C syntax

The syntax of the C programming language is a set of rules that specifies whether the sequence of characters in a file is conforming C source code. The rules specify how the character sequences are to be chunked into tokens (the lexical grammar), the permissible sequences of these tokens and some of the meaning to be attributed to these permissible token sequences (additional meaning is assigned by the semantics of the language).

C syntax makes use of the maximal munch principle.

Data structures

Primitive data types

The C language represents numbers in three forms: "integral", "real" and "complex". This distinction reflects similar distinctions in the instruction set architecture of most central processing units. "Integral" data types store numbers in the set of integers, while "real" and "complex" numbers represent numbers (or pair of numbers) in the set of real numbers in floating point form.

All C integer types have signed and unsigned variants. If signed or unsigned is not specified explicitly, in most circumstances signed is assumed. However, for historic reasons plain char is a type distinct from both signed char and unsigned char. It may be a signed type or an unsigned type, depending on the compiler and the character set (C guarantees that members of the C basic character set have positive values). Also, bit field types specified as plain int may be signed or unsigned, depending on the compiler.

Integral types

The integral types come in different sizes, with varying amounts of memory usage and range of representable numbers. Modifiers are used to designate the size: short, long and long longThe long long modifier was introduced in the C99 standard; some compilers had already supported it.] . The character type, whose specifier is char, represents the smallest addressable storage unit, which is most often an 8-bit byte (its size must be at least 7-bit to store the basic character set, or larger) The standard header limits.h defines the minimum and maximum values of the integral primitive data types, amongst other limits.

The following table provides a list of the integral types and their "typical" storage sizes and acceptable ranges of values, which may vary from one compiler and platform to another. ISO C defines different limits in section 5.2.4.2.1 of the standard. For integral types of "guaranteed" sizes, the standard provides the stdint.h header.

The use of other backslash escapes is not defined by the C standard.

tring literal concatenation

Adjacent string literals are concatenated at compile time; this allows long strings to be split over multiple lines, and also allows string literals resulting from C preprocessor defines and macros to be appended to strings at compile time: printf(__FILE__ ": %d: Hello " "world ", __LINE__);will expand to printf("helloworld.c" ": %d: Hello " "world ", 10);which is syntactically equivalent to printf("helloworld.c: %d: Hello world ", 10);

Character constants

Individual character constants are represented by single-quotes, e.g. 'A', and have type int (not char). The difference is that "A" represents a pointer to the first element of a null-terminated array, whereas 'A' directly represents the code value (65 if ASCII is used). The same backslash-escapes are supported as for strings, except that (of course) " can validly be used as a character without being escaped, whereas ' must now be escaped. A character constant cannot be empty (i.e. " is invalid syntax), although a string may be (it still has the null terminating character). Multi-character constants (e.g. 'xy') are valid, although rarely useful — they let one store several characters in an integer (e.g. 4 ASCII characters can fit in a 32-bit integer, 8 in a 64-bit one). Since the order in which the characters are packed into one int is not specified, portable use of multi-character constants is difficult.

Wide character strings

Since type char is usually 1 byte wide, a single char value typically can represent at most 255 distinct character codes, not nearly enough for all the characters in use worldwide. To provide better support for international characters, the first C standard (C89) introduced wide characters (encoded in type wchar_t) and wide character strings, which are written as L"Hello world!"

Wide characters are most commonly either 2 bytes (using a 2-byte encoding such as UTF-16) or 4 bytes (usually UTF-32), but Standard C does not specify the width for wchar_t, leaving the choice to the implementor. Microsoft Windows generally uses UTF-16, thus the above string would be 26 bytes long for a Microsoft compiler; the Unix world prefers UTF-32, thus compilers such as GCC would generate a 52-byte string. A 2-byte wide wchar_t suffers the same limitation as char, in that certain characters (those outside the BMP) cannot be represented in a single wchar_t; but must be represented using surrogate pairs.

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 <wchar.h>, with <wctype.h> containing wide-character classification and mapping functions.

