16.6 Limitations of C Arrays

Arrays have quirks in C because they are not “first-class objects”: there is no way in C to operate on an array as a unit.

The other composite objects in C, structures and unions, are first-class objects: a C program can copy a structure or union value in an assignment, or pass one as an argument to a function, or make a function return one. You can’t do those things with an array in C. That is because a value you can operate on never has an array type.

An expression in C can have an array type, but that doesn’t produce the array as a value. Instead it is converted automatically to a pointer to the array’s element at index zero. The code can operate on the pointer, and through that on individual elements of the array, but it can’t get and operate on the array as a unit.

There are three exceptions to this conversion rule, but none of them offers a way to operate on the array as a whole.

First, ‘&’ applied to an expression with array type gives you the address of the array, as an array type. However, you can’t operate on the whole array that way—if you apply ‘*’ to get the array back, that expression converts, as usual, to a pointer to its zeroth element.

Second, the operators sizeof, _Alignof, and typeof do not convert the array to a pointer; they leave it as an array. But they don’t operate on the array’s data—they only give information about its type.

Third, a string constant used as an initializer for an array is not converted to a pointer—rather, the declaration copies the contents of that string in that one special case.

You can copy the contents of an array, just not with an assignment operator. You can do it by calling the library function memcpy or memmove (see The GNU C Library in The GNU C Library Reference Manual). Also, when a structure contains just an array, you can copy that structure.

An array itself is an lvalue if it is a declared variable, or part of a structure or union that is an lvalue. When you construct an array from elements (see Constructing Array Values), that array is not an lvalue.