GNU Fortran 7.3.0 使用指南

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"GNU Fortran 7.3.0 手册" GNU Fortran是GCC（GNU Compiler Collection）的一部分，是一个开源的FORTRAN编译器，旨在替代或作为Unix系统中的f95命令的免费替代品。手册详细介绍了如何使用gfortran以及其特性与不兼容性，帮助用户理解和掌握这个强大的编程工具。在gcc版本7.3.0中，gfortran提供了对FORTRAN语言的全面支持，包括最新的FORTRAN 2003和2008标准。FORTRAN是一种历史悠久的编程语言，尤其适合科学计算和数值分析。gfortran不仅支持标准FORTRAN语法，还包含了各种优化选项和调试工具，以提高程序性能和简化开发过程。手册的第一部分，"Introduction"，通常会介绍gfortran的基本概念，包括它的历史、目标以及相对于其他FORTRAN编译器的优势。这部分可能会讲解如何安装和配置gfortran环境，以及它与其他GCC组件的集成。 "Invoking GNU Fortran"章节详细说明了如何使用gfortran命令行工具。用户将学习如何编译、链接FORTRAN源代码，以及如何传递编译选项来控制编译过程。这些选项可能涉及优化级别、错误警告、目标平台等。例如，-O2选项可以启用二级优化，而-Wall选项则会让编译器报告所有可能的警告。第三部分，“Runtime: Influencing runtime behavior with environment variables”，探讨了如何通过设置环境变量来影响程序运行时的行为。这可能包括内存管理策略、并行执行控制和错误处理方式。例如，用户可以通过设置环境变量来指定堆栈大小或者启用或禁用特定的运行时检查。 "Fortran 2003 and 2008 Status"章节会列出gfortran对FORTRAN 2003和2008标准的支持程度。这些新标准引入了许多增强功能，如对象导向编程、协程和ISO C绑定，允许FORTRAN程序与C语言更紧密地交互。手册会详细说明哪些特性已被实现，哪些仍在发展中，以及如何在实际编程中利用这些新特性。 "Compiler Characteristics"部分将详细介绍gfortran的编译器特性，包括诊断信息、代码生成选项、以及可能遇到的编译问题和解决方法。此外，手册可能还会涵盖如何使用gfortran进行调试，比如使用-g选项生成调试信息，以及与GDB等调试工具的配合使用。 "GNU Fortran 7.3.0 Manual"是FORTRAN程序员的重要参考资源，无论你是初学者还是经验丰富的开发者，都能从中获取到关于gfortran的深入知识和实用技巧。这份文档不仅涵盖了基本的编译和运行时设置，还提供了语言特性的详细解析，有助于编写高效、可靠的FORTRAN程序。

xiv The GNU Fortran Compiler

Funding Free Software . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

Option Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

Keyword Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

Chapter 1: Introduction 1

1 Introduction

This manual documents the use of gfortran, the GNU Fortran compiler. You can ﬁnd in

this manual how to invoke gfortran, as well as its features and incompatibilities.

The GNU Fortran compiler front end was designed initially as a free replacement for,

or alternative to, the Unix f95 command; gfortran is the command you will use to invoke

the compiler.

1.1 About GNU Fortran

The GNU Fortran compiler supports the Fortran 77, 90 and 95 standards completely, parts

of the Fortran 2003 and Fortran 2008 standards, and several vendor extensions. The devel-

opment goal is to provide the following features:

• Read a user’s program, stored in a ﬁle and containing instructions written in Fortran

77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008. This ﬁle contains source

code.

• 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. The result after compilation

of a program is machine code, code designed to be eﬃciently translated and processed

by a machine such as your computer. Humans usually are not as good writing machine

code as they are at writing Fortran (or C++, Ada, or Java), because it is easy to make

tiny mistakes writing machine code.

• Provide the user with information about the reasons why the compiler is unable to

create a binary from the source code. Usually this will be the case if the source code

is ﬂawed. The Fortran 90 standard requires that the compiler can point out mistakes

to the user. An incorrect usage of the language causes an error message.

The compiler will also attempt to diagnose cases where the user’s program contains a

correct usage of the language, but instructs the computer to do something questionable.

This kind of diagnostics message is called a warning message.

• Provide optional information about the translation passes from the source code to

machine code. This can help a user of the compiler to ﬁnd the cause of certain bugs

which may not be obvious in the source code, but may be more easily found at a lower

level compiler output. It also helps developers to ﬁnd bugs in the compiler itself.

• 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 the GNU Debugger

gdb).