Variable width strings

A common alternative to wchar_t is to use a variable-width encoding, whereby a logical character may extend over multiple positions of the string. Variable-width strings may be encoded into literals verbatim, at the risk of confusing the compiler, or using numerical backslash escapes (e.g. "xc3xa9" for "é" in UTF-8). The UTF-8 encoding was specifically designed (under Plan 9) for compatibility with the standard library string functions; supporting features of the encoding include a lack of embedded nulls, no valid interpretations for subsequences, and trivial resynchronisation. Encodings lacking these features are likely to prove incompatible with the standard library functions; encoding-aware string functions are often used in such case.

Library functions

strings, both constant and variable, may be manipulated without using the standard library. However, the library contains many useful functions for working with null-terminated strings. It is the programmer's responsibility to ensure that enough storage has been allocated to hold the resulting strings.

The most commonly used string functions are:

* strcat(dest, source) - appends the string source to the end of string dest
* strchr(s, c) - finds the first instance of character c in string s and returns a pointer to it or a null pointer if c is not found
* strcmp(a, b) - compares strings a and b (lexicographical ordering); returns negative if a is less than b, 0 if equal, positive if greater.
* strcpy(dest, source) - copies the string source onto the string dest
* strlen(st) - return the length of string st
* strncat(dest, source, n) - appends a maximum of n characters from the string source to the end of string dest and null terminates the string at the end of input or at index n+1 when the max length is reached
* strncmp(a, b, n) - compares a maximum of n characters from strings a and b (lexical ordering); returns negative if a is less than b, 0 if equal, positive if greater
* strrchr(s, c) - finds the last instance of character c in string s and returns a pointer to it or a null pointer if c is not found

Other standard string functions include:

* strcoll(s1, s2) - compare two strings according to a locale-specific collating sequence
* strcspn(s1, s2) - returns the index of the first character in s1 that matches any character in s2
* strerror(errno) - returns a string with an error message corresponding to the code in errno
* strncpy(dest, source, n) - copies n characters from the string source onto the string dest, substituting null bytes once past the end of source; does not null terminate if max length is reached
* strpbrk(s1, s2) - returns a pointer to the first character in s1 that matches any character in s2 or a null pointer if not found
* strspn(s1, s2) - returns the index of the first character in s1 that matches no character in s2
* strstr(st, subst) - returns a pointer to the first occurrence of the string subst in st or a null pointer if no such substring exists
* strtok(s1, s2) - returns a pointer to a token within s1 delimited by the characters in s2
* strxfrm(s1, s2, n) - transforms s2 onto s1, such that s1 used with strcmp gives the same results as s2 used with strcoll

There is a similar set of functions for handling wide character strings.

tructures and unions

tructures

Structures in C are defined as data containers consisting of a sequence of named members of various types. They are similar to records in other programming 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. The size of a structure is equal to the sum of the sizes of its members, plus the size of the padding.

Unions

Unions in C are related to structures and are defined as objects that may hold (at different times) objects of different types and sizes. They are analogous to variant records in other programming languages. Unlike structures, the components of a union all refer to the same location in memory. In this way, a union can be used at various times to hold different types of objects, without the need to create a separate object for each new type. The size of a union is equal to the size of its largest component type.

Declaration

Structures are declared with the struct keyword and unions are declared with the union keyword. The specifier keyword is followed by an optional identifier name, which is used to identify the form of the structure or union. The identifier is followed by the declaration of the structure or union's body: a list of member declarations, contained within curly braces, with each declaration terminated by a semicolon. Finally, the declaration concludes with an optional list of identifier names, which are declared as instances of the structure or union.

For example, the following statement declares a structure named s that contains three members; it will also declare an instance of the structure known as t:

struct s{ int x; float y; char *z;} t;

And the following statement will declare a similar union named u and an instance of it named n:

union u{ int x; float y; char *z;} n;

Once a structure or union body has been declared and given a name, it can be considered a new data type using the specifier struct or union, as appropriate, and the name. For example, the following statement, given the above structure declaration, declares a new instance of the structure s named r:

struct s r;

It is also common to use the typedef specifier to eliminate the need for the struct or union keyword in later references to the structure. The first identifier after the body of the structure is taken as the new name for the structure type. For example, the following statement will declare a new type known as s_type that will contain some structure:

typedef struct {…} s_type;

Future statements can then use the specifier s_type (instead of the expanded struct … specifier) to refer to the structure.

