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1. Introduction | Purpose of the GNU C Library. | |
2. Error Reporting | How library functions report errors. | |
3. Virtual Memory Allocation And Paging | Allocating virtual memory and controlling paging. | |
4. Character Handling | Character testing and conversion functions. | |
5. String and Array Utilities | Utilities for copying and comparing strings and arrays. | |
6. Character Set Handling | Support for extended character sets. | |
7. Locales and Internationalization | The country and language can affect the behavior of library functions. | |
8. Message Translation | How to make the program speak the user’s language. | |
9. Searching and Sorting | General searching and sorting functions. | |
10. Pattern Matching | Matching shell “globs” and regular expressions. | |
11. Input/Output Overview | Introduction to the I/O facilities. | |
12. Input/Output on Streams | High-level, portable I/O facilities. | |
13. Low-Level Input/Output | Low-level, less portable I/O. | |
14. File System Interface | Functions for manipulating files. | |
15. Pipes and FIFOs | A simple interprocess communication mechanism. | |
16. Sockets | A more complicated IPC mechanism, with networking support. | |
17. Low-Level Terminal Interface | How to change the characteristics of a terminal device. | |
18. Syslog | System logging and messaging. | |
19. Mathematics | Math functions, useful constants, random numbers. | |
20. Arithmetic Functions | Low level arithmetic functions. | |
21. Date and Time | Functions for getting the date and time and formatting them nicely. | |
22. Resource Usage And Limitation | Functions for examining resource usage and getting and setting limits. | |
23. Non-Local Exits | Jumping out of nested function calls. | |
24. Signal Handling | How to send, block, and handle signals. | |
25. The Basic Program/System Interface | Writing the beginning and end of your program. | |
26. Processes | How to create processes and run other programs. | |
27. Job Control | All about process groups and sessions. | |
28. System Databases and Name Service Switch | Accessing system databases. | |
29. Users and Groups | How users are identified and classified. | |
30. System Management | Controlling the system and getting information about it. | |
31. System Configuration Parameters | Parameters describing operating system limits. | |
32. DES Encryption and Password Handling | DES encryption and password handling. | |
33. Debugging support | Functions to help debugging applications. | |
Appendices | ||
---|---|---|
A. C Language Facilities in the Library | C language features provided by the library. | |
B. Summary of Library Facilities | A summary showing the syntax, header file, and derivation of each library feature. | |
C. Installing the GNU C Library | How to install the GNU C library. | |
D. Library Maintenance | How to enhance and port the GNU C Library. | |
E. Contributors to the GNU C Library | Who wrote what parts of the GNU C library. | |
F. Free Software Needs Free Documentation | ||
G. GNU Lesser General Public License | The GNU Lesser General Public License says how you can copy and share the GNU C Library. | |
H. GNU Free Documentation License | This manual is under the GNU Free Documentation License. | |
Indices | ||
Concept Index | Index of concepts and names. | |
Type Index | Index of types and type qualifiers. | |
Function and Macro Index | Index of functions and function-like macros. | |
Variable and Constant Macro Index | Index of variables and variable-like macros. | |
Program and File Index | Index of programs and files. | |
— The Detailed Node Listing — Introduction | ||
1.1 Getting Started | What this manual is for and how to use it. | |
1.2 Standards and Portability | Standards and sources upon which the GNU C library is based. | |
1.3 Using the Library | Some practical uses for the library. | |
1.4 Roadmap to the Manual | Overview of the remaining chapters in this manual. | |
Standards and Portability | ||
1.2.1 ISO C | The international standard for the C programming language. | |
1.2.2 POSIX (The Portable Operating System Interface) | The ISO/IEC 9945 (aka IEEE 1003) standards for operating systems. | |
1.2.3 Berkeley Unix | BSD and SunOS. | |
1.2.4 SVID (The System V Interface Description) | The System V Interface Description. | |
1.2.5 XPG (The X/Open Portability Guide) | The X/Open Portability Guide. | |
Using the Library | ||
1.3.1 Header Files | How to include the header files in your programs. | |
1.3.2 Macro Definitions of Functions | Some functions in the library may really be implemented as macros. | |
1.3.3 Reserved Names | The C standard reserves some names for the library, and some for users. | |
1.3.4 Feature Test Macros | How to control what names are defined. | |
Error Reporting | ||
2.1 Checking for Errors | How errors are reported by library functions. | |
2.2 Error Codes | Error code macros; all of these expand into integer constant values. | |
2.3 Error Messages | Mapping error codes onto error messages. | |
Memory | ||
3.1 Process Memory Concepts | An introduction to concepts and terminology. | |
3.2 Allocating Storage For Program Data | Allocating storage for your program data | |
3.4 Locking Pages | Preventing page faults | |
3.3 Resizing the Data Segment | brk , sbrk
| |
Memory Allocation | ||
3.2.1 Memory Allocation in C Programs | How to get different kinds of allocation in C. | |
3.2.2 Unconstrained Allocation | The malloc facility allows fully general
dynamic allocation.
