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What the hell is this? How is this allowed and legal? I swear, every time I think I'm getting a grip of how this language works, some real cute looking syntax like this shows up.

This is from Semtech's example project for one of their radios.

From within radio.h:

   void    ( *SetRxConfig )( RadioModems_t modem, uint32_t bandwidth,
                              uint32_t datarate, uint8_t coderate,
                              uint32_t bandwidthAfc, uint16_t preambleLen,
                              uint16_t symbTimeout, bool fixLen,
                              uint8_t payloadLen,
                              bool crcOn, bool FreqHopOn, uint8_t HopPeriod,
                              bool iqInverted, bool rxContinuous );

If there's justice in Heaven, how is it OK to write something like this? What is this chunk of code doing and what is it called so I can look up more about it?

testname123
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1 Answers1

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This declares a function:

int foo(int, float, double);

And this defines (or declares - depending on context) a function pointer, that can be assigned the address of function foo:

int (*function_pointer)(int, float, double);

This is what you are seeing here.

A more verbose explanation with additional examples on how to use, pass as argument and return function pointers can be found in an answer to a similar question suggested by user2357112.

Possible uses include:

  • A library defines hooks to it's function where you can make it call your code. Either by defining a global variable (like void * (*mymegalib_malloc)(size_t size) where you can replace the allocator by your own), or by providing setter functions for the function pointer like in signal,
  • Passing a function to do stuff to another function, like in qsort, where you pass an array, the number of elements and a function, that compares two indices of that array - giving you the opportunity to let qsort operate on whatever data structure you want,

  • Creating some jump tables, like this:

    struct jmp {
    char op;
    double (*func)(double a, double b);
} jtable = {
    { '+', add },
    { '-', sub },
    { '*', mul },
    { '/', div },
    { 0, NUUL },
};
// in some function:
for (size_t i = 0; jtable[i].op; ++i) {
    if (jtable[i].op == requested_op) {
        result = jtable[i].func(a, b);
    }
}
Community
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Bodo Thiesen
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    Why would you ever want to do this? – testname123 Dec 04 '16 at 23:30
  • "And this defines a function pointer" - without further context, this is a declaration. We cannot know if it is also a definition. – too honest for this site Dec 04 '16 at 23:33
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    @Olaf: `int foo(int, float, double);` always only declares a function. `int (*function_pointer)(int, float, double);` however can declare (e.g. in a struct or argument list) or define (e.g. in file or function scope) a function pointer. I defined it to define a function pointer, this narrows down the places, where I expect this construct to show up. – Bodo Thiesen Dec 04 '16 at 23:47
  • It it **always** a declaration, regardless if it is also a definition. But it is only a definition for variables with block-scope. And your `struct` is not a defintion at all, but a _type-specifier_ in the first place (specifically a _struct-or-union-specifier_), and forms a _declaration_ without _declarator_. Feel free to read the standard, if you want to use the correct terms. Btw.: what is an "argument list"? This does not exist in C. – too honest for this site Dec 05 '16 at 00:19
  • I talk english not standardish. – Bodo Thiesen Dec 05 '16 at 00:50
  • this is what two wizards fighting looks like. – testname123 Dec 05 '16 at 00:53