Accessing members

Members are accessed using the name of the instance of a structure or union, a period (.), and the name of the member. For example, given the declaration of t from above, the member known as y (of type float) can be accessed using the following syntax:

t.y

Structures are commonly accessed through pointers. Consider the following example that defines a pointer to t, known as ptr_to_t:

struct s *ptr_to_t = &t;

Member y of t can then be accessed by dereferencing ptr_to_t and using the result as the left operand:

(*ptr_to_t).y

Which is identical to the simpler t.y above as long as ptr_to_t points to t. Because this operation is common, C provides an abbreviated syntax for accessing a member directly from a pointer. With this syntax, the name of the instance is replaced with the name of the pointer and the period is replaced with the character sequence ->. Thus, the following method of accessing y is identical to the previous two:

ptr_to_t->y

Members of unions are accessed in the same way.

Initialization

A structure can be initialized in its declarations using an initializer list, similar to arrays. If a structure is not initialized, the values of its members are undefined until assigned. The components of the initializer list must agree, in type and number, with the components of the structure itself.

The following statement will initialize a new instance of the structure s from above known as pi:

struct s pi = { 3, 3.1415, "Pi" };

C99 introduces a more flexible initialization syntax for structures, which allows members to be initialized by name. The following initialization using this syntax is equivalent to the previous one. Initialization using this syntax may initialize members in any order:

struct s pi = { .x = 3, .y = 3.1415, .z = "Pi" };

In C89, a union can only be initialized with a value of the type of its first member. That is, the union u from above can only be initialized with a value of type int.

union u value = 3;

In C99, any one member of a union may be initialized using the new syntax described above.

union u value = { .y = 3.1415 }; // C99

Assignment

Assigning values to individual members of structures and unions is syntactically identical to assigning values to any other object. The only difference is that the "lvalue" of the assignment is the name of the member, as accessed by the syntax mentioned above.

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.

For example, the following statement assigns the value of 74 (the ASCII code point for the letter 't') to the member named x in the structure t, from above:

t.x = 74;

And the same assignment, using ptr_to_t in place of t, would look like:

ptr_to_t->x = 74;

Assignment with members of unions is identical, except that each new assignment changes the current type of the union, and the previous type and value are lost.

Other operations

According to the C standard, the only legal operations that can be performed on a structure are copying it, assigning to it as a unit (or initializing it), taking its address with the address-of (&) unary operator, and accessing its members. Unions have the same restrictions. One of the operations implicitly forbidden is comparison: structures and unions cannot be compared using C's standard comparison facilities (=, >, <, etc.).

Bit fields

C also provides a special type of structure member known as a bit field, which is an integer with an explicitly specified number of bits. A bit field is declared as a structure member of type int, signed int, unsigned int, or (in C99 only) _Bool, following the member name by a colon (:) and the number of bits it should occupy. The total number of bits in a single bit field must not exceed the total number of bits in its declared type.

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.

Empty entries consisting of just a colon followed by a number of bits are also allowed; these indicate padding.

The members of bit fields do not have addresses, and as such cannot be used with the address-of (&) unary operator. The sizeof operator may not be applied to bit fields.

The following declaration declares a new structure type known as f and an instance of it known as g. Comments provide a description of each of the members:

struct f{ unsigned int flag : 1; /* a bit flag: can either be on (1) or off (0) */ signed int num : 4; /* a signed 4-bit field; range -7...7 or -8...7 */ : 3; /* 3 bits of padding to round out 8 bits */} g;

Incomplete types

The body of a struct or union declaration, or a typedef thereof, may be omitted, yielding an "incomplete type". 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).

Incomplete types are used to implement recursive structures; the body of the type declaration may be deferred to later in the translation unit:typedef struct Bert Bert;typedef struct Wilma Wilma;

struct Bert{ Wilma *wilma;};

struct Wilma{ Bert *bert;};

Incomplete types are also used for data hiding; the incomplete type is defined in a header file, and the body only within the relevant source file.

Operators

:"Main article: Operators in C and C++"

Control structures

C is a free-form language.

Bracing style varies from programmer to programmer and can be the subject of debate ("flame wars"). See Indent style for more details.

Compound statements

In the items in this section, any can be replaced with a compound statement. In C89, compound statements in C have the form:{ }and are used as the body of a function or anywhere that a single statement is expected. The declaration-list declares variables to be used in that scope, and the statement-list are the actions to be performed. Brackets define their own scope, and variables defined inside those brackets will be automatically deallocated at the closing bracket. C99 extends this syntax to allow declarations and statements to be freely intermixed within a compound statement (as does C++).

election statements

C has two types of selection statements: the if statement and the switch statement.

