22.7.1 Variable-Length Array Parameters

An array parameter can have variable length: simply declare the array type with a size that isn’t constant. In a nested function, the length can refer to a variable defined in a containing scope. In any function, it can refer to a previous parameter, like this:

struct entry
tester (int len, char data[len][len])
{
  …
}

Alternatively, in function declarations (but not in function definitions), you can use [*] to denote that the array parameter is of a variable length, such that these two declarations mean the same thing:

struct entry
tester (int len, char data[len][len]);

struct entry
tester (int len, char data[*][*]);

The two forms of input are equivalent in GNU C, but emphasizing that the array parameter is variable-length may be helpful to those studying the code.

You can also omit the length parameter, and instead use some other in-scope variable for the length in the function definition:

struct entry
tester (char data[*][*]);
…
int dataLength = 20;
…
struct entry
tester (char data[dataLength][dataLength])
{
  …
}

In GNU C, to pass the array first and the length afterward, you can use a parameter forward declaration, like this:

struct entry
tester (int len; char data[len][len], int len)
{
  …
}

The ‘int len’ before the semicolon is the parameter forward declaration; it serves the purpose of making the name len known when the declaration of data is parsed.

You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the “real” parameter declarations. Each forward declaration must match a subsequent “real” declaration in parameter name and data type.

Standard C does not support parameter forward declarations.