Table Of Content
The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted.
Structures and unions
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Semicolons terminate statements, while curly braces are used to group statements into blocks. An aspect of the C standard (not unique to C) is that the behavior of certain code is said to be "undefined". In practice, this means that the program produced from this code can do anything, from working as the programmer intended, to crashing every time it is run. The first two definitions are equivalent (and both are compatible with C++). It is probably up to individual preference which one is used (the current C standard contains two examples of main() and two of main(void), but the draft C++ standard uses main()).
Pointers
The latest C standard (C11) allows multi-national Unicode characters to be embedded portably within C source text by using \uXXXX or \UXXXXXXXX encoding (where the X denotes a hexadecimal character), although this feature is not yet widely implemented. A structure can also be assigned as a unit to another structure of the same type. Structures (and pointers to structures) may also be used as function parameter and return types. Members of structures and unions cannot have an incomplete or function type.
C (programming language)
These functions are detailed in various standards such as POSIX and the Single UNIX Specification. The basic C execution character set contains the same characters, along with representations for alert, backspace, and carriage return. Run-time support for extended character sets has increased with each revision of the C standard. In early versions of C, only functions that return types other than int must be declared if used before the function definition; functions used without prior declaration were presumed to return type int. Some implementations are not hosted, usually because they are not intended to be used with an operating system. A free-standing implementation is free to specify how it handles program startup; in particular it need not require a program to define a main function.
Rationale for use in systems programming
It was created in the 1970s by Dennis Ritchie, and remains very widely used and influential. By design, C's features cleanly reflect the capabilities of the targeted CPUs. Since C99, the programmer can specify that a function takes an array of a certain size by using the keyword static. In void setArray(int array[static 4], int index, int value) the first parameter must be a pointer to the first element of an array of length at least 4. It is also possible to add qualifiers (const, volatile and restrict) to the pointer type that the array is converted to by putting them between the brackets.
Unless otherwise specified, static objects contain zero or null pointer values upon program startup. If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn about this problem, but both false positives and false negatives can occur. C does not have a special provision for declaring multi-dimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multi-dimensional array" can be thought of as increasing in row-major order. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices.
In this call, the printf function is passed (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The return value of the printf function is of type int, but it is silently discarded since it is not used. (A more careful program might test the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.
Such a type may not be instantiated (its size is not known), nor may its members be accessed (they, too, are unknown); however, the derived pointer type may be used (but not dereferenced). Objects with automatic storage are local to the block in which they were declared and are discarded when the block is exited. Additionally, objects declared with the register storage class may be given higher priority by the compiler for access to registers; although the compiler may choose not to actually store any of them in a register. Objects with this storage class may not be used with the address-of (&) unary operator. In this way, the same object can be accessed by a function across multiple calls.
Reserved keywords
The type qualifier volatile indicates to an optimizing compiler that it may not remove apparently redundant reads or writes, as the value may change even if it was not modified by any expression or statement, or multiple writes may be necessary, such as for memory-mapped I/O. In some cases, we are only interested in the memory address itself, and we might just want to pass that address around, for example to a function that doesn’t care how the data itself is arranged. If we use the void pointer, we can simply assign to and from it without any explicit type cast necessary, which can help us keep the code a bit cleaner.
Interfaces for the latter are usually declared by including header files—with the #include preprocessing directive—and the library objects are linked into the final executable image. Certain library functions, such as printf, are defined by the C standard; these are referred to as the standard library functions. C supports the use of pointers, a type of reference that records the address or location of an object or function in memory.
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 argument. Often, this POSIX-specified functionality will be regarded as part of the library; the basic C library may be identified as the ANSI or ISO C library. C enables programmers to create efficient implementations of algorithms and data structures, because the layer of abstraction from hardware is thin, and its overhead is low, an important criterion for computationally intensive programs. For example, the GNU Multiple Precision Arithmetic Library, the GNU Scientific Library, Mathematica, and MATLAB are completely or partially written in C.
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 in Linux). The C standard library provides macros, type definitions and functions for tasks such as string handling, mathematical computations, input/output processing, memory management, and several other operating system services. There are also compilers, libraries, and operating system level mechanisms for performing actions that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflow, serialization, dynamic memory tracking, and automatic garbage collection. Additional multi-byte encoded characters may be used in string literals, but they are not entirely portable.
Structures, unions and arrays can be initialized in their declarations using an initializer list. Unless designators are used, the components of an initializer correspond with the elements in the order they are defined and stored, thus all preceding values must be provided before any particular element's value. As a special exception to the usual C syntax rules, it is implementation-defined whether a bit field declared as type int, without specifying signed or unsigned, is signed or unsigned. Thus, it is recommended to explicitly specify signed or unsigned on all structure members for portability. Like string literals, character constants can also be modified by prefixes, for example L'A' has type wchar_t and represents the character value of "A" in the wide character encoding. A character constant cannot be empty (i.e. '' is invalid syntax), although a string may be (it still has the null terminating character).
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