The if statement is in the form:

if () else

In the if statement, if the in parentheses is nonzero (true), control passes to . If the else clause is present and the is zero (false), control will pass to . The "else part is optional, and if absent, a false will simply result in skipping over the . An else always matches the nearest previous unmatched if; braces may be used to override this when necessary, or for clarity.

The switch statement causes control to be transferred to one of several statements depending on the value of an expression, which must have integral type. The substatement controlled by a switch is typically compound. Any statement within the substatement may be labeled with one or more case labels, which consist of the keyword case followed by a constant expression and then a colon (:). The syntax is as follows:

switch (){ case : case : break; default : }

No two of the case constants associated with the same switch may have the same value. There may be at most one default label associated with a switch - if none of the case labels are equal to the expression in the parentheses following switch, control passes to the default label, or if there is no default label, execution resumes just beyond the entire construct.Switches may be nested; a case or default label is associated with the innermost switch that contains it. Switch statements can "fall through", that is, when one case section has completed its execution, statements will continue to be executed downward until a break; statement is encountered. Fall-through is useful in some circumstances, but is usually not desired.In the preceding example, if is reached, the statements are executed and nothing more inside the braces. However if is reached, both and are executed since there is no break to separate the two case statements.

Iteration statements

C has three forms of iteration statement:

do while ( ) ;

while ( )

for ( ; ; )

In the while and do statements, the substatement is executed repeatedly so long as the value of the expression remains nonzero (true). With while, the test, including all side effects from the expression, occurs before each execution of the statement; with do, the test follows each iteration. Thus, a do statement always executes its substatement at least once, whereas while may not execute the substatement at all.

If all three expressions are present in a for, the statementfor (e1; e2; e3) s;is equivalent toe1;while (e2){ s; e3;}except for the behavior of a continue; statement (which in the for loop jumps to e3 instead of e2).

Any of the three expressions in the for loop may be omitted. A missing second expression makes the while test always nonzero, creating a potentially infinite loop.

C99 generalizes the for loop by allowing the first expression to take the form of a declaration (typically including an initializer). The declaration's scope is limited to the extent of the for loop.

Jump statements

Jump statements transfer control unconditionally. There are four types of jump statements in C: goto, continue, break, and return.

The goto statement looks like this:goto ;

The identifier must be a label (followed by a colon) located in the current function. Control transfers to the labeled statement.

A continue statement may appear only within an iteration statement and causes control to pass to the loop-continuation portion of the innermost enclosing iteration statement. That is, within each of the statements

while (expression){ /* ... */ cont: ;}

do{ /* ... */ cont: ;} while (expression);

for (expr1; expr2; expr3) { /* ... */ cont: ;}

a continue not contained within a nested iteration statement is the same as goto cont.

The break statement is used to end a for loop, while loop, do loop, or switch statement. Control passes to the statement following the terminated statement.

A function returns to its caller by the return statement. When return is followed by an expression, the value is returned to the caller as the value of the function. Encountering the end of the function is equivalent to a return with no expression. In that case, if the function is declared as returning a value and the caller tries to use the returned value, the result is undefined.

toring the address of a label

GCC extends the C language with a unary && operator that returns the address of a label. This address can be stored in a void* variable type and may be used later in a goto instruction. For example, the following prints "hi " in an infinite loop:

void *ptr = &&J1; J1: printf("hi "); goto *ptr;

This feature can be used to implement a jump table.

Functions

yntax

A C function definition consists of a return type (void if no value is returned), a unique name, a list of parameters in parentheses (void if there are none), and various statements. A function with non-void return type should include at least one return statement:

functionName( ){ return ;}

where of n variables is declared as data type and variable name separated by a comma: var1, var2, ... varN

It is possible to define a function as taking a variable number of parameters by providing the ... keyword asthe last parameter instead of a data type and variable name. A commonly used function that does this is the standard library function printf, which has the declaration:int printf (const char*, ...);

Manipulation of these parameters can be done by using the routines in the standard library header .