| |
3.2.3 Allocation Debugging | Finding memory leaks and not freed memory. | |
3.2.4 Obstacks | Obstacks are less general than malloc but more efficient and convenient. | |
3.2.5 Automatic Storage with Variable Size | Allocation of variable-sized blocks of automatic storage that are freed when the calling function returns. | |
Unconstrained Allocation | ||
3.2.2.1 Basic Memory Allocation | Simple use of malloc .
| |
3.2.2.2 Examples of malloc | Examples of malloc . xmalloc .
| |
3.2.2.3 Freeing Memory Allocated with malloc | Use free to free a block you
got with malloc .
| |
3.2.2.4 Changing the Size of a Block | Use realloc to make a block
bigger or smaller.
| |
3.2.2.5 Allocating Cleared Space | Use calloc to allocate a
block and clear it.
| |
3.2.2.6 Efficiency Considerations for malloc | Efficiency considerations in use of these functions. | |
3.2.2.7 Allocating Aligned Memory Blocks | Allocating specially aligned memory. | |
3.2.2.8 Malloc Tunable Parameters | Use mallopt to adjust allocation
parameters.
| |
3.2.2.9 Heap Consistency Checking | Automatic checking for errors. | |
3.2.2.10 Memory Allocation Hooks | You can use these hooks for debugging
programs that use malloc .
| |
3.2.2.11 Statistics for Memory Allocation with malloc | Getting information about how much memory your program is using. | |
3.2.2.12 Summary of malloc -Related Functions | Summary of malloc and related functions.
| |
Allocation Debugging | ||
3.2.3.1 How to install the tracing functionality | ||
3.2.3.2 Example program excerpts | Example programs excerpts. | |
3.2.3.3 Some more or less clever ideas | ||
3.2.3.4 Interpreting the traces | What do all these lines mean? | |
Obstacks | ||
3.2.4.1 Creating Obstacks | How to declare an obstack in your program. | |
3.2.4.2 Preparing for Using Obstacks | Preparations needed before you can use obstacks. | |
3.2.4.3 Allocation in an Obstack | Allocating objects in an obstack. | |
3.2.4.4 Freeing Objects in an Obstack | Freeing objects in an obstack. | |
3.2.4.5 Obstack Functions and Macros | The obstack functions are both functions and macros. | |
3.2.4.6 Growing Objects | Making an object bigger by stages. | |
3.2.4.7 Extra Fast Growing Objects | Extra-high-efficiency (though more complicated) growing objects. | |
3.2.4.8 Status of an Obstack | Inquiries about the status of an obstack. | |
3.2.4.9 Alignment of Data in Obstacks | Controlling alignment of objects in obstacks. | |
3.2.4.10 Obstack Chunks | How obstacks obtain and release chunks; efficiency considerations. | |
3.2.4.11 Summary of Obstack Functions | ||
Variable Size Automatic | ||
3.2.5.1 alloca Example | Example of using alloca .