• Locate and gather machine code already generated to perform actions requested by

statements in the user’s program. This machine code is organized into modules and is

located and linked to the user program.

The GNU Fortran compiler consists of several components:

• A version of the gcc command (which also might be installed as the system’s cc com-

mand) that also understands and accepts Fortran source code. The gcc command is

the driver program for all the languages in the GNU Compiler Collection (GCC); With

gcc, you can compile the source code of any language for which a front end is available

in GCC.

2 The GNU Fortran Compiler

• The gfortran command itself, which also might be installed as the system’s f95 com-

mand. gfortran is just another driver program, but speciﬁcally for the Fortran com-

piler only. The diﬀerence with gcc is that gfortran will automatically link the correct

libraries to your program.

• A collection of run-time libraries. These libraries contain the machine code needed

to support capabilities of the Fortran language that are not directly provided by the

machine code generated by the gfortran compilation phase, such as intrinsic functions

and subroutines, and routines for interaction with ﬁles and the operating system.

• The Fortran compiler itself, (f951). This is the GNU Fortran parser and code generator,

linked to and interfaced with the GCC backend library. f951 “translates” the source

code to assembler code. You would typically not use this program directly; instead,

the gcc or gfortran driver programs will call it for you.

1.2 GNU Fortran and GCC

GNU Fortran is a part of GCC, the GNU Compiler Collection. GCC consists of a collec-

tion of front ends for various languages, which translate the source code into a language-

independent form called GENERIC. This is then processed by a common middle end which

provides optimization, and then passed to one of a collection of back ends which generate

code for diﬀerent computer architectures and operating systems.

Functionally, this is implemented with a driver program (gcc) which provides the

command-line interface for the compiler. It calls the relevant compiler front-end program

(e.g., f951 for Fortran) for each ﬁle in the source code, and then calls the assembler and

linker as appropriate to produce the compiled output. In a copy of GCC which has been

compiled with Fortran language support enabled, gcc will recognize ﬁles with ‘.f’, ‘.for’,

‘.ftn’, ‘.f90’, ‘.f95’, ‘.f03’ and ‘.f08’ extensions as Fortran source code, and compile it

accordingly. A gfortran driver program is also provided, which is identical to gcc except

that it automatically links the Fortran runtime libraries into the compiled program.

Source ﬁles with ‘.f’, ‘.for’, ‘.fpp’, ‘.ftn’, ‘.F’, ‘.FOR’, ‘.FPP’, and ‘.FTN’ extensions

are treated as ﬁxed form. Source ﬁles with ‘.f90’, ‘.f95’, ‘.f03’, ‘.f08’, ‘.F90’, ‘.F95’,

‘.F03’ and ‘.F08’ extensions are treated as free form. The capitalized versions of either

form are run through preprocessing. Source ﬁles with the lower case ‘.fpp’ extension are

also run through preprocessing.

This manual speciﬁcally documents the Fortran front end, which handles the program-

ming language’s syntax and semantics. The aspects of GCC which relate to the optimization

passes and the back-end code generation are documented in the GCC manual; see

Section

“Introduction” in Using the GNU Compiler Collection (GCC). The two manuals together

provide a complete reference for the GNU Fortran compiler.

1.3 Preprocessing and conditional compilation

Many Fortran compilers including GNU Fortran allow passing the source code through a

C preprocessor (CPP; sometimes also called the Fortran preprocessor, FPP) to allow for

conditional compilation. In the case of GNU Fortran, this is the GNU C Preprocessor

in the traditional mode. On systems with case-preserving ﬁle names, the preprocessor is

automatically invoked if the ﬁlename extension is ‘.F’, ‘.FOR’, ‘.FTN’, ‘.fpp’, ‘.FPP’, ‘.F90’,

Chapter 1: Introduction 3

‘.F95’, ‘.F03’ or ‘.F08’. To manually invoke the preprocessor on any ﬁle, use ‘-cpp’, to

disable preprocessing on ﬁles where the preprocessor is run automatically, use ‘-nocpp’.

If a preprocessed ﬁle includes another ﬁle with the Fortran INCLUDE statement, the in-

cluded ﬁle is not preprocessed. To preprocess included ﬁles, use the equivalent preprocessor

statement #include.

If GNU Fortran invokes the preprocessor, __GFORTRAN__ is deﬁned and __GNUC__, _

_GNUC_MINOR__ and __GNUC_PATCHLEVEL__ can be used to determine the version of the

compiler. See Section “Overview” in The C Preprocessor for details.

