| C standard library (libc) |
|---|
| General topics |
| Miscellaneous headers |
TheC standard library, sometimes referred to aslibc,[1] is thestandard library for theC programming language, as specified in theISO C standard.[2] Starting from the originalANSI C standard, it was developed at the same time as theC POSIX library, which is a superset of it.[3] Since ANSI C was adopted by theInternational Organization for Standardization,[4] the C standard library is also called theISO C library.[5]
The C standard library providesmacros,type definitions andfunctions for tasks such asstring manipulation, mathematical computation, input/output processing,memory management, andinput/output.
Theapplication programming interface (API) of the C standard library is declared in a number ofheader files. Each header file contains one or more function declarations, data type definitions, and macros.
After a long period of stability, three new header files (iso646.h,wchar.h, andwctype.h) were added withNormative Addendum 1 (NA1), an addition to the C Standard ratified in 1995. Six more header files (complex.h,fenv.h,inttypes.h,stdbool.h,stdint.h, andtgmath.h) were added withC99, a revision to the C Standard published in 1999, five more files (stdalign.h,stdatomic.h,stdnoreturn.h,threads.h, anduchar.h) withC11 in 2011 and one more file (stdbit.h) withC23 in 2023. In total, there are now 30 header files:
| Name | From | Description |
|---|---|---|
<assert.h> | Declares theassert macro, used to assist with detecting logical errors and other types of bugs whiledebugging a program. | |
<complex.h> | C99 | Defines aset of functions for manipulatingcomplex numbers. |
<ctype.h> | Definesset of functions used to classify characters by their types or to convert between upper and lower case in a way that is independent of the usedcharacter set (typicallyASCII or one of its extensions, although implementations utilizingEBCDIC are also known). | |
<errno.h> | For testing error codes reported by library functions. | |
<fenv.h> | C99 | Defines aset of functions for controllingfloating-point environment. |
<float.h> | Definesmacro constants specifying the implementation-specific properties of thefloating-point library. | |
<inttypes.h> | C99 | Definesexact-width integer types. |
<iso646.h> | NA1 | Definesseveral macros that implement alternative ways to express several standard tokens. For programming inISO 646 variant character sets. |
<limits.h> | Definesmacro constants specifying the implementation-specific properties of the integer types. | |
<locale.h> | Defineslocalization functions. | |
<math.h> | Definescommon mathematical functions. | |
<setjmp.h> | Declares the macrossetjmp andlongjmp, which are used for non-local exits. | |
<signal.h> | Definessignal-handling functions. | |
<stdalign.h> | C11 | For querying and specifying thealignment of objects. |
<stdarg.h> | For accessing a varying number of arguments passed to functions. | |
<stdatomic.h> | C11 | Foratomic operations on data shared between threads. |
<stdbit.h> | C23 | For byte ordening and bit representation. |
<stdbool.h> | C99 | Definesa Boolean data type. |
<stddef.h> | Definesseveral useful types and macros. | |
<stdint.h> | C99 | Definesexact-width integer types. |
<stdio.h> | Definescore input and output functions | |
<stdlib.h> | Definesnumeric conversion functions,pseudo-random numbers generation functions,memory allocation,process control functions | |
<stdnoreturn.h> | C11 | For specifying non-returning functions |
<string.h> | Definesstring-handling functions | |
<tgmath.h> | C99 | Definestype-generic mathematical functions. |
<threads.h> | C11 | Defines functions for managing multiplethreads,mutexes andcondition variables |
<time.h> | Definesdate- and time-handling functions | |
<uchar.h> | C11 | Types and functions for manipulatingUnicode characters |
<wchar.h> | NA1 | Defineswide-string-handling functions |
<wctype.h> | NA1 | Definesset of functions used to classify wide characters by their types or to convert between upper and lower case |
Three of the header files (complex.h,stdatomic.h, andthreads.h) are conditional features that implementations are not required to support.
