Pointer declaration

Pointer is a type of an object that refers to a function or an object of another type, possibly adding qualifiers. Pointer may also refer to nothing, which is indicated by the special null pointer value.

Syntax

In the declaration grammar of a pointer declaration, the type-specifier sequence designates the pointed-to type (which may be function or object type and may be incomplete), and the declarator

where declarator may be the identifier that names the pointer being declared, including another pointer declarator (which would indicate a pointer to a pointer):

float *p, **pp; // p is a pointer to float
                // pp is a pointer to a pointer to float
int (*fp)(int); // fp is a pointer to function with type int(int)

The qualifiers that appear between * and the identifier (or other nested declarator) qualify the type of the pointer that is being declared:

int n;
const int * pc = &n; // pc is a non-const pointer to a const int
// *pc = 2; // Error: n cannot be changed through pc without a cast
pc = NULL; // OK: pc itself can be changed
 
int * const cp = &n; // cp is a const pointer to a non-const int
*cp = 2; // OK to change n through cp
// cp = NULL; // Error: cp itself cannot be changed
 
int * const * pcp = &cp; // non-const pointer to const pointer to non-const int

The attr-spec-seq(C23) is an optional list of attributes, applied to the declared pointer.

Explanation

Pointers are used for indirection, which is a ubiquitous programming technique; they can be used to implement pass-by-reference semantics, to access objects with dynamic storage duration

Pointers to objects

A pointer to object can be initialized with the result of the address-of operator applied to an expression of object type (which may be incomplete):

int n;
int *np = &n; // pointer to int
int *const *npp = &np; // non-const pointer to const pointer to non-const int
 
int a[2];
int (*ap)[2] = &a; // pointer to array of int
 
struct S { int n; } s = {1}
int* sp = &s.n; // pointer to the int that is a member of s

Pointers may appear as operands to the indirection operator (unary *), which returns the lvalue

int n;
int* p = &n; // pointer p is pointing to n
*p = 7; // stores 7 in n
printf("%d\n", *p); // lvalue-to-rvalue conversion reads the value from n

Pointers to objects of struct and union type may also appear as the left-hand operands of the member access through pointer operator ->

Because of the array-to-pointer implicit conversion, pointer to the first element of an array can be initialized with an expression of array type:

int a[2];
int *p = a; // pointer to a[0]
 
int b[3][3];
int (*row)[3] = b; // pointer to b[0]

Certain addition, subtraction, compound assignment, increment, and decrement

Comparison operators

Many implementations also provide strict total ordering

Pointers to functions

A pointer to function can be initialized with an address of a function. Because of the function-to-pointer conversion, the address-of operator is optional:

void f(int);
void (*pf1)(int) = &f;
void (*pf2)(int) = f; // same as &f

Unlike functions, pointers to functions are objects and thus can be stored in arrays, copied, assigned, passed to other functions as arguments, etc.

A pointer to function can be used on the left-hand side of the function call operator; this invokes the pointed-to function:

#include <stdio.h>
 
int f(int n)
{
    printf("%d\n", n);
    return n * n;
}
 
int main(void)
{
    int (*p)(int) = f;
    int x = p(7);
}

Dereferencing a function pointer yields the function designator for the pointed-to function:

int f();
int (*p)() = f;    // pointer p is pointing to f
(*p)(); // function f invoked through the function designator
p();    // function f invoked directly through the pointer

Equality comparison operators are defined for pointers to functions (they compare equal if pointing to the same function).

Because compatibility of function types

int f(int), fc(const int);
int (*pc)(const int) = f; // OK
int (*p)(int) = fc;       // OK
pc = p;                   // OK

Pointers to void

Pointer to object of any type can be implicitly converted to pointer to void (optionally const or volatile

int n=1, *p=&n;
void* pv = p; // int* to void*
int* p2 = pv; // void* to int*
printf("%d\n", *p2); // prints 1

Pointers to void are used to pass objects of unknown type, which is common in generic interfaces: malloc returns void*, qsort expects a user-provided callback that accepts two const void* arguments. pthread_create expects a user-provided callback that accepts and returns void*

Null pointers

Pointers of every type have a special value known as null pointer value of that type. A pointer whose value is null does not point to an object or a function (dereferencing a null pointer is undefined behavior), and compares equal to all pointers of the same type whose value is also null

To initialize a pointer to null or to assign the null value to an existing pointer, a null pointer constant (NULL, or any other integer constant with the value zero) may be used. static initialization

Null pointers can indicate the absence of an object or can be used to indicate other types of error conditions. In general, a function that receives a pointer argument almost always needs to check if the value is null and handle that case differently (for example, free

Notes

Although any pointer to object can be cast to pointer to object of a different type, dereferencing a pointer to the type different from the declared type of the object is almost always undefined behavior. See strict aliasing

lvalue expressions of array type, when used in most contexts, undergo an implicit conversion to the pointer to the first element of the array. See array

char *str = "abc"; // "abc" is a char[4] array, str is a pointer to 'a'

Pointers to char are often used to represent strings

References

See also