Function Pointers

A pointer to a function can be declared as follows:

(*functionName)();

The following program shows use of a function pointer for selecting between addition and subtraction:


#include int (*operation)(int x, int y); int add(int x, int y){ return x + y;} int subtract(int x, int y) { return x - y;} int main(int argc, char* args [] ){ int foo = 1, bar = 1;

operation = add; printf("%d + %d = %d ", foo, bar, operation(foo, bar)); operation = subtract; printf("%d - %d = %d ", foo, bar, operation(foo, bar)); return 0;}

Global structure

After preprocessing, at the highest level a C program consists of a sequence of declarations at file scope. These may be partitioned into several separate source files, which may be compiled separately; the resulting object modules are then linked along with implementation-provided run-time support modules to produce an executable image.

The declarations introduce functions, variables and types. C functions are akin to the subroutines of Fortran or the procedures of Pascal.

A definition is a special type of declaration. A variable definition sets aside storage and possibly initializes it, a function definition provides its body.

An implementation of C providing all of the standard library functions is called a "hosted implementation". Programs written for hosted implementations are required to define a special function called main, which is the first function called when execution of the program begins.

Hosted implementations of C start program execution by invoking the main function, which must be defined in a fashion compatible with one of the following prototypes:

int main(void)int main(int argc, char *argv [] )

In particular, the function main must be declared as having an int return type according to the C Standard. The C standard defines return values 0 and EXIT_SUCCESS as indicating success and EXIT_FAILURE as indicating failure. (EXIT_SUCCESS and EXIT_FAILURE are defined in <stdlib.h>). Other return values have implementation defined meanings; for example, under Linux a program killed by a signal yields a return code of the numerical value of the signal plus 128.

Here is a minimal C program:

int main(void){ return 0;}

The main function will usually call other functions to help it perform its job.

Some implementations are not hosted, usually because they are not intended to be used with an operating system. Such implementations are called "free-standing" in the C standard. 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.

Functions may be written by the programmer or provided by existing libraries. 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.

A function may return a value to the environment that called it. This is usually another C function; however, the calling environment of the main function is the parent process in Unix-like systems or the operating system itself in other cases. By definition, the return value zero (or the value of the EXIT_SUCCESS macro) from main signifies successful completion of the program. (There is also an EXIT_FAILURE macro to signify failure.) The printf function mentioned above returns how many characters were printed, but this value is often ignored.

Argument passing

In C, arguments are passed to functions by value while other languages may pass variables by reference.This means that the receiving function gets copies of the values and has no direct way of altering the original variables.For a function to alter a variable passed from another function, the caller must pass its "address" (a "pointer" to it), which can then be dereferenced in the receiving function (see Pointers for more info):

void incInt(int *y){ (*y)++; // Increase the value of 'x', in main, by one} int main(void){ int x = 0; incInt(&x); // pass a reference to the var 'x' return 0;}

The function scanf works the same way:int x;scanf("%d", &x);

In order to pass an editable pointer to a function you have to pass a pointer to "that" pointer; its address:


#include
#include void setInt(int **p, int n){ *p = malloc(sizeof(int)); // allocate a memory area, saving the pointer in the // location pointed to by the parameter "p" if (*p = NULL) { perror("malloc"); exit(EXIT_FAILURE); } // dereference the given pointer that has been assigned an address // of dynamically allocated memory and set the int to the value of n (42) **p = n; } int main(void){ int *p; // create a pointer to an integer setInt(&p, 42); // pass the address of 'p' return 0;}

int **p defines a pointer to a pointer, which is the address to the pointer p in this case.

Array parameters

Function parameters of array type may at first glance appear to be an exception to C's pass-by-value rule. The following program will print 2, not 1:


#include void setArray(int array [] , int index, int value){ array [index] = value;} int main(void){ int a [1] = {1}; setArray(a, 0, 2); printf ("a [0] =%d ", a [0] ); return 0;}

However, there is a different reason for this behavior. In fact, a function parameter declared with an array type is treated almost exactly like one declared to be a pointer. That is, the preceding declaration of setArray is equivalent to the following:void setArray(int *array, int index, int value)

At the same time, C rules for the use of arrays in expressions cause the value of a in the call to setArray to be converted to a pointer to the first element of array a. Thus, in fact this is still an example of pass-by-value, with the caveat that it is the address of the first element of the array being passed by value, not the contents of the array.