ThePOSIX standard added several nonstandard C headers for Unix-specific functionality. Many have found their way to other architectures. Examples includefcntl.h andunistd.h. A number of other groups are using other nonstandard headers – theGNU C Library hasalloca.h, andOpenVMS has theva_count() function.
On Unix-like systems, the authoritative documentation of the API is provided in the form ofman pages. On most systems, man pages on standard library functions are in section 3; section 7 may contain some more generic pages on underlying concepts (e.g.man 7 math_error inLinux).
Unix-like systems typically have a C library inshared library form, but the header files (and compiler toolchain) may be absent from an installation so C development may not be possible. The C library is considered part of the operating system on Unix-like systems; in addition to functions specified by the C standard, it includes other functions that are part of the operating system API, such as functions specified in thePOSIX standard. The C library functions, including the ISO C standard ones, are widely used by programs, and are regarded as if they were not only an implementation of something in the C language, but alsode facto part of the operating system interface. Unix-like operating systems generally cannot function if the C library is erased. This is true for applications which are dynamically as opposed to statically linked. Further, the kernel itself (at least in the case of Linux) operates independently of any libraries.
On Microsoft Windows, the core system dynamic libraries (DLLs) provide an implementation of the C standard library for theMicrosoft Visual C++ compiler v6.0; the C standard library for newer versions of the Microsoft Visual C++ compiler is provided by each compiler individually, as well asredistributable packages. Compiled applications written in C are either statically linked with a C library, or linked to a dynamic version of the library that is shipped with these applications, rather than relied upon to be present on the targeted systems. Functions in a compiler's C library are not regarded as interfaces to Microsoft Windows.
Many C library implementations exist, provided with both various operating systems and C compilers. Some of the popular implementations are the following:
Some compilers (for example,GCC[8]) 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 compiledobject file, and the program calls the built-in versions instead of the functions in the C libraryshared object file. This reduces function-call overhead, especially if function calls are replaced withinline variants, and allows other forms ofoptimization (as the compiler knows thecontrol-flow characteristics of the built-in variants), but may cause confusion when debugging (for example, the built-in versions cannot be replaced withinstrumented variants).
However, the built-in functions must behave like ordinary functions in accordance with ISO C. The main implication is that the program must be able to create a pointer to these functions by taking their address, and invoke the function by means of that pointer. If two pointers to the same function are derived in two different translation units in the program, these two pointers must compare equal; that is, the address comes by resolving the name of the function, which has external (program-wide) linkage.
Under FreeBSD[9] and glibc,[10] some functions such as sin() are not linked in by default and are instead bundled in the mathematical librarylibm. If any of them are used, the linker must be given the directive-lm. POSIX requires that the c99 compiler supports-lm, and that the functions declared in the headersmath.h,complex.h, andfenv.h are available for linking if-lm is specified, but does not specify if the functions are linked by default.[11] musl satisfies this requirement by putting everything into a single libc library and providing an empty libm.[12]
According to the C standard the macro__STDC_HOSTED__ shall be defined to1 if the implementation is hosted. A hosted implementation has all the headers specified by the C standard. An implementation can also befreestanding which means that these headers will not be present. If an implementation isfreestanding, it shall define__STDC_HOSTED__ to0.
Some functions in the C standard library have been notorious for havingbuffer overflow vulnerabilities and generally encouraging buggy programming ever since their adoption.[13][a] The most criticized items are:
strcpy() andstrcat(), for lack ofbounds checking and possible buffer overflows if the bounds are not checked manually;printf() family of routines, for spoiling theexecution stack when the format string does not match the arguments given. This fundamental flaw created an entire class of attacks:format string attacks;gets() andscanf() family of I/O routines, for lack of (either any or easy) input length checking.Except the extreme case withgets(), all the security vulnerabilities can be avoided by introducing auxiliary code to perform memory management, bounds checking, input checking, etc. This is often done in the form of wrappers that make standard library functions safer and easier to use. This dates back to as early asThe Practice of Programming book by B. Kernighan and R. Pike where the authors commonly use wrappers that print error messages and quit the program if an error occurs.
