path: root/manual/intro.texi

@node Introduction, Error Reporting, Top, Top
@chapter Introduction
@c %MENU% Purpose of the GNU C Library

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 @w{ISO 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 non-portable
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 ISO standard C
(@pxref{ISO C}), rather than ``traditional'' pre-ISO 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 @w{ISO 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
* ISO C::                       The international standard for the C
                                 programming language.
* POSIX::                       The ISO/IEC 9945 (aka IEEE 1003) standards
                                 for operating systems.
* Berkeley Unix::               BSD and SunOS.
* SVID::                        The System V Interface Description.
* XPG::                         The X/Open Portability Guide.
@end menu

@node ISO C, POSIX,  , Standards and Portability
@subsection ISO C
@cindex ISO 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''} and later
by the International Standardization Organization (ISO):
@cite{ISO/IEC 9899:1990, ``Programming languages---C''}.
We here refer to the standard as @w{ISO C} since this is the more
general standard in respect of ratification.
The header files and library facilities that make up the GNU library are
a superset of those specified by the @w{ISO C} standard.@refill

@pindex gcc
If you are concerned about strict adherence to the @w{ISO 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} ISO
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 @w{ISO C} features is
important because @w{ISO 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 @w{ISO 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, ISO C, Standards and Portability
@subsection POSIX (The Portable Operating System Interface)
@cindex POSIX
@cindex POSIX.1
@cindex IEEE Std 1003.1
@cindex ISO/IEC 9945-1
@cindex POSIX.2
@cindex IEEE Std 1003.2
@cindex ISO/IEC 9945-2

The GNU library is also compatible with the ISO @dfn{POSIX} family of
standards, known more formally as the @dfn{Portable Operating System
Interface for Computer Environments} (ISO/IEC 9945).  They were also
published as ANSI/IEEE Std 1003.  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 @w{ISO C}; POSIX specifies additional features for
@w{ISO 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{ISO/IEC 9945-1:1996, the POSIX System Application Program
Interface}, commonly referred to as POSIX.1.  The primary extensions to
the @w{ISO 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{ISO/IEC 9945-2:1993, 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 @w{ISO C} 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, XPG, 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 most of the facilities required by the SVID
that are not also required by the @w{ISO C} 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.)

The supported facilities from System V include the methods for
inter-process communication and shared memory, the @code{hsearch} and
@code{drand48} families of functions, @code{fmtmsg} and several of the
mathematical functions.

@node XPG, , SVID, Standards and Portability
@subsection XPG (The X/Open Portability Guide)

The X/Open Portability Guide, published by the X/Open Company, Ltd., is
a more general standard than POSIX.  X/Open owns the Unix copyright and
the XPG specifies the requirements for systems which are intended to be
a Unix system.

The GNU C library complies to the X/Open Portability Guide, Issue 4.2,
with all extensions common to XSI (X/Open System Interface)
compliant systems and also all X/Open UNIX extensions.

The additions on top of POSIX are mainly derived from functionality
available in @w{System V} and BSD systems.  Some of the really bad
mistakes in @w{System V} systems were corrected, though.  Since
fulfilling the XPG standard with the Unix extensions is a
precondition for getting the Unix brand chances are good that the
functionality is available on commercial systems.


@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