| |
3.2.5.2 Advantages of alloca | Reasons to use alloca .
| |
3.2.5.3 Disadvantages of alloca | Reasons to avoid alloca .
| |
3.2.5.4 GNU C Variable-Size Arrays | Only in GNU C, here is an alternative method of allocating dynamically and freeing automatically. | |
Locking Pages | ||
3.4.1 Why Lock Pages | Reasons to read this section. | |
3.4.2 Locked Memory Details | Everything you need to know locked memory | |
3.4.3 Functions To Lock And Unlock Pages | Here’s how to do it. | |
Character Handling | ||
4.1 Classification of Characters | Testing whether characters are letters, digits, punctuation, etc. | |
4.2 Case Conversion | Case mapping, and the like. | |
4.3 Character class determination for wide characters | ||
4.4 Notes on using the wide character classes | ||
4.5 Mapping of wide characters. | ||
String and Array Utilities | ||
5.1 Representation of Strings | Introduction to basic concepts. | |
5.2 String and Array Conventions | Whether to use a string function or an arbitrary array function. | |
5.3 String Length | Determining the length of a string. | |
5.4 Copying and Concatenation | Functions to copy the contents of strings and arrays. | |
5.5 String/Array Comparison | Functions for byte-wise and character-wise comparison. | |
5.6 Collation Functions | Functions for collating strings. | |
5.7 Search Functions | Searching for a specific element or substring. | |
5.8 Finding Tokens in a String | Splitting a string into tokens by looking for delimiters. | |
5.9 strfry | Function for flash-cooking a string. | |
5.10 Trivial Encryption | Obscuring data. | |
5.11 Encode Binary Data | Encoding and Decoding of Binary Data. | |
5.12 Argz and Envz Vectors | Null-separated string vectors. | |
Argz and Envz Vectors | ||
5.12.1 Argz Functions | Operations on argz vectors. | |
5.12.2 Envz Functions | Additional operations on environment vectors. | |
Character Set Handling | ||
6.1 Introduction to Extended Characters | ||
6.2 Overview about Character Handling Functions | ||
6.3 Restartable Multibyte Conversion Functions | Restartable multibyte conversion Functions. | |
6.4 Non-reentrant Conversion Function | ||
6.5 Generic Charset Conversion | ||
Restartable multibyte conversion | ||
6.3.1 Selecting the conversion and its properties | ||
6.3.2 Representing the state of the conversion | ||
6.3.3 Converting Single Characters | ||
6.3.4 Converting Multibyte and Wide Character Strings | ||
6.3.5 A Complete Multibyte Conversion Example | ||
Non-reentrant Conversion | ||
6.4.1 Non-reentrant Conversion of Single Characters | ||
6.4.2 Non-reentrant Conversion of Strings | ||
6.4.3 States in Non-reentrant Functions | ||
Generic Charset Conversion | ||
6.5.1 Generic Character Set Conversion Interface | ||
6.5.2 A complete iconv example | ||
6.5.3 Some Details about other iconv Implementations | ||
6.5.4 The iconv Implementation in the GNU C library | ||
Locales | ||
7.1 What Effects a Locale Has | Actions affected by the choice of locale. | |
7.2 Choosing a Locale | How the user specifies a locale. | |
7.3 Categories of Activities that Locales Affect | Different purposes for which you can select a locale. | |
7.4 How Programs Set the Locale | How a program specifies the locale with library functions. | |
7.5 Standard Locales | Locale names available on all systems. | |
7.6 Accessing Locale Information | How to access the information for the locale. | |
7.7 A dedicated function to format numbers | ||
7.8 Yes-or-No Questions | Check a Response against the locale. | |
Locale Information | ||
7.6.1 localeconv : It is portable but … | ISO C’s localeconv .
| |
7.6.2 Pinpoint Access to Locale Data | X/Open’s nl_langinfo .
| |
The Lame Way to Locale Data | ||
7.6.1.1 Generic Numeric Formatting Parameters | Parameters for formatting numbers and currency amounts. | |
7.6.1.2 Printing the Currency Symbol | How to print the symbol that identifies an amount of money (e.g. ‘$’). | |
7.6.1.3 Printing the Sign of a Monetary Amount | How to print the (positive or negative) sign for a monetary amount, if one exists. | |
Message Translation | ||
8.1 X/Open Message Catalog Handling | The catgets family of functions.
| |
8.2 The Uniforum approach to Message Translation | The gettext family of functions.
| |
Message catalogs a la X/Open | ||
8.1.1 The catgets function family | ||
8.1.2 Format of the message catalog files | ||
8.1.3 Generate Message Catalogs files | How to generate message catalogs files which can be used by the functions. | |
8.1.4 How to use the catgets interface | ||
The Uniforum approach | ||
8.2.1 The gettext family of functions | ||
8.2.2 Programs to handle message catalogs for gettext | ||
Message catalogs with gettext | ||
8.2.1.1 What has to be done to translate a message? | ||
8.2.1.2 How to determine which catalog to be used | ||
8.2.1.3 Additional functions for more complicated situations | ||
8.2.1.4 How to specify the output character set gettext uses | ||
8.2.1.5 How to use gettext in GUI programs | ||
8.2.1.6 User influence on gettext | The possibilities of the user to influence
the way gettext works.
