22.5.1 Declaring Function Pointers

The declaration of a function pointer variable (or structure field) looks almost like a function declaration, except it has an additional ‘*’ just before the variable name. Proper nesting requires a pair of parentheses around the two of them. For instance, int (*a) (); says, “Declare a as a pointer such that *a is an int-returning function.”

Contrast these three declarations:

/* Declare a function returning char *.  */
char *a (char *);
/* Declare a pointer to a function returning char.  */
char (*a) (char *);
/* Declare a pointer to a function returning char *.  */
char *(*a) (char *);

The possible argument types of the function pointed to are the same as in a function declaration. You can write a prototype that specifies all the argument types:

rettype (*function) (arguments…);

or one that specifies some and leaves the rest unspecified:

rettype (*function) (arguments…, ...);

or one that says there are no arguments:

rettype (*function) (void);

You can also write a non-prototype declaration that says nothing about the argument types:

rettype (*function) ();

For example, here’s a declaration for a variable that should point to some arithmetic function that operates on two doubles:

double (*binary_op) (double, double);

Structure fields, union alternatives, and array elements can be function pointers; so can parameter variables. The function pointer declaration construct can also be combined with other operators allowed in declarations. For instance,

int **(*foo)();

declares foo as a pointer to a function that returns type int **, and

int **(*foo[30])();

declares foo as an array of 30 pointers to functions that return type int **.

int **(**foo)();

declares foo as a pointer to a pointer to a function that returns type int **.