The ISO C committee published Technical reports TR 24731-1[14] and is working on TR 24731-2[15] to propose adoption of some functions with bounds checking and automatic buffer allocation, correspondingly. The former has met severe criticism with some praise,[16][17] and the latter saw mixed response.
Despite concerns, TR 24731-1 was integrated into the C standards track in ISO/IEC 9899:2011 (C11), Annex K (Bounds-checking interfaces), and implemented approximately in Microsoft’s C/++ runtime (CRT) library for the Win32 and Win64 platforms.
(By default, Microsoft Visual Studio’s C and C++ compilers issue warnings when using older, "insecure" functions. However, Microsoft’s implementation of TR 24731-1 is subtly incompatible with both TR 24731-1 and Annex K,[18] so it’s common for portable projects to disable or ignore these warnings. They can be disabled directly by issuing
#pragma warning(disable : 4996)before/around the call site[s] in question, or indirectly by issuing
#define _CRT_SECURE_NO_WARNINGS 1before including any headers.[19] Command-line option/D_CRT_NO_SECURE_WARNINGS=1 should have the same effect as this#define.)
Thestrerror() routine is criticized for beingthread unsafe and otherwise vulnerable torace conditions.
The error handling of the functions in the C standard library is not consistent and sometimes confusing. According to the Linux manual pagemath_error, "The current (version 2.8) situation under glibc is messy. Most (but not all) functions raise exceptions on errors. Some also seterrno. A few functions seterrno, but do not raise an exception. A very few functions do neither."[20]
The originalC language provided no built-in functions such as I/O operations, unlike traditional languages such asCOBOL andFortran.[citation needed] Over time, user communities of C shared ideas and implementations of what is now called C standard libraries. Many of these ideas were incorporated eventually into the definition of the standardized C language.
BothUnix and C were created atAT&T's Bell Laboratories in the late 1960s and early 1970s. During the 1970s the C language became increasingly popular. Many universities and organizations began creating their own variants of the language for their own projects. By the beginning of the 1980s compatibility problems between the various C implementations became apparent. In 1983 theAmerican 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 ofsoftware libraries called the ANSI C standard library.
POSIX, as well asSUS, specify a number of routines that should be available over and above those in the basic C standard library. The POSIX specification includes header files for, among other uses,multi-threading,networking, andregular expressions. These are often implemented alongside the C standard library functionality, with varying degrees of closeness. For example,glibc implements functions such asfork withinlibc.so, but beforeNPTL was merged into glibc it constituted a separate library with its own linker flag argument. Often, this POSIX-specified functionality will be regarded as part of the library; the basic C library may be identified as the ANSI orISO C library.
BSD libc is a superset of the POSIX standard library supported by the C libraries included withBSDoperating systems such asFreeBSD,NetBSD,OpenBSD andmacOS. BSD libc has some extensions that are not defined in the original standard, many of which first appeared in 1994's4.4BSD release (the first to be largely developed after the first standard was issued in 1989). Some of the extensions of BSD libc are:
sys/tree.h – contains an implementation ofred–black tree andsplay tree[21][22]sys/queue.h – implementations ofLinked list,queues,tail queue, etc.[23][24]fgetln() – defined instdio.h. This can be used to read a file line by line.[25][26][27]fts.h – contains some functions to traverse a file hierarchy[28][29]db.h – some functions to connect to theBerkeley DB[30][31]strlcat() andstrlcpy() – secure alternatives forstrncat() andstrncpy()[32][33][34][35][36]err.h – contains some functions to print formatted error messages[37][38]vis.h – contains thevis() function. This function is used to display non-printable characters in a visual format.[39][40][41]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 theoperational semantics are kept similar.
TheC++ language incorporates the majority of the C standard library’s constructs into its own, excluding C-specific machinery. C standard library functions are exported from the C++ standard library in two ways.