| |
Searching and Sorting | ||
9.1 Defining the Comparison Function | Defining how to compare two objects. Since the sort and search facilities are general, you have to specify the ordering. | |
9.2 Array Search Function | The bsearch function.
| |
9.3 Array Sort Function | The qsort function.
| |
9.4 Searching and Sorting Example | An example program. | |
9.5 The hsearch function. | ||
9.6 The tsearch function. | ||
Pattern Matching | ||
10.1 Wildcard Matching | Matching a wildcard pattern against a single string. | |
10.2 Globbing | Finding the files that match a wildcard pattern. | |
10.3 Regular Expression Matching | Matching regular expressions against strings. | |
10.4 Shell-Style Word Expansion | Expanding shell variables, nested commands, arithmetic, and wildcards. This is what the shell does with shell commands. | |
Globbing | ||
10.2.1 Calling glob | Basic use of glob .
| |
10.2.2 Flags for Globbing | Flags that enable various options in glob .
| |
10.2.3 More Flags for Globbing | GNU specific extensions to glob .
| |
Regular Expressions | ||
10.3.1 POSIX Regular Expression Compilation | Using regcomp to prepare to match.
| |
10.3.2 Flags for POSIX Regular Expressions | Syntax variations for regcomp .
| |
10.3.3 Matching a Compiled POSIX Regular Expression | Using regexec to match the compiled
pattern that you get from regcomp .
| |
10.3.4 Match Results with Subexpressions | Finding which parts of the string were matched. | |
10.3.5 Complications in Subexpression Matching | Find points of which parts were matched. | |
10.3.6 POSIX Regexp Matching Cleanup | Freeing storage; reporting errors. | |
Word Expansion | ||
10.4.1 The Stages of Word Expansion | What word expansion does to a string. | |
10.4.2 Calling wordexp | How to call wordexp .
| |
10.4.3 Flags for Word Expansion | Options you can enable in wordexp .
| |
10.4.4 wordexp Example | A sample program that does word expansion. | |
10.4.5 Details of Tilde Expansion | Details of how tilde expansion works. | |
10.4.6 Details of Variable Substitution | Different types of variable substitution. | |
I/O Overview | ||
11.1 Input/Output Concepts | Some basic information and terminology. | |
11.2 File Names | How to refer to a file. | |
I/O Concepts | ||
11.1.1 Streams and File Descriptors | The GNU Library provides two ways to access the contents of files. | |
11.1.2 File Position | The number of bytes from the beginning of the file. | |
File Names | ||
11.2.1 Directories | Directories contain entries for files. | |
11.2.2 File Name Resolution | A file name specifies how to look up a file. | |
11.2.3 File Name Errors | Error conditions relating to file names. | |
11.2.4 Portability of File Names | File name portability and syntax issues. | |
I/O on Streams | ||
12.1 Streams | About the data type representing a stream. | |
12.2 Standard Streams | Streams to the standard input and output devices are created for you. | |
12.3 Opening Streams | How to create a stream to talk to a file. | |
12.4 Closing Streams | Close a stream when you are finished with it. | |
12.5 Streams and Threads | Issues with streams in threaded programs. | |
12.6 Streams in Internationalized Applications | Streams in internationalized applications. | |
12.7 Simple Output by Characters or Lines | Unformatted output by characters and lines. | |
12.8 Character Input | Unformatted input by characters and words. | |
12.9 Line-Oriented Input | Reading a line or a record from a stream. | |
12.10 Unreading | Peeking ahead/pushing back input just read. | |
12.11 Block Input/Output | Input and output operations on blocks of data. | |
12.12 Formatted Output | printf and related functions.
| |
12.13 Customizing printf | You can define new conversion specifiers for
printf and friends.
