g77.texi

gcc-2.95.3 Linux下最常用的C编译器

@cindex translation of user programs
@cindex machine code
@cindex code, machine
@cindex mistakes
@item
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 @dfn{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.

@cindex debugger
@cindex bugs, finding
@cindex @code{gdb}, command
@cindex commands, @code{gdb}
@item
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 @dfn{debugger},
such as @code{gdb}).

@cindex libraries
@cindex linking
@cindex @code{ld} command
@cindex commands, @code{ld}
@item
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 @dfn{libraries} and is located and gathered
during the @dfn{link} phase of the compilation
process.
(Linking often is thought of as a separate
step, because it can be directly invoked via the
@code{ld} command.
However, the @code{g77} and @code{gcc}
commands, as with most compiler commands, automatically
perform the linking step by calling on @code{ld}
directly, unless asked to not do so by the user.)

@cindex language, incorrect use of
@cindex incorrect use of language
@item
They attempt to diagnose cases where the user's
program contains incorrect usages of the language.
The @dfn{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.
@cindex diagnostics, incorrect
@cindex incorrect diagnostics
@cindex error messages, incorrect
@cindex incorrect error messages
(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.)

@cindex warnings
@cindex questionable instructions
@item
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 @dfn{warnings},
instead of the @dfn{errors} that indicate incorrect
usage of the language.
@end itemize

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.

@cindex components of g77
@cindex @code{g77}, components of
@code{g77} consists of several components:

@cindex @code{gcc}, command
@cindex commands, @code{gcc}
@itemize @bullet
@item
A modified version of the @code{gcc} command, which also might be
installed as the system's @code{cc} command.
(In many cases, @code{cc} refers to the
system's ``native'' C compiler, which
might be a non-GNU compiler, or an older version
of @code{gcc} considered more stable or that is
used to build the operating system kernel.)

@cindex @code{g77}, command
@cindex commands, @code{g77}
@item
The @code{g77} command itself, which also might be installed as the
system's @code{f77} command.

@cindex libg2c library
@cindex libf2c library
@cindex libraries, libf2c
@cindex libraries, libg2c
@cindex run-time, library
@item
The @code{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 @code{g77}
compilation phase.

@code{libg2c} is just the unique name @code{g77} gives
to its version of @code{libf2c} to distinguish it from
any copy of @code{libf2c} installed from @code{f2c}
(or versions of @code{g77} that built @code{libf2c} under
that same name)
on the system.

The maintainer of @code{libf2c} currently is
@email{dmg@@bell-labs.com}.

@cindex @code{f771}, program
@cindex programs, @code{f771}
@cindex assembler
@cindex @code{as} command
@cindex commands, @code{as}
@cindex assembly code
@cindex code, assembly
@item
The compiler itself, internally named @code{f771}.

Note that @code{f771} does not generate machine code directly---it
generates @dfn{assembly code} that is a more readable form
of machine code, leaving the conversion to actual machine code
to an @dfn{assembler}, usually named @code{as}.
@end itemize

@code{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, @code{gcc} determines which actions to perform, including
preprocessing, compiling (in a variety of possible languages), assembling,
and linking.

@cindex driver, gcc command as
@cindex @code{gcc}, command as driver
@cindex executable file
@cindex files, executable
@cindex cc1 program
@cindex programs, cc1
@cindex preprocessor
@cindex cpp program
@cindex programs, cpp
For example, the command @samp{gcc foo.c} @dfn{drives} the file
@file{foo.c} through the preprocessor @code{cpp}, then
the C compiler (internally named
@code{cc1}), then the assembler (usually @code{as}), then the linker
(@code{ld}), producing an executable program named @file{a.out} (on
UNIX systems).

@cindex cc1plus program
@cindex programs, cc1plus
As another example, the command @samp{gcc foo.cc} would do much the same as
@samp{gcc foo.c}, but instead of using the C compiler named @code{cc1},
@code{gcc} would use the C++ compiler (named @code{cc1plus}).

@cindex @code{f771}, program
@cindex programs, @code{f771}
In a GNU Fortran installation, @code{gcc} recognizes Fortran source
files by name just like it does C and C++ source files.
It knows to use the Fortran compiler named @code{f771}, instead of
@code{cc1} or @code{cc1plus}, to compile Fortran files.

