intro.texi

一个C源代码分析器

@node Introduction, Error Reporting, Top, Top
@chapter Introduction

The C language provides no built-in facilities for performing such
common operations as input/output, memory management, string
manipulation, and the like.  Instead, these facilities are defined
in a standard @dfn{library}, which you compile and link with your
programs.
@cindex library

The GNU C library, described in this document, defines all of the
library functions that are specified by the ANSI C standard, as well as
additional features specific to POSIX and other derivatives of the Unix
operating system, and extensions specific to the GNU system.

The purpose of this manual is to tell you how to use the facilities
of the GNU library.  We have mentioned which features belong to which
standards to help you identify things that are potentially nonportable
to other systems.  But the emphasis in this manual is not on strict
portability.

@menu
* Getting Started::             What this manual is for and how to use it.
* Standards and Portability::   Standards and sources upon which the GNU
                                 C library is based.
* Using the Library::           Some practical uses for the library.
* Roadmap to the Manual::       Overview of the remaining chapters in
                                 this manual.
@end menu

@node Getting Started, Standards and Portability,  , Introduction
@section Getting Started

This manual is written with the assumption that you are at least
somewhat familiar with the C programming language and basic programming
concepts.  Specifically, familiarity with ANSI standard C
(@pxref{ANSI C}), rather than ``traditional'' pre-ANSI C dialects, is
assumed.

The GNU C library includes several @dfn{header files}, each of which
provides definitions and declarations for a group of related facilities;
this information is used by the C compiler when processing your program.
For example, the header file @file{stdio.h} declares facilities for
performing input and output, and the header file @file{string.h}
declares string processing utilities.  The organization of this manual
generally follows the same division as the header files.

If you are reading this manual for the first time, you should read all
of the introductory material and skim the remaining chapters.  There are
a @emph{lot} of functions in the GNU C library and it's not realistic to
expect that you will be able to remember exactly @emph{how} to use each
and every one of them.  It's more important to become generally familiar
with the kinds of facilities that the library provides, so that when you
are writing your programs you can recognize @emph{when} to make use of
library functions, and @emph{where} in this manual you can find more
specific information about them.


@node Standards and Portability, Using the Library, Getting Started, Introduction
@section Standards and Portability
@cindex standards

This section discusses the various standards and other sources that the
GNU C library is based upon.  These sources include the ANSI C and
POSIX standards, and the System V and Berkeley Unix implementations.

The primary focus of this manual is to tell you how to make effective
use of the GNU library facilities.  But if you are concerned about
making your programs compatible with these standards, or portable to
operating systems other than GNU, this can affect how you use the
library.  This section gives you an overview of these standards, so that
you will know what they are when they are mentioned in other parts of
the manual.

@xref{Library Summary}, for an alphabetical list of the functions and
other symbols provided by the library.  This list also states which
standards each function or symbol comes from.

@menu
* ANSI C::                      The American National Standard for the
				 C programming language.  
* POSIX::                       The IEEE 1003 standards for operating
                                 systems. 
* Berkeley Unix::               BSD and SunOS.
* SVID::                        The System V Interface Description.  
@end menu

@node ANSI C, POSIX,  , Standards and Portability
@subsection ANSI C
@cindex ANSI C

The GNU C library is compatible with the C standard adopted by the
American National Standards Institute (ANSI):
@cite{American National Standard X3.159-1989---``ANSI C''}.
The header files and library facilities that make up the GNU library are
a superset of those specified by the ANSI C standard.@refill

@pindex gcc
If you are concerned about strict adherence to the ANSI C standard, you
should use the @samp{-ansi} option when you compile your programs with
the GNU C compiler.  This tells the compiler to define @emph{only} ANSI
standard features from the library header files, unless you explicitly
ask for additional features.  @xref{Feature Test Macros}, for
information on how to do this.

Being able to restrict the library to include only ANSI C features is
important because ANSI C puts limitations on what names can be defined
by the library implementation, and the GNU extensions don't fit these
limitations.  @xref{Reserved Names}, for more information about these
restrictions.

This manual does not attempt to give you complete details on the
differences between ANSI C and older dialects.  It gives advice on how
to write programs to work portably under multiple C dialects, but does
not aim for completeness.