| |
12.14 Formatted Input | scanf and related functions.
| |
12.15 End-Of-File and Errors | How you can tell if an I/O error happens. | |
12.16 Recovering from errors | What you can do about errors. | |
12.17 Text and Binary Streams | Some systems distinguish between text files and binary files. | |
12.18 File Positioning | About random-access streams. | |
12.19 Portable File-Position Functions | Random access on peculiar ISO C systems. | |
12.20 Stream Buffering | How to control buffering of streams. | |
12.21 Other Kinds of Streams | Streams that do not necessarily correspond to an open file. | |
12.22 Formatted Messages | Print strictly formatted messages. | |
Unreading | ||
12.10.1 What Unreading Means | An explanation of unreading with pictures. | |
12.10.2 Using ungetc To Do Unreading | How to call ungetc to do unreading.
| |
Formatted Output | ||
12.12.1 Formatted Output Basics | Some examples to get you started. | |
12.12.2 Output Conversion Syntax | General syntax of conversion specifications. | |
12.12.3 Table of Output Conversions | Summary of output conversions and what they do. | |
12.12.4 Integer Conversions | Details about formatting of integers. | |
12.12.5 Floating-Point Conversions | Details about formatting of floating-point numbers. | |
12.12.6 Other Output Conversions | Details about formatting of strings, characters, pointers, and the like. | |
12.12.7 Formatted Output Functions | Descriptions of the actual functions. | |
12.12.8 Dynamically Allocating Formatted Output | Functions that allocate memory for the output. | |
12.12.9 Variable Arguments Output Functions | vprintf and friends.
| |
12.12.10 Parsing a Template String | What kinds of args does a given template call for? | |
12.12.11 Example of Parsing a Template String | Sample program using parse_printf_format .
| |
Customizing Printf | ||
12.13.1 Registering New Conversions | Using register_printf_function
to register a new output conversion.
| |
12.13.2 Conversion Specifier Options | The handler must be able to get the options specified in the template when it is called. | |
12.13.3 Defining the Output Handler | Defining the handler and arginfo
functions that are passed as arguments
to register_printf_function .
| |
12.13.4 printf Extension Example | How to define a printf
handler function.
| |
12.13.5 Predefined printf Handlers | Predefined printf handlers.
| |
Formatted Input | ||
12.14.1 Formatted Input Basics | Some basics to get you started. | |
12.14.2 Input Conversion Syntax | Syntax of conversion specifications. | |
12.14.3 Table of Input Conversions | Summary of input conversions and what they do. | |
12.14.4 Numeric Input Conversions | Details of conversions for reading numbers. | |
12.14.5 String Input Conversions | Details of conversions for reading strings. | |
12.14.6 Dynamically Allocating String Conversions | String conversions that malloc the buffer.
| |
12.14.7 Other Input Conversions | Details of miscellaneous other conversions. | |
12.14.8 Formatted Input Functions | Descriptions of the actual functions. | |
12.14.9 Variable Arguments Input Functions | vscanf and friends.
| |
Stream Buffering | ||
12.20.1 Buffering Concepts | Terminology is defined here. | |
12.20.2 Flushing Buffers | How to ensure that output buffers are flushed. | |
12.20.3 Controlling Which Kind of Buffering | How to specify what kind of buffering to use. | |
Other Kinds of Streams | ||
12.21.1 String Streams | Streams that get data from or put data in a string or memory buffer. | |
12.21.2 Programming Your Own Custom Streams | Defining your own streams with an arbitrary input data source and/or output data sink. | |
Custom Streams | ||
12.21.2.1 Custom Streams and Cookies | The cookie records where to fetch or store data that is read or written. | |
12.21.2.2 Custom Stream Hook Functions |
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 library, which you compile and link with your programs.
The GNU C library, described in this document, defines all of the library functions that are specified by the 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.
1.1 Getting Started | What this manual is for and how to use it. | |
1.2 Standards and Portability | Standards and sources upon which the GNU C library is based. | |
1.3 Using the Library | Some practical uses for the library. | |
1.4 Roadmap to the Manual | Overview of the remaining chapters in this manual. |
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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 (see section ISO C), rather than “traditional” pre-ISO C dialects, is assumed.
