const type qualifier

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Each individual type in the C type system has several qualified versions of that type, corresponding to one, two, or all three of the const, volatile, and, for pointers to object types, restrict qualifiers. This page describes the effects of the const qualifier.

Objects declared with const-qualified types may be placed in read-only memory by the compiler, and if the address of a const object is never taken in a program, it may not be stored at all.

Any attempt to modify an object whose type is const-qualified results in undefined behavior. 

const int n = 1; // object of const-qualified type
int* p = (int*)&n;
*p = 2; // undefined behavior

const semantics apply to lvalue expressions only; whenever a const lvalue expression is used in context that does not require an lvalue, its const qualifier is lost (note that volatile qualifier, if present, isn't lost).

The lvalue expressions that designate objects of const-qualified type and the lvalue expressions that designate objects of struct or union type with at least one member of const-qualified type (including members of recursively contained aggregates or unions), are not modifiable lvalues. In particular, they are not assignable: 

const int n = 1; // object of const type
n = 2; // error: the type of n is const-qualified
 
int x = 2; // object of unqualified type
const int* p = &x;
*p = 3; // error: the type of the lvalue *p is const-qualified
 
struct {int a; const int b; } s1 = {.b=1}, s2 = {.b=2};
s1 = s2; // error: the type of s1 is unqualified, but it has a const member

A member of a const-qualified structure or union type acquires the qualification of the type it belongs to (both when accessed using the . operator or the -> operator). 

struct s { int i; const int ci; } s;
// the type of s.i is int, the type of s.ci is const int
const struct s cs;
// the types of cs.i and cs.ci are both const int

If an array type is declared with the const type qualifier (through the use of typedef), the array type is not const-qualified, but its element type is.

(until C23)

An array type and its element type are always considered to be identically const-qualified.

(since C23)
typedef int A[2][3];
const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of const int
int* pi = a[0]; // Error: a[0] has type const int*
void *unqual_ptr = a; // OK until C23; error since C23
// Notes: clang applies the rule in C++/C23 even in C89-C17 modes

If a function type is declared with the const type qualifier (through the use of typedef), the behavior is undefined.

In a function declaration, the keyword const may appear inside the square brackets that are used to declare an array type of a function parameter. It qualifies the pointer type to which the array type is transformed.

The following two declarations declare the same function: 

void f(double x[const], const double y[const]);
void f(double * const x, const double * const y);
 (since C99)

const-qualified compound literals do not necessarily designate distinct objects; they may share storage with other compound literals and with string literals that happen to have the same or overlapping representation. 

const int* p1 = (const int[]){1, 2, 3};
const int* p2 = (const int[]){2, 3, 4}; // the value of p2 may equal p1+1
_Bool b = "foobar" + 3 == (const char[]){"bar"}; // the value of b may be 1
 (since C99)

A pointer to a non-const type can be implicitly converted to a pointer to const-qualified version of the same or compatible type. The reverse conversion requires a cast expression. 

int* p = 0;
const int* cp = p; // OK: adds qualifiers (int to const int)
p = cp; // Error: discards qualifiers (const int to int)
p = (int*)cp; // OK: cast

Note that pointer to pointer to T is not convertible to pointer to pointer to const T; for two types to be compatible, their qualifications must be identical. 

char *p = 0;
const char **cpp = &p; // Error: char* and const char* are not compatible types
char * const *pcp = &p; // OK, adds qualifiers (char* to char*const)

Keywords

const

Notes

C adopted the const qualifier from C++, but unlike in C++, expressions of const-qualified type in C are not constant expressions; they may not be used as case labels or to initialize static and thread storage duration objects, enumerators, or bit-field sizes. When they are used as array sizes, the resulting arrays are VLAs.

References

  • C17 standard (ISO/IEC 9899:2018):
  • 6.7.3 Type qualifiers (p: 87-90)
  • C11 standard (ISO/IEC 9899:2011):
  • 6.7.3 Type qualifiers (p: 121-123)
  • C99 standard (ISO/IEC 9899:1999):
  • 6.7.3 Type qualifiers (p: 108-110)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 6.5.3 Type qualifiers

See also

C++ documentation for cv (const and volatile) type qualifiers
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