glibc/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 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 @w{ISO 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.
@end itemize

For example, suppose the header file @file{stdlib.h} declares a function
named @code{abs} with

@smallexample
extern int abs (int);
@end smallexample

@noindent
and also provides a macro definition for @code{abs}.  Then, in:

@smallexample
#include <stdlib.h>
int f (int *i) @{ return abs (++*i); @}
@end smallexample

@noindent
the reference to @code{abs} might refer to either a macro or a function.
On the other hand, in each of the following examples the reference is
to a function and not a macro.

@smallexample
#include <stdlib.h>
int g (int *i) @{ return (abs) (++*i); @}

#undef abs
int h (int *i) @{ return abs (++*i); @}
@end smallexample

Since macro definitions that double for a function behave in
exactly the same way as the actual function version, there is usually no
need for any of these methods.  In fact, removing macro definitions usually
just makes your program slower.


@node Reserved Names, Feature Test Macros, Macro Definitions, Using the Library
@subsection Reserved Names
@cindex reserved names
@cindex name space

The names of all library types, macros, variables and functions that
come from the @w{ISO C} standard are reserved unconditionally; your program
@strong{may not} redefine these names.  All other library names are
reserved if your program explicitly includes the header file that
defines or declares them.  There are several reasons for these
restrictions:

@itemize @bullet
@item
Other people reading your code could get very confused if you were using
a function named @code{exit} to do something completely different from
what the standard @code{exit} function does, for example.  Preventing
this situation helps to make your programs easier to understand and
contributes to modularity and maintainability.

@item
It avoids the possibility of a user accidentally redefining a library
function that is called by other library functions.  If redefinition
were allowed, those other functions would not work properly.

@item
It allows the compiler to do whatever special optimizations it pleases
on calls to these functions, without the possibility that they may have
been redefined by the user.  Some library facilities, such as those for
dealing with variadic arguments (@pxref{Variadic Functions})
and non-local exits (@pxref{Non-Local Exits}), actually require a
considerable amount of cooperation on the part of the C compiler, and
implementationally it might be easier for the compiler to treat these as
built-in parts of the language.
@end itemize

In addition to the names documented in this manual, reserved names
include all external identifiers (global functions and variables) that
begin with an underscore (@samp{_}) and all identifiers regardless of
use that begin with either two underscores or an underscore followed by
a capital letter are reserved names.  This is so that the library and
header files can define functions, variables, and macros for internal
purposes without risk of conflict with names in user programs.

Some additional classes of identifier names are reserved for future
extensions to the C language or the POSIX.1 environment.  While using these
names for your own purposes right now might not cause a problem, they do
raise the possibility of conflict with future versions of the C
or POSIX standards, so you should avoid these names.

@itemize @bullet
@item
Names beginning with a capital @samp{E} followed a digit or uppercase
letter may be used for additional error code names.  @xref{Error
Reporting}.

@item
Names that begin with either @samp{is} or @samp{to} followed by a
lowercase letter may be used for additional character testing and
conversion functions.  @xref{Character Handling}.

@item
Names that begin with @samp{LC_} followed by an uppercase letter may be
used for additional macros specifying locale attributes.
@xref{Locales}.

@item
Names of all existing mathematics functions (@pxref{Mathematics})
suffixed with @samp{f} or @samp{l} are reserved for corresponding
functions that operate on @code{float} and @code{long double} arguments,
respectively.

@item
Names that begin with @samp{SIG} followed by an uppercase letter are
reserved for additional signal names.  @xref{Standard Signals}.

@item
Names that begin with @samp{SIG_} followed by an uppercase letter are
reserved for additional signal actions.  @xref{Basic Signal Handling}.

@item
Names beginning with @samp{str}, @samp{mem}, or @samp{wcs} followed by a
lowercase letter are reserved for additional string and array functions.
@xref{String and Array Utilities}.

@item
Names that end with @samp{_t} are reserved for additional type names.
@end itemize

In addition, some individual header files reserve names beyond
those that they actually define.  You only need to worry about these
restrictions if your program includes that particular header file.

@itemize @bullet
@item
The header file @file{dirent.h} reserves names prefixed with
@samp{d_}.
@pindex dirent.h

@item
The header file @file{fcntl.h} reserves names prefixed with
@samp{l_}, @samp{F_}, @samp{O_}, and @samp{S_}.
@pindex fcntl.h

@item
The header file @file{grp.h} reserves names prefixed with @samp{gr_}.
@pindex grp.h

@item
The header file @file{limits.h} reserves names suffixed with @samp{_MAX}.
@pindex limits.h

@item
The header file @file{pwd.h} reserves names prefixed with @samp{pw_}.
@pindex pwd.h

@item
The header file @file{signal.h} reserves names prefixed with @samp{sa_}
and @samp{SA_}.
@pindex signal.h

@item
The header file @file{sys/stat.h} reserves names prefixed with @samp{st_}
and @samp{S_}.
@pindex sys/stat.h

@item
The header file @file{sys/times.h} reserves names prefixed with @samp{tms_}.
@pindex sys/times.h

@item
The header file @file{termios.h} reserves names prefixed with @samp{c_},
@samp{V}, @samp{I}, @samp{O}, and @samp{TC}; and names prefixed with
@samp{B} followed by a digit.
@pindex termios.h
@end itemize

@comment Include the section on Creature Nest Macros.
@comment It is in a separate file so it can be formatted into ../NOTES.
@include creature.texi

@node Roadmap to the Manual,  , Using the Library, Introduction
@section Roadmap to the Manual

Here is an overview of the contents of the remaining chapters of
this manual.