The GNU C library includes several 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 ‘stdio.h’ declares facilities for performing input and output, and the header 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 lot of functions in the GNU C library and it’s not realistic to expect that you will be able to remember exactly 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 when to make use of library functions, and where in this manual you can find more specific information about them.
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This section discusses the various standards and other sources that the GNU C library is based upon. These sources include the 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.
See section Summary of Library Facilities, 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.
1.2.1 ISO C | The international standard for the C programming language. | |
1.2.2 POSIX (The Portable Operating System Interface) | The ISO/IEC 9945 (aka IEEE 1003) standards for operating systems. | |
1.2.3 Berkeley Unix | BSD and SunOS. | |
1.2.4 SVID (The System V Interface Description) | The System V Interface Description. | |
1.2.5 XPG (The X/Open Portability Guide) | The X/Open Portability Guide. |
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The GNU C library is compatible with the C standard adopted by the American National Standards Institute (ANSI): American National Standard X3.159-1989—“ANSI C” and later by the International Standardization Organization (ISO): ISO/IEC 9899:1990, “Programming languages—C”. We here refer to the standard as 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 ISO C standard.
If you are concerned about strict adherence to the ISO C standard, you should use the ‘-ansi’ option when you compile your programs with the GNU C compiler. This tells the compiler to define only ISO standard features from the library header files, unless you explicitly ask for additional features. See section Feature Test Macros, for information on how to do this.
Being able to restrict the library to include only ISO C features is important because ISO C puts limitations on what names can be defined by the library implementation, and the GNU extensions don’t fit these limitations. See section Reserved Names, for more information about these restrictions.
This manual does not attempt to give you complete details on the differences between 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.
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The GNU library is also compatible with the ISO POSIX family of standards, known more formally as the 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 ISO C; POSIX specifies additional features for 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.
The GNU C library implements all of the functions specified in ISO/IEC 9945-1:1996, the POSIX System Application Program Interface, commonly referred to as POSIX.1. The primary extensions to the ISO C facilities specified by this standard include file system interface primitives (see section File System Interface), device-specific terminal control functions (see section Low-Level Terminal Interface), and process control functions (see section Processes).
Some facilities from 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 (see section Pattern Matching).
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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 Berkeley Unix) and from SunOS (a popular 4.2 BSD derivative that includes some Unix System V functionality). These systems support most of the 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 (see section Symbolic Links), the
select
function (see section Waiting for Input or Output), the BSD signal
functions (see section BSD Signal Handling), and sockets (see section Sockets).
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The 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 (see section POSIX (The Portable Operating System Interface)).
The GNU C library defines most of the facilities required by the SVID that are not also required by the 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 hsearch
and
drand48
families of functions, fmtmsg
and several of the
mathematical functions.
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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 System V and BSD systems. Some of the really bad mistakes in 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.
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This section describes some of the practical issues involved in using the GNU C library.
1.3.1 Header Files | How to include the header files in your programs. | |
1.3.2 Macro Definitions of Functions | Some functions in the library may really be implemented as macros. | |
1.3.3 Reserved Names | The C standard reserves some names for the library, and some for users. | |
1.3.4 Feature Test Macros | How to control what names are defined. |
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Libraries for use by C programs really consist of two parts: header files that define types and macros and declare variables and functions; and the actual library or archive that contains the definitions of the variables and functions.
(Recall that in C, a 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 definition, on the other hand, actually allocates storage for a variable or says what a function does.)
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 ‘#include’ preprocessor directive. The C language supports two forms of this directive; the first,
#include "header" |
is typically used to include a header file header that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast,
#include <file.h> |
is typically used to include a header 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, ‘#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 ‘#include’ directives immediately afterwards, following the feature test macro definition (see section Feature Test Macros).
For more information about the use of header files and ‘#include’ directives, see (cpp.info)Header Files section ‘Header Files’ in The GNU C Preprocessor Manual.
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.
Compatibility Note: Inclusion of standard header files in any order and any number of times works in any ISO C implementation. However, this has traditionally not been the case in many older C implementations.
Strictly speaking, you don’t 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.
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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:
For example, suppose the header file ‘stdlib.h’ declares a function
named abs
with
extern int abs (int); |
and also provides a macro definition for abs
. Then, in:
#include <stdlib.h> int f (int *i) { return abs (++*i); } |
the reference to 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.