@node POSIX, Berkeley Unix, ANSI C, Standards and Portability
@subsection POSIX (The Portable Operating System Interface)
@cindex POSIX
@cindex POSIX.1
@cindex IEEE Std 1003.1
@cindex POSIX.2
@cindex IEEE Std 1003.2

The GNU library is also compatible with the IEEE @dfn{POSIX} family of
standards, known more formally as the @dfn{Portable Operating System
Interface for Computer Environments}.  POSIX is derived mostly from
various versions of the Unix operating system.

The library facilities specified by the POSIX standards are a superset
of those required by ANSI C; POSIX specifies additional features for
ANSI C functions, as well as specifying new additional functions.  In
general, the additional requirements and functionality defined by the
POSIX standards are aimed at providing lower-level support for a
particular kind of operating system environment, rather than general
programming language support which can run in many diverse operating
system environments.@refill

The GNU C library implements all of the functions specified in
@cite{IEEE Std 1003.1-1990, the POSIX System Application Program
Interface}, commonly referred to as POSIX.1.  The primary extensions to
the ANSI C facilities specified by this standard include file system
interface primitives (@pxref{File System Interface}), device-specific
terminal control functions (@pxref{Low-Level Terminal Interface}), and
process control functions (@pxref{Processes}).

Some facilities from @cite{IEEE Std 1003.2-1992, the POSIX Shell and
Utilities standard} (POSIX.2) are also implemented in the GNU library.
These include utilities for dealing with regular expressions and other
pattern matching facilities (@pxref{Pattern Matching}).

@comment Roland sez:
@comment The GNU C library as it stands conforms to 1003.2 draft 11, which
@comment specifies:
@comment 
@comment Several new macros in <limits.h>.
@comment popen, pclose
@comment <regex.h> (which is not yet fully implemented--wait on this)
@comment fnmatch
@comment getopt
@comment <glob.h>
@comment <wordexp.h> (not yet implemented)
@comment confstr


@node Berkeley Unix, SVID, POSIX, Standards and Portability
@subsection Berkeley Unix
@cindex BSD Unix
@cindex 4.@var{n} BSD Unix
@cindex Berkeley Unix
@cindex SunOS
@cindex Unix, Berkeley

The GNU C library defines facilities from some versions of Unix which
are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD,
and 4.4 BSD Unix systems (also known as @dfn{Berkeley Unix}) and from
@dfn{SunOS} (a popular 4.2 BSD derivative that includes some Unix System
V functionality).  These systems support most of the ANSI and POSIX
facilities, and 4.4 BSD and newer releases of SunOS in fact support them all.

The BSD facilities include symbolic links (@pxref{Symbolic Links}), the
@code{select} function (@pxref{Waiting for I/O}), the BSD signal
functions (@pxref{BSD Signal Handling}), and sockets (@pxref{Sockets}).

@node SVID,  , Berkeley Unix, Standards and Portability
@subsection SVID (The System V Interface Description)
@cindex SVID
@cindex System V Unix
@cindex Unix, System V

The @dfn{System V Interface Description} (SVID) is a document describing
the AT&T Unix System V operating system.  It is to some extent a
superset of the POSIX standard (@pxref{POSIX}).

The GNU C library defines some of the facilities required by the SVID
that are not also required by the ANSI or POSIX standards, for
compatibility with  System V Unix and other Unix systems (such as
SunOS) which include these facilities.  However, many of the more
obscure and less generally useful facilities required by the SVID are
not included.  (In fact, Unix System V itself does not provide them all.)

@c !!! mention sysv ipc/shmem when it is there.


@node Using the Library, Roadmap to the Manual, Standards and Portability, Introduction
@section Using the Library

This section describes some of the practical issues involved in using
the GNU C library.

@menu
* Header Files::                How to include the header files in your
                                 programs. 
* Macro Definitions::           Some functions in the library may really
                                 be implemented as macros.
* Reserved Names::              The C standard reserves some names for
                                 the library, and some for users.
* Feature Test Macros::         How to control what names are defined.
@end menu

@node Header Files, Macro Definitions,  , Using the Library
@subsection Header Files
@cindex header files

Libraries for use by C programs really consist of two parts: @dfn{header
files} that define types and macros and declare variables and
functions; and the actual library or @dfn{archive} that contains the
definitions of the variables and functions.

