Table Of Content
Some compilers warn if an object with enumerated type is assigned a value that is not one of its constants. However, such an object can be assigned any values in the range of their compatible type, and enum constants can be used anywhere an integer is expected. For this reason, enum values are often used in place of preprocessor #define directives to create named constants.
Standardization
A standards-compliant C program written with portability in mind can be compiled for a wide variety of computer platforms and operating systems with few changes to its source code. C supports the use of pointers, a type of reference that records the address or location of an object or function in memory. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to function.
The C Programming Language
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. The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such as embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and time values.
Character set
The int type specifiers which are commented out could be omitted in K&R C, but are required in later standards. To make sure we’re on the same page, we start with a recap of pointer syntax, how to assign and use them, and how not to use them. Expressions before a sequence point are always evaluated before those after a sequence point. In the case of short-circuit evaluation, the second expression may not be evaluated depending on the result of the first expression. For example, in the expression (a() || b()), if the first argument evaluates to nonzero (true), the result of the entire expression cannot be anything else than true, so b() is not evaluated. Similarly, in the expression (a() && b()), if the first argument evaluates to zero (false), the result of the entire expression cannot be anything else than false, so b() is not evaluated.
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The influence of The C Programming Language on programmers, a generation of whom first worked with C in universities and industry, has led many to accept the authors' programming style and conventions as recommended practice, if not normative practice. For example, the coding and formatting style of the programs presented in both editions of the book is often referred to as "K&R style" or the "One True Brace Style" and became the coding style used by convention in the source code for the Unix and Linux kernels. Appendix A, the reference manual, is not the standard, but our attempt to convey the essentials of the standard in a smaller space.
Pointers
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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. Thus members cannot be an instance of the structure or union being declared (because it is incomplete at that point) but can be pointers to the type being declared. The now generally recommended method[note 3] of supporting international characters is through UTF-8, which is stored in char arrays, and can be written directly in the source code if using a UTF-8 editor, because UTF-8 is a direct ASCII extension. 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.
So if you’ve always been a little fuzzy on pointers, read on for some real-world scenarios of where and how pointers are used. The first part starts with regular pointers, their basics and common pitfalls, and some general and microcontroller specific examples. But before you storm off to the comment section now, pointers are indeed a polarizing subject and are both C’s biggest strength, and its major source of problems.
The C standard library in other languages
The members of a structure are stored in consecutive locations in memory, although the compiler is allowed to insert padding between or after members (but not before the first member) for efficiency or as padding required for proper alignment by the target architecture. The size of a structure is equal to the sum of the sizes of its members, plus the size of the padding. The application programming interface (API) of the C standard library is declared in a number of header files. Each header file contains one or more function declarations, data type definitions, and macros.
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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. 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. The original C standard specified only minimal functions for operating with wide character strings; in 1995 the standard was modified to include much more extensive support, comparable to that for char strings. The relevant functions are mostly named after their char equivalents, with the addition of a "w" or the replacement of "str" with "wcs"; they are specified in , with containing wide-character classification and mapping functions.
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This syntax produces an array whose size is fixed until the end of the block. The largest allowed array subscript is therefore equal to the number of elements in the array minus 1. To illustrate this, consider an array a declared as having 10 elements; the first element would be a[0] and the last element would be a[9]. Contemporary C compilers include checks which may generate warnings to help identify many potential bugs.
Newline indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it as one. ANSI, like other national standards bodies, no longer develops the C standard independently, but defers to the international C standard, maintained by the working group ISO/IEC JTC1/SC22/WG14. National adoption of an update to the international standard typically occurs within a year of ISO publication.
Because they are typically unchecked, a pointer variable can be made to point to any arbitrary location, which can cause undesirable effects. In general, C is permissive in allowing manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these problems by using more restrictive reference types. Unix-like systems typically have a C library in shared 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 the POSIX 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 also de facto part of the operating system interface.
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