#include <stdlib.h> int g (int *i) { return (abs) (++*i); } #undef abs int h (int *i) { return abs (++*i); } |
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.
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The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program 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:
exit
to do something completely different from
what the standard exit
function does, for example. Preventing
this situation helps to make your programs easier to understand and
contributes to modularity and maintainability.
In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (‘_’) 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.
float
and long double
arguments,
respectively.
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.
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The exact set of features available when you compile a source file is controlled by which feature test macros you define.
If you compile your programs using ‘gcc -ansi’, you get only the ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. See (gcc.info)Invoking GCC section ‘GNU CC Command Options’ in The GNU CC Manual, for more information about GCC options.
You should define these macros by using ‘#define’ preprocessor
directives at the top of your source code files. These directives
must come before any #include
of a system header file. It
is best to make them the very first thing in the file, preceded only by
comments. You could also use the ‘-D’ option to GCC, but it’s
better if you make the source files indicate their own meaning in a
self-contained way.
This system exists to allow the library to conform to multiple standards.
Although the different standards are often described as supersets of each
other, they are usually incompatible because larger standards require
functions with names that smaller ones reserve to the user program. This
is not mere pedantry — it has been a problem in practice. For instance,
some non-GNU programs define functions named getline
that have
nothing to do with this library’s getline
. They would not be
compilable if all features were enabled indiscriminately.
This should not be used to verify that a program conforms to a limited standard. It is insufficient for this purpose, as it will not protect you from including header files outside the standard, or relying on semantics undefined within the standard.
If you define this macro, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is available, as well as all of the ISO C facilities.
The state of _POSIX_SOURCE
is irrelevant if you define the
macro _POSIX_C_SOURCE
to a positive integer.
Define this macro to a positive integer to control which POSIX functionality is made available. The greater the value of this macro, the more functionality is made available.
If you define this macro to a value greater than or equal to 1
,
then the functionality from the 1990 edition of the POSIX.1 standard
(IEEE Standard 1003.1-1990) is made available.
If you define this macro to a value greater than or equal to 2
,
then the functionality from the 1992 edition of the POSIX.2 standard
(IEEE Standard 1003.2-1992) is made available.
If you define this macro to a value greater than or equal to 199309L
,
then the functionality from the 1993 edition of the POSIX.1b standard
(IEEE Standard 1003.1b-1993) is made available.
Greater values for _POSIX_C_SOURCE
will enable future extensions.
The POSIX standards process will define these values as necessary, and
the GNU C Library should support them some time after they become standardized.
The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that
if you define _POSIX_C_SOURCE
to a value greater than
or equal to 199506L
, then the functionality from the 1996
edition is made available.
If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ISO C, POSIX.1, and POSIX.2 material.
Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions.
Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1,
you need to use a special BSD compatibility library when linking
programs compiled for BSD compatibility. This is because some functions
must be defined in two different ways, one of them in the normal C
library, and one of them in the compatibility library. If your program
defines _BSD_SOURCE
, you must give the option ‘-lbsd-compat’
to the compiler or linker when linking the program, to tell it to find
functions in this special compatibility library before looking for them in
the normal C library.
If you define this macro, functionality derived from SVID is included as well as the ISO C, POSIX.1, POSIX.2, and X/Open material.
If you define this macro, functionality described in the X/Open
Portability Guide is included. This is a superset of the POSIX.1 and
POSIX.2 functionality and in fact _POSIX_SOURCE
and
_POSIX_C_SOURCE
are automatically defined.
As the unification of all Unices, functionality only available in BSD and SVID is also included.
If the macro _XOPEN_SOURCE_EXTENDED
is also defined, even more
functionality is available. The extra functions will make all functions
available which are necessary for the X/Open Unix brand.
If the macro _XOPEN_SOURCE
has the value 500 this includes
all functionality described so far plus some new definitions from the
Single Unix Specification, version 2.
If this macro is defined some extra functions are available which
rectify a few shortcomings in all previous standards. Specifically,
the functions fseeko
and ftello
are available. Without
these functions the difference between the ISO C interface
(fseek
, ftell
) and the low-level POSIX interface
(lseek
) would lead to problems.