(Recall that in C, a @dfn{declaration} merely provides information that
a function or variable exists and gives its type.  For a function
declaration, information about the types of its arguments might be
provided as well.  The purpose of declarations is to allow the compiler
to correctly process references to the declared variables and functions.
A @dfn{definition}, on the other hand, actually allocates storage for a
variable or says what a function does.)
@cindex definition (compared to declaration)
@cindex declaration (compared to definition)

In order to use the facilities in the GNU C library, you should be sure
that your program source files include the appropriate header files.
This is so that the compiler has declarations of these facilities
available and can correctly process references to them.  Once your
program has been compiled, the linker resolves these references to
the actual definitions provided in the archive file.

Header files are included into a program source file by the
@samp{#include} preprocessor directive.  The C language supports two
forms of this directive; the first,

@smallexample
#include "@var{header}"
@end smallexample

@noindent
is typically used to include a header file @var{header} that you write
yourself; this would contain definitions and declarations describing the
interfaces between the different parts of your particular application.
By contrast,

@smallexample
#include <file.h>
@end smallexample

@noindent
is typically used to include a header file @file{file.h} that contains
definitions and declarations for a standard library.  This file would
normally be installed in a standard place by your system administrator.
You should use this second form for the C library header files.

Typically, @samp{#include} directives are placed at the top of the C
source file, before any other code.  If you begin your source files with
some comments explaining what the code in the file does (a good idea),
put the @samp{#include} directives immediately afterwards, following the
feature test macro definition (@pxref{Feature Test Macros}).

For more information about the use of header files and @samp{#include}
directives, @pxref{Header Files,,, cpp.info, The GNU C Preprocessor
Manual}.@refill

The GNU C library provides several header files, each of which contains
the type and macro definitions and variable and function declarations
for a group of related facilities.  This means that your programs may
need to include several header files, depending on exactly which
facilities you are using.

Some library header files include other library header files
automatically.  However, as a matter of programming style, you should
not rely on this; it is better to explicitly include all the header
files required for the library facilities you are using.  The GNU C
library header files have been written in such a way that it doesn't
matter if a header file is accidentally included more than once;
including a header file a second time has no effect.  Likewise, if your
program needs to include multiple header files, the order in which they
are included doesn't matter.

@strong{Compatibility Note:} Inclusion of standard header files in any
order and any number of times works in any ANSI C implementation.
However, this has traditionally not been the case in many older C
implementations.

Strictly speaking, you don't @emph{have to} include a header file to use
a function it declares; you could declare the function explicitly
yourself, according to the specifications in this manual.  But it is
usually better to include the header file because it may define types
and macros that are not otherwise available and because it may define
more efficient macro replacements for some functions.  It is also a sure
way to have the correct declaration.

@node Macro Definitions, Reserved Names, Header Files, Using the Library
@subsection Macro Definitions of Functions
@cindex shadowing functions with macros
@cindex removing macros that shadow functions
@cindex undefining macros that shadow functions

If we describe something as a function in this manual, it may have a
macro definition as well.  This normally has no effect on how your
program runs---the macro definition does the same thing as the function
would.  In particular, macro equivalents for library functions evaluate
arguments exactly once, in the same way that a function call would.  The
main reason for these macro definitions is that sometimes they can
produce an inline expansion that is considerably faster than an actual
function call.

Taking the address of a library function works even if it is also
defined as a macro.  This is because, in this context, the name of the
function isn't followed by the left parenthesis that is syntactically
necessary to recognize a macro call.

You might occasionally want to avoid using the macro definition of a
function---perhaps to make your program easier to debug.  There are
two ways you can do this:

@itemize @bullet
@item 
You can avoid a macro definition in a specific use by enclosing the name
of the function in parentheses.  This works because the name of the
function doesn't appear in a syntactic context where it is recognizable
as a macro call.

@item
You can suppress any macro definition for a whole source file by using
the @samp{#undef} preprocessor directive, unless otherwise stated
explicitly in the description of that facility.