For backwards-/cross-compatibility to C and pre-Standard C++, functions can be accessed in the globalnamespace (::), after#includeing the C standard header name as in C.[42] Thus, the C++98 program
#include<stdio.h>intmain(){return::puts("Hello, world!")==EOF;}
should exhibit (apparently-)identical behavior toC95 program
#include<stdio.h>intmain(void){returnputs("Hello, world!")==EOF;}
FromC++98 on, C functions are also made available in namespace::std (e.g., C printf as C++ ::std::printf,atoi as::std::atoi,feof as::std::feof), by including header<chdrname> instead of corresponding C header<hdrname.h>. E.g.,<cstdio> substitutes for<stdio.h> and<cmath> for<math.h>; note lack of.h extension on C++ header names.
Thus, an equivalent (generally preferable) C++≥98 program to the above two is:
#include<cstdio>intmain(){returnstd::puts("Hello, world")==EOF;}
Ausingnamespace::std declaration above or withinmain can be issued to apply the::std:: prefix automatically, although it’s generally considered poor practice to use it globally in headers because it pollutes the global namespace.[43]
A few of the C++≥98 versions of C’s headers are missing; e.g., C≥11<stdnoreturn.h> and<threads.h> have no C++ counterparts.[44]
Others are reduced to placeholders, such as (untilC++20)<ciso646> for C95<iso646.h>, all of whose requisite macros are rendered as keywords in C++98. C-specific syntactic constructs aren’t generally supported, even if their header is.[45]
Several C headers exist primarily for C++ compatibility, and these tend to be near-empty in C++. For example,C99–17<stdbool.h> require only
#define bool _Bool#define false 0#define true 1#define __bool_true_false_are_defined 1
in order to feign support for the C++98bool,false, andtrue keywords in C.C++11 requires<stdbool.h> and<cstdbool> for compatibility, but all they need to define is__bool_true_false_are_defined.C23 obsoletes older_Bool keyword in favor of new, C++98-equivalentbool,false, andtrue keywords, so the C≥23 and C++≥11<stdbool.h>/<cstdbool> headers are fully equivalent. (In particular, C23 doesn’t require any__STDC_VERSION_BOOL_H__ macro for<stdbool.h>.)
Access to C library functions via namespace::std and the C++≥98 header names is preferred where possible. To encourage adoption,C++98 obsoletes the C (*.h) header names, so it’s possible that use of C compatibility headers will cause an especially strict C++98–20 preprocessor to raise a diagnostic of some sort. However,C++23 (unusually)de-obsoletes these headers, so newer C++ implementations/modes shouldn’t complain without being asked to specifically.[46]
Other languages take a similar approach, placing C compatibility functions/routines under a common namespace; these includeD,Perl, andRuby.
CPython includes wrappers for some of the C library functions in its own common library, and it also grants more direct access to C functions and variables via itsctypes package.[47]
More generally,Python 2.x specifies the built-in file objects as being “implemented using C'sstdio package,"[48] and frequent reference is made to C standard library behaviors; the available operations (open,read,write , etc.) are expected to have the same behavior as the corresponding C functions (fopen,fread,fwrite, etc.).
Python 3’s specification relies considerably less on C specifics thanPython 2, however.
Rust offers cratelibc, which allows various C standard (and other) library functions and type definitions to be used.[49]
The C standard library is small compared to the standard libraries of some other languages. The C 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 theC++Standard Template Library, let alone the completegraphical user interface (GUI) toolkits, networking tools, and profusion of other functionality thatJava and the.NET Framework provide as standard. The main advantage of the small standard library is that providing a working ISO C environment is much easier than it is with other languages, and consequently porting C to a new platform is comparatively easy.
gets() have been created as early as in 1988.A singleusing namespace std;statement in a single header file in a complex project can make a mess out of the namespace management for the whole project.So, no top level [using namespace] statements in a header file!
File objects are implemented using C'sstdio package and can be created with the built-inopen() function.| Features | |||||
|---|---|---|---|---|---|
| Standard library |
| ||||
| Compilers | |||||
| IDEs | |||||
| Comparison with other languages | |||||
| Descendant languages | |||||
| Designer | |||||
