GNU Fortran.  It contains
the procedures needed by Fortran programs while they are running.
   For example, while code generated by `g77' is likely to do
additions, subtractions, and multiplications "in line"--in the actual
compiled code--it is not likely to do trigonometric functions this way.
   Instead, operations like trigonometric functions are compiled by the
`f771' compiler (invoked by `g77' when compiling Fortran code) into
machine code that, when run, calls on functions in `libg2c', so
`libg2c' must be linked with almost every useful program having any
component compiled by GNU Fortran.  (As mentioned above, the `g77'
command takes care of all this for you.)
   The `f771' program represents most of what is unique to GNU Fortran.
While much of the `libg2c' component comes from the `libf2c' component
of `f2c', a free Fortran-to-C converter distributed by Bellcore (AT&T),
plus `libU77', provided by Dave Love, and the `g77' command is just a
small front-end to `gcc', `f771' is a combination of two rather large
chunks of code.
   One chunk is the so-called "GNU Back End", or GBE, which knows how
to generate fast code for a wide variety of processors.  The same GBE
is used by the C, C++, and Fortran compiler programs `cc1', `cc1plus',
and `f771', plus others.  Often the GBE is referred to as the "gcc back
end" or even just "gcc"--in this manual, the term GBE is used whenever
the distinction is important.
   The other chunk of `f771' is the majority of what is unique about
GNU Fortran--the code that knows how to interpret Fortran programs to
determine what they are intending to do, and then communicate that
knowledge to the GBE for actual compilation of those programs.  This
chunk is called the "Fortran Front End" (FFE).  The `cc1' and `cc1plus'
programs have their own front ends, for the C and C++ languages,
respectively.  These fronts ends are responsible for diagnosing
incorrect usage of their respective languages by the programs the
process, and are responsible for most of the warnings about
questionable constructs as well.  (The GBE handles producing some
warnings, like those concerning possible references to undefined
variables.)
   Because so much is shared among the compilers for various languages,
much of the behavior and many of the user-selectable options for these
compilers are similar.  For example, diagnostics (error messages and
warnings) are similar in appearance; command-line options like `-Wall'
have generally similar effects; and the quality of generated code (in
terms of speed and size) is roughly similar (since that work is done by
the shared GBE).