将函数调用为fun（）和fun之间有什么区别？

Question

I came across a piece of code which is as below

qsort(array, 10, sizeof(int), sort);

Here sort is again a function. But I was always under the impression that any function which is called should have () after the function name or was I missing something.

I just wanted to know how will this work and what are the differences in calling functions like these. Thanks!

Answer 1

qsort(array, 10, sizeof(int), sort);

这会传递一个指向函数“sort”的指针，以便函数“qsort”可以调用此函数。

Answer 2

That function is not called it is passed as a function pointer. see Function Pointers in C and C++

Answer 3

You are passing a pointer to the function sort to function qsort , so that qsort can use the function as comparison function.

The correct syntax should include a & before sort (since sort is a function, not a pointer, so we need to reference it), but the compiler "adds" it for you anyway.

Answer 4

If you look at the C grammar (eg in the C Standard) you find that there is an operator written as () , which is the function call operator . It is similar to the array subscript operator [] in that it is applied to an identifier, in this case, the name of the function. If the identifier for a function is not followed by the function call operator, the identifier represents a pointer to that function. So now it becomes clear, what qsort 's last arg is: a pointer to the comparison function. Whenever it needs to compare two elements it calls the function pointed to by its last arg.

The sort function in your example is termed a callback function, qsort calls it back (several times, with different arguments).