@itemize @bullet
@item
@ref{Error Reporting}, describes how errors detected by the library
are reported.

@item
@ref{Language Features}, contains information about library support for
standard parts of the C language, including things like the @code{sizeof}
operator and the symbolic constant @code{NULL}, how to write functions
accepting variable numbers of arguments, and constants describing the
ranges and other properties of the numerical types.  There is also a simple
debugging mechanism which allows you to put assertions in your code, and
have diagnostic messages printed if the tests fail.

@item
@ref{Memory Allocation}, describes the GNU library's facilities for
dynamic allocation of storage.  If you do not know in advance how much
storage your program needs, you can allocate it dynamically instead,
and manipulate it via pointers.

@item
@ref{Character Handling}, contains information about character
classification functions (such as @code{isspace}) and functions for
performing case conversion.

@item
@ref{String and Array Utilities}, has descriptions of functions for
manipulating strings (null-terminated character arrays) and general
byte arrays, including operations such as copying and comparison.

@item
@ref{I/O Overview}, gives an overall look at the input and output
facilities in the library, and contains information about basic concepts
such as file names.

@item
@ref{I/O on Streams}, describes I/O operations involving streams (or
@w{@code{FILE *}} objects).  These are the normal C library functions
from @file{stdio.h}.

@item
@ref{Low-Level I/O}, contains information about I/O operations
on file descriptors.  File descriptors are a lower-level mechanism
specific to the Unix family of operating systems.

@item
@ref{File System Interface}, has descriptions of operations on entire
files, such as functions for deleting and renaming them and for creating
new directories.  This chapter also contains information about how you
can access the attributes of a file, such as its owner and file protection
modes.

@item
@ref{Pipes and FIFOs}, contains information about simple interprocess
communication mechanisms.  Pipes allow communication between two related
processes (such as between a parent and child), while FIFOs allow
communication between processes sharing a common file system on the same
machine.

@item
@ref{Sockets}, describes a more complicated interprocess communication
mechanism that allows processes running on different machines to
communicate over a network.  This chapter also contains information about
Internet host addressing and how to use the system network databases.

@item
@ref{Low-Level Terminal Interface}, describes how you can change the
attributes of a terminal device.  If you want to disable echo of
characters typed by the user, for example, read this chapter.

@item
@ref{Mathematics}, contains information about the math library
functions.  These include things like random-number generators and
remainder functions on integers as well as the usual trigonometric and
exponential functions on floating-point numbers.

@item
@ref{Arithmetic,, Low-Level Arithmetic Functions}, describes functions
for simple arithmetic, analysis of floating-point values, and reading
numbers from strings.

@item
@ref{Searching and Sorting}, contains information about functions
for searching and sorting arrays.  You can use these functions on any
kind of array by providing an appropriate comparison function.

@item
@ref{Pattern Matching}, presents functions for matching regular expressions
and shell file name patterns, and for expanding words as the shell does.

@item
@ref{Date and Time}, describes functions for measuring both calendar time
and CPU time, as well as functions for setting alarms and timers.

@item
@ref{Extended Characters}, contains information about manipulating
characters and strings using character sets larger than will fit in
the usual @code{char} data type.

@item
@ref{Locales}, describes how selecting a particular country
or language affects the behavior of the library.  For example, the locale
affects collation sequences for strings and how monetary values are
formatted.

@item
@ref{Non-Local Exits}, contains descriptions of the @code{setjmp} and
@code{longjmp} functions.  These functions provide a facility for
@code{goto}-like jumps which can jump from one function to another.

@item
@ref{Signal Handling}, tells you all about signals---what they are,
how to establish a handler that is called when a particular kind of
signal is delivered, and how to prevent signals from arriving during
critical sections of your program.

@item
@ref{Process Startup}, tells how your programs can access their
command-line arguments and environment variables.

@item
@ref{Processes}, contains information about how to start new processes
and run programs.

@item
@ref{Job Control}, describes functions for manipulating process groups
and the controlling terminal.  This material is probably only of
interest if you are writing a shell or other program which handles job
control specially.

@item
@ref{Name Service Switch}, describes the services which are available
for looking up names in the system databases, how to determine which
service is used for which database, and how these services are
implemented so that contributors can design their own services.

@item
@ref{User Database}, and @ref{Group Database}, tell you how to access
the system user and group databases.

@item
@ref{System Information}, describes functions for getting information
about the hardware and software configuration your program is executing
under.

@item
@ref{System Configuration}, tells you how you can get information about
various operating system limits.  Most of these parameters are provided for
compatibility with POSIX.

@item
@ref{Library Summary}, gives a summary of all the functions, variables, and
macros in the library, with complete data types and function prototypes,
and says what standard or system each is derived from.

@item
@ref{Maintenance}, explains how to build and install the GNU C library on
your system, how to report any bugs you might find, and how to add new
functions or port the library to a new system.
@end itemize

If you already know the name of the facility you are interested in, you
can look it up in @ref{Library Summary}.  This gives you a summary of
its syntax and a pointer to where you can find a more detailed
description.  This appendix is particularly useful if you just want to
verify the order and type of arguments to a function, for example.  It
also tells you what standard or system each function, variable, or macro
is derived from.