C standard library

The C standard library (also known as libc) is a now-standardized collection of header files and library routines used to implement common operations, such as input/output and string handling, in the C programming language. Unlike other languages such as COBOL, Fortran, and PL/I, C does not include builtin keywords for these tasks, so nearly all C programs rely on the standard library to function.

[edit] Design

The name and characteristic of each function are in a computer file called a header file but the actual implementation of functions are separated into a library file. The naming and scope of headers have become common but the organization of libraries still remains diverse. The standard library is usually shipped along with a compiler. Since C compilers often provide extra functionalities that are not specified in ANSI C, a standard library with a particular compiler is mostly incompatible with standard libraries of other compilers.

[edit] Design considerations

Much of the C standard library has been shown to have been well-designed. A few parts, with the benefit of hindsight, are regarded as mistakes. The string input functions gets() (and the use of scanf() to read string input) are the source of many buffer overflows, and most programming guides recommend avoiding this usage. strcpy() also infamously shares this reputation. Another oddity is strtok(), a function that is designed as a primitive lexical analyser but is highly "fragile" and difficult to use.

The choice of using a size_t type instead of an int for the number of elements specified for fread() and fwrite() appears inconsistent with the generally assumed semantics for size_t (to represent byte quantities).

stdio is limited (too high level for use in many situations) and the standard does not allow the user to override or extend its internals portably. As a result many applications develop their own wrapper libraries around lower level, OS-provided functions such as the POSIX ones. For instance, stdio does not mix well with signals or asynchroneous, non-blocking I/O modes, which are widely used in networking servers. As a result only servers using a full-process per client model can reliably depend on stdio functions to serve them on a POSIX system, in blocking I/O mode.

Certain standard library functions have to be avoided in multithreaded applications. Thread control primitives are intended to remain part of the OS and despite common standards like the POSIX threads library, care is expected of C programmers to deal with reentrancy and synchronization. Neither the C language or its standard library are expected to be concerned about these system-specific issues.

[edit] History

The C programming language, before it was standardized, did not provide built-in functionalities such as I/O operations (unlike traditional languages such as Cobol and Fortran). Over time, user communities of C shared ideas and implementations of what is now called C standard libraries to provide that functionality. Many of these ideas were incorporated eventually into the definition of the standardized C programming language.

Both Unix and C were created at AT&T's Bell Laboratories in the late 1960s and early 1970s. During the 1970s the C programming language became increasingly popular. Many universities and organizations began creating their own variations of the language for their own projects. By the beginning of the 1980s compatibility problems between the various C implementations became apparent. In 1983 the American National Standards Institute (ANSI) formed a committee to establish a standard specification of C known as "ANSI C". This work culminated in the creation of the so-called C89 standard in 1989. Part of the resulting standard was a set of software libraries called the ANSI C standard library.

Later revisions of the C standard have added several new required header files to the library. Support for these new extensions varies between implementations.

The headers <iso646.h>, <wchar.h>, and <wctype.h> were added with Normative Addendum 1 (hereafter abbreviated as NA1), an addition to the C Standard ratified in 1995.

The headers <complex.h>, <fenv.h>, <inttypes.h>, <stdbool.h>, <stdint.h>, and <tgmath.h> were added with C99, a revision to the C Standard published in 1999.

[edit] ANSI Standard

The ANSI C standard library consists of 24 C header files which can be included into a programmer's project with a single directive. Each header file contains one or more function declarations, data type definitions and macros. The contents of these header files follows.

In comparison to some other languages (for example Java) the standard library is minuscule. The library provides a basic set of mathematical functions, string manipulation, type conversions, and file and console-based I/O. It does not include a standard set of "container types" like the C++ Standard Template Library, let alone the complete graphical user interface (GUI) toolkits, networking tools, and profusion of other functionality that Java provides as standard. The main advantage of the small standard library is that providing a working ANSI C environment is much easier than it is with other languages, and consequently porting C to a new platform is relatively easy.

Many other libraries have been developed to supply equivalent functionality to that provided by other languages in their standard library. For instance, the GNOME desktop environment project has developed the GTK+ graphics toolkit and GLib, a library of container data structures, and there are many other well-known examples. The variety of libraries available has meant that some superior toolkits have proven themselves through history. The considerable downside is that they often do not work particularly well together, programmers are often familiar with different sets of libraries, and a different set of them may be available on any particular platform.

[edit] ANSI C library header files

[edit] The C standard library in other languages

Some languages include the functionality of the standard C library in their own libraries. The library may be adapted to better suit the language's structure, but the operation semantics are kept similar. The C++ programming language, for example, includes the functionality of the ANSI C standard library in the namespace std (like std::printf, std::atoi, std::feof, etc.), in header files with similar names to the C ones ("cstdio", "cmath", "cstdlib", etc). Other languages that take similar approaches are D and Python. In the latter, for example, the built-in file object is defined as "implemented using C's stdio package"[1], so that the available operations (open, read, write, etc.) are expected to have the same behavior as the corresponding C functions.

[edit] Common support libraries

While not standardized, C programs may depend on a runtime library of routines which contain code the compiler uses at runtime. The code that initializes the process for the operating system, for example, before calling main(), is implemented in the C Run-Time Library for a given vendor's compiler. The Run-Time Library code might help with other language feature implementations, like handling uncaught exceptions or implementing floating point code.

The C standard library only documents that the specific routines mentioned in this article are available, and how they behave. Because the compiler implementation might depend on these additional implementation-level functions to be available, it is likely the vendor-specific routines are packaged with the C Standard Library in the same module, because they're both likely to be needed by any program built with their toolset.

Though often confused with the C Standard Library because of this packaging, the C Runtime Library is not a standardized part of the language and is vendor-specific.

[edit] Compiler built-in functions

Some compilers (for example, GCC^[1]) provide built-in versions of many of the functions in the C standard library; that is, the implementations of the functions are written into the compiled object file, and the program calls the built-in versions instead of the functions in the C library shared object file. This reduces function call overhead, especially if function calls are replaced with inline variants, and allows other forms of optimisation (as the compiler knows the control-flow characteristics of the built-in variants), but may cause confusion when debugging (for example, the built-in versions cannot be replaced with instrumented variants).

[edit] POSIX standard library

Main article: C POSIX library

POSIX (and SUS) specifies a number of routines that should be available over and above those in the C standard library proper; these are often implemented alongside the C standard library functionality, with varying degrees of closeness. For example, glibc implements functions such as fork within libc.so, but before NPTL was merged into glibc it constituted a separate library with its own linker flag. Often, this POSIX-specified functionality will be regarded as part of the library; the C library proper may be identified as the ANSI or ISO C library.

[edit] Implementations

Many implementations exist, provided with both various operating systems and C compilers. On BSD systems, for instance, the system library is embedded with the operating system and is maintained in the common source repository. On most systems the library can be found under the name "libc".

Although there exist too many implementations to list, some popular implementations follow:

• BSD libc
• GNU C Library, the most common implementation used with Linux
• Microsoft C Run-time Library
• dietlibc, an alternative small implementation of the C standard library
• uClibc, a C standard library for embedded Linux systems (MMU-less)
• Newlib, a C standard library for embedded systems (MMU-less)^[2]
• klibc, primarily for booting Linux systems.
• EGLIBC, variant of glibc for embedded systems.

[edit] See also

• List of C functions
• C++ standard library

[edit] References

[edit] External links

• The C Standard Library: A detailed description of the header files
• The C Library Reference Guide
• Microsoft C Run-Time Libraries on MSDN
• NetBSD C libraries manual and full C library source: A good academic reference.