Miscellaneous

Reserved keywords

The following words are reserved, and may not be used as identifiers:

:auto:_Bool:break:case:char:_Complex:const:continue:default:do:double:else:enum:extern:float:for:goto:if:_Imaginary:inline:int:long:register:restrict:return:short:signed:sizeof:static:struct:switch:typedef:union:unsigned:void:volatile:while

Implementations may reserve other keywords, such as asm, although implementations typically provide non-standard keywords that begin with one or two underscores.

Case sensitivity

C identifiers are case sensitive (e.g., foo, FOO, and Foo are the names of different objects). Some linkers may map external identifiers to a single case, although this is uncommon in most modern linkers.

Comments

Text starting with /* is treated as a comment and ignored. The comment ends at the next */; it can occur within expressions, and can span multiple lines. Accidental omission of the comment terminator is problematic in that the next comment's properly constructed comment terminator will be used to terminate the initial comment, and all code in between the comments will be considered as a comment. C-style comments do not "nest".

The C99 standard introduced C++ style line comments. These start with // and extend to the end of the line.

// this line will be ignored by the compiler (C99)

/* these lines will be ignored by the compiler */

x = *p/*q; /* note: this comment starts after the 'p' */

Command-line arguments

The parameters given on a command line are passed to a C program with two predefined variables - the count of the command-line arguments in argc and the individual arguments as character strings in the pointer array argv.So the command myFilt p1 p2 p3results in something like

(Note: While individual strings are contiguous arrays of char, there is no guarantee that the strings are stored as a contiguous group.)

The name of the program, argv [0] , may be useful when printing diagnostic messages. The individual values of the parameters may be accessed with argv [1] , argv [2] , and argv [3] , as shown in the following program:


#include int main(int argc, char *argv [] ){ int i;

printf ("argc = %i ", argc); for (i = 0; i < argc; i++) printf ("argv [%i] = %s ", i, argv [i] ); return 0;}

Evaluation order

In any reasonably complex expression, there arises a choice as to the order in which to evaluate the parts of the expression: (1+1)+(3+3) may be evaluated in the order (1+1)+(3+3) → (2)+(3+3) → (2)+(6) → 8 or in the order (1+1)+(3+3) → (1+1)+(6) → (2)+(6) → 8. Formally, a conforming C compiler may evaluate expressions in "any" order between "sequence points". Sequence points are defined by:
*"Statement ends" at semicolons.
*The "sequencing operator": a comma. However, commas that delimit function arguments are not sequence points.
*The "short-circuit operators": logical "and" (&&) and logical "or" (||).
*The "conditional operator" (?:): This operator evaluates its first sub-expression first, and then its second or third (never both of them) based on the value of the first.
*Entry to and exit from a "function call" (but not between evaluations of the arguments).

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 will also be true, so b() is not evaluated.

The arguments to a function call may be evaulated in any order, as long as they are all evaluated by the time the function call takes place. The following expression, for example, has undefined behaviour: printf("%s %s ", argv [i = 0] , argv [++i] );

Undefined behavior

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.

For example, the following code produces undefined behavior, because the variable b is modified more than once with no intervening sequence point:


#include int main(void){ int a, b = 1;

a = b++ + b++; printf("%d ", a); return 0;}

Because there is no sequence point between the modifications of b in b++ + b++, it is possible to perform the evaluation steps in more than one order, resulting in an ambiguous statement. This can be fixed by rewriting the code to insert a sequence point:a = b++;a += b++;

ee also

*C programming language
*C variable types and declarations
*Operators in C and C++

References

*cite book |last=Kernighan |first=Brian W. |authorlink= |coauthors=Ritchie, Dennis M. |title=The C Programming Language |edition=2nd Edition |year=1988 |publisher=Prentice Hall PTR |location=Upper Saddle River, New Jersey |isbn=0131103709
* American National Standard for Information Systems - Programming Language - C - ANSI X3.159-1989

External links

* [http://www.math.grin.edu/~stone/courses/languages/C-syntax.xhtml "The syntax of C in Backus-Naur form"]
* [http://www.cs.cf.ac.uk/Dave/C/CE.html Programming in C]
* [http://c-faq.com/ "The comp.lang.c Frequently Asked Questions Page"]