While CPP is the de-facto standard for preprocessing Fortran code, Part 3 of the Fortran

95 standard (ISO/IEC 1539-3:1998) deﬁnes Conditional Compilation, which is not widely

used and not directly supported by the GNU Fortran compiler. You can use the program

coco to preprocess such ﬁles (http://www.daniellnagle.com/coco.html).

1.4 GNU Fortran and G77

The GNU Fortran compiler is the successor to g77, the Fortran 77 front end included in GCC

prior to version 4. It is an entirely new program that has been designed to provide Fortran

95 support and extensibility for future Fortran language standards, as well as providing

backwards compatibility for Fortran 77 and nearly all of the GNU language extensions

supported by g77.

1.5 Project Status

As soon as gfortran can parse all of the statements correctly, it will be in the

“larva” state. When we generate code, the “puppa” state. When gfortran is

done, we’ll see if it will be a beautiful butterﬂy, or just a big bug....

–Andy Vaught, April 2000

The start of the GNU Fortran 95 project was announced on the GCC homepage in March

18, 2000 (even though Andy had already been working on it for a while, of course).

The GNU Fortran compiler is able to compile nearly all standard-compliant Fortran 95,

Fortran 90, and Fortran 77 programs, including a number of standard and non-standard

extensions, and can be used on real-world programs. In particular, the supported extensions

include OpenMP, Cray-style pointers, some old vendor extensions, and several Fortran 2003

and Fortran 2008 features, including TR 15581. However, it is still under development and

has a few remaining rough edges. There also is initial support for OpenACC. Note that this

is an experimental feature, incomplete, and subject to change in future versions of GCC.

See https://gcc.gnu.org/wiki/OpenACC for more information.

At present, the GNU Fortran compiler passes the NIST Fortran 77 Test Suite, and pro-

duces acceptable results on the LAPACK Test Suite. It also provides respectable perfor-

mance on the Polyhedron Fortran compiler benchmarks and the Livermore Fortran Kernels

test. It has been used to compile a number of large real-world programs, including the

HARMONIE and HIRLAM weather forecasting code and the Tonto quantum chemistry

package; see https://gcc.gnu.org/wiki/GfortranApps for an extended list.

Among other things, the GNU Fortran compiler is intended as a replacement for G77.

At this point, nearly all programs that could be compiled with G77 can be compiled with

GNU Fortran, although there are a few minor known regressions.

4 The GNU Fortran Compiler

The primary work remaining to be done on GNU Fortran falls into three categories:

bug ﬁxing (primarily regarding the treatment of invalid code and providing useful error

messages), improving the compiler optimizations and the performance of compiled code,

and extending the compiler to support future standards—in particular, Fortran 2003 and

Fortran 2008.

1.6 Standards

The GNU Fortran compiler implements ISO/IEC 1539:1997 (Fortran 95). As such, it can

also compile essentially all standard-compliant Fortran 90 and Fortran 77 programs. It also

supports the ISO/IEC TR-15581 enhancements to allocatable arrays.

GNU Fortran also have a partial support for ISO/IEC 1539-1:2004 (Fortran

2003), ISO/IEC 1539-1:2010 (Fortran 2008), the Technical Speciﬁcation Further

Interoperability of Fortran with C (ISO/IEC TS 29113:2012). Full support of those

standards and future Fortran standards is planned. The current status of the support is

can be found in the Section 4.1 [Fortran 2003 status], page 35, Section 4.2 [Fortran 2008

status], page 36, Section 4.3 [TS 29113 status], page 38 and Section 4.4 [TS 18508 status],

page 38 sections of the documentation.

Additionally, the GNU Fortran compilers supports the OpenMP speciﬁcation

(version 4.0 and most of the features of the 4.5 version, http: / / openmp . org / wp /

openmp-specifications/). There also is initial support for the OpenACC speciﬁcation

(targeting version 2.0, http://www.openacc.org/). Note that this is an experimental

feature, incomplete, and subject to change in future versions of GCC. See https://gcc.

gnu.org/wiki/OpenACC for more information.

1.6.1 Varying Length Character Strings

The Fortran 95 standard speciﬁes in Part 2 (ISO/IEC 1539-2:2000) varying length character

strings. While GNU Fortran currently does not support such strings directly, there exist

two Fortran implementations for them, which work with GNU Fortran. They can be found

at http://www.fortran.com/iso_varying_string.f95 and at ftp:// ftp.nag.co.uk/

sc22wg5/ISO_VARYING_STRING/.

Deferred-length character strings of Fortran 2003 supports part of the features of ISO_

VARYING_STRING and should be considered as replacement. (Namely, allocatable or pointers

of the type character(len=:).)

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