This macro was introduced as part of the Large File Support extension (LFS).
If you define this macro an additional set of functions is made available which enables 32 bit systems to use files of sizes beyond the usual limit of 2GB. This interface is not available if the system does not support files that large. On systems where the natural file size limit is greater than 2GB (i.e., on 64 bit systems) the new functions are identical to the replaced functions.
The new functionality is made available by a new set of types and
functions which replace the existing ones. The names of these new objects
contain 64
to indicate the intention, e.g., off_t
vs. off64_t
and fseeko
vs. fseeko64
.
This macro was introduced as part of the Large File Support extension
(LFS). It is a transition interface for the period when 64 bit
offsets are not generally used (see _FILE_OFFSET_BITS
).
This macro determines which file system interface shall be used, one
replacing the other. Whereas _LARGEFILE64_SOURCE
makes the 64
bit interface available as an additional interface,
_FILE_OFFSET_BITS
allows the 64 bit interface to
replace the old interface.
If _FILE_OFFSET_BITS
is undefined, or if it is defined to the
value 32
, nothing changes. The 32 bit interface is used and
types like off_t
have a size of 32 bits on 32 bit
systems.
If the macro is defined to the value 64
, the large file interface
replaces the old interface. I.e., the functions are not made available
under different names (as they are with _LARGEFILE64_SOURCE
).
Instead the old function names now reference the new functions, e.g., a
call to fseeko
now indeed calls fseeko64
.
This macro should only be selected if the system provides mechanisms for
handling large files. On 64 bit systems this macro has no effect
since the *64
functions are identical to the normal functions.
This macro was introduced as part of the Large File Support extension (LFS).
Until the revised ISO C standard is widely adopted the new features
are not automatically enabled. The GNU libc nevertheless has a complete
implementation of the new standard and to enable the new features the
macro _ISOC99_SOURCE
should be defined.
If you define this macro, everything is included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions. In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence.
If you want to get the full effect of _GNU_SOURCE
but make the
BSD definitions take precedence over the POSIX definitions, use this
sequence of definitions:
#define _GNU_SOURCE #define _BSD_SOURCE #define _SVID_SOURCE |
Note that if you do this, you must link your program with the BSD compatibility library by passing the ‘-lbsd-compat’ option to the compiler or linker. NB: If you forget to do this, you may get very strange errors at run time.
If you define one of these macros, reentrant versions of several functions get declared. Some of the functions are specified in POSIX.1c but many others are only available on a few other systems or are unique to GNU libc. The problem is the delay in the standardization of the thread safe C library interface.
Unlike on some other systems, no special version of the C library must be used for linking. There is only one version but while compiling this it must have been specified to compile as thread safe.
We recommend you use _GNU_SOURCE
in new programs. If you don’t
specify the ‘-ansi’ option to GCC and don’t define any of these
macros explicitly, the effect is the same as defining
_POSIX_C_SOURCE
to 2 and _POSIX_SOURCE
,
_SVID_SOURCE
, and _BSD_SOURCE
to 1.
When you define a feature test macro to request a larger class of features,
it is harmless to define in addition a feature test macro for a subset of
those features. For example, if you define _POSIX_C_SOURCE
, then
defining _POSIX_SOURCE
as well has no effect. Likewise, if you
define _GNU_SOURCE
, then defining either _POSIX_SOURCE
or
_POSIX_C_SOURCE
or _SVID_SOURCE
as well has no effect.
Note, however, that the features of _BSD_SOURCE
are not a subset of
any of the other feature test macros supported. This is because it defines
BSD features that take precedence over the POSIX features that are
requested by the other macros. For this reason, defining
_BSD_SOURCE
in addition to the other feature test macros does have
an effect: it causes the BSD features to take priority over the conflicting
POSIX features.
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Here is an overview of the contents of the remaining chapters of this manual.
sizeof
operator and the symbolic constant 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.
isspace
) and functions for
performing case conversion.
FILE *
objects). These are the normal C library functions
from ‘stdio.h’.
char
data type.
setjmp
and
longjmp
functions. These functions provide a facility for
goto
-like jumps which can jump from one function to another.
If you already know the name of the facility you are interested in, you can look it up in Summary of Library Facilities. 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.
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