What happens if I define a 0-size array in C/C++?

Question

Just curious, what actually happens if I define a zero-length array int array[0]; in code? GCC doesn\'t complain at all.

Sample Program

Answer 1

Another use of zero-length arrays is for making variable-length object (pre-C99). Zero-length arrays are different from flexible arrays which have [] without 0.

Quoted from gcc doc:

Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure that is really a header for a variable-length object:

 struct line {
   int length;
   char contents[0];
 };
 
 struct line *thisline = (struct line *)
   malloc (sizeof (struct line) + this_length);
 thisline->length = this_length;

In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:

• Flexible array members are written as contents[] without the 0.
• Flexible array members have incomplete type, and so the sizeof operator may not be applied.

A real-world example is zero-length arrays of struct kdbus_item in kdbus.h (a Linux kernel module).