@cindex @code{gcc}, not recognizing Fortran source
@cindex unrecognized file format
@cindex file format not recognized
Non-Fortran-related operation of @code{gcc} is generally
unaffected by installing the GNU Fortran version of @code{gcc}.
However, without the installed version of @code{gcc} being the
GNU Fortran version, @code{gcc} will not be able to compile
and link Fortran programs---and since @code{g77} uses @code{gcc}
to do most of the actual work, neither will @code{g77}!

@cindex @code{g77}, command
@cindex commands, @code{g77}
The @code{g77} command is essentially just a front-end for
the @code{gcc} command.
Fortran users will normally use @code{g77} instead of @code{gcc},
because @code{g77}
knows how to specify the libraries needed to link with Fortran programs
(@code{libg2c} and @code{lm}).
@code{g77} can still compile and link programs and
source files written in other languages, just like @code{gcc}.

@cindex printing version information
@cindex version information, printing
The command @samp{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 @samp{gcc -v} currently does.
(If it produces an error message near the end of the output---diagnostics
from the linker, usually @code{ld}---you might
have an out-of-date @code{libf2c} that improperly handles
complex arithmetic.)
In the output of this command, the line beginning @samp{GNU Fortran Front
End} identifies the version number of GNU Fortran; immediately
preceding that line is a line identifying the version of @code{gcc}
with which that version of @code{g77} was built.

@cindex libf2c library
@cindex libraries, libf2c
The @code{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.

@cindex in-line code
@cindex code, in-line
For example, while code generated by @code{g77} is likely
to do additions, subtractions, and multiplications @dfn{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 @code{f771} compiler
(invoked by @code{g77} when compiling Fortran code) into machine
code that, when run, calls on functions in @code{libg2c}, so
@code{libg2c} must be linked with almost every useful program
having any component compiled by GNU Fortran.
(As mentioned above, the @code{g77} command takes
care of all this for you.)

The @code{f771} program represents most of what is unique to GNU Fortran.
While much of the @code{libg2c} component comes from
the @code{libf2c} component of @code{f2c},
a free Fortran-to-C converter distributed by Bellcore (AT&T),
plus @code{libU77}, provided by Dave Love,
and the @code{g77} command is just a small front-end to @code{gcc},
@code{f771} is a combination of two rather
large chunks of code.

@cindex GNU Back End (GBE)
@cindex GBE
@cindex @code{gcc}, back end
@cindex back end, gcc
@cindex code generator
One chunk is the so-called @dfn{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 @code{cc1},
@code{cc1plus}, and @code{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.

@cindex GNU Fortran Front End (FFE)
@cindex FFE
@cindex @code{g77}, front end
@cindex front end, @code{g77}
The other chunk of @code{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 @dfn{Fortran Front End} (FFE).
The @code{cc1} and @code{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 @samp{-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).

@node G77 and GCC
@chapter Compile Fortran, C, or Other Programs
@cindex compiling programs
@cindex programs, compiling

@cindex @code{gcc}, command
@cindex commands, @code{gcc}
A GNU Fortran installation includes a modified version of the @code{gcc}
command.

In a non-Fortran installation, @code{gcc} recognizes C, C++,
and Objective-C source files.

In a GNU Fortran installation, @code{gcc} also recognizes Fortran source
files and accepts Fortran-specific command-line options, plus some
command-line options that are designed to cater to Fortran users
but apply to other languages as well.

@xref{G++ and GCC,,Compile C; C++; or Objective-C,gcc,Using and Porting GNU CC},
for information on the way different languages are handled
by the GNU CC compiler (@code{gcc}).

@cindex @code{g77}, command
@cindex commands, @code{g77}
Also provided as part of GNU Fortran is the @code{g77} command.
The @code{g77} command is designed to make compiling and linking Fortran
programs somewhat easier than when using the @code{gcc} command for
these tasks.
It does this by analyzing the command line somewhat and changing it
appropriately before submitting it to the @code{gcc} command.

@cindex -v option
@cindex @code{g77} options, -v
@cindex options, -v
Use the @samp{-v} option with @code{g77}
to see what is going on---the first line of output is the invocation
of the @code{gcc} command.