_Static_assert declaration (C11)

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Static assertions can be declared to detect and diagnose common usage errors at compile time. A _Static_assert declaration takes the following form:

_Static_assert declaration syntax


1  _Static_assert  (  constant_expression  ,  string_literal  )  ;

The constant_expression must be an integer constant expression. If the integer constant expression evaluates to 0, the compiler issues a severe error containing the string literal with the source location of the _Static_assert declaration. Otherwise, the _Static_assert declaration has no effect.

The declaration of static assertions does not declare a new type or object, and does not imply any size or time cost at run time.

static_assert is a macro defined in assert.h for C.

The addition of static assertions to the C language has the following benefits:
  • Libraries can detect common usage errors at compile time.
  • Implementations of the C Standard Library can detect and diagnose common usage errors, improving usability.
You can declare static assertions to check important program invariants at compile time.

Examples: _Static_assert declaration

Example 1: The following example demonstrates the use of a _Static_assert declaration inside a structure.
 #include <stddef.h>
 struct __attribute__((packed)) B{
    char a;
    int i;
 };
 
 struct A{
    struct B b;
    _Static_assert(offsetof(struct B,i)==1,"S not packed");
 };
Example 2: The following example contains static assertions declared with static_assert, so the assert.h header file must be included.
 /* static_assert requires <assert.h> */
 #include <assert.h>
 static_assert(sizeof(long) >= 8, "64-bit not enabled.");
Example 3: The following example shows the use of a _Static_assert declaration with an invalid constant expression.
 _Static_assert(1 / 0, "never shows up!");
When you compile this program, the compiler does not show the string literal in the _Static_assert declaration. Instead, the compiler issues an error message indicating that the divisor cannot be zero.