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How can we use callbacks in ANSI-C?

I have already tried it, see here: https://code.sololearn.com/cg5LJ8X2P10Z/?ref=app But this methode doesn't worked. How I use callbacks in C?

25th Sep 2018, 1:21 PM
BraveHornet
BraveHornet - avatar
3 odpowiedzi
+ 1
You have to use function pointers instead of regular pointers. A function pointer is defined as follows: function_return_type (*variable_name) ( parameter_types, ... ); Might take a bit to get used to. This would be the corrected program: #include <stdio.h> // function prototype int sum( int, int ); int main() { printSum(2,4, sum); return 0; } int sum(int a, int b){ // No need for += return a+b; } void printSum(int a, int b, int (*doSum)(int, int)){ // doSum is the function pointer name printf("%d",doSum(a,b)); }
25th Sep 2018, 1:41 PM
Dennis
Dennis - avatar
+ 1
@Hornetfly The file is processed from top to bottom, assuming there is no prototype, when it reaches this line: printSum(2,4, sum); C has to know what printSum is and what sum is. Since the compiler hasn't come across sum yet it has no idea what that variable is. So with a prototype we tell C that sum is a function and that it exists somewhere. But we don't have a prototype for printSum either and that still compiles? You may ask. Well, C is a little weird. When you call a function, C assumes the function already exists and has a int return type. It's later the linkers job to find the correct function. If you would output printSum's return type, you could, even though it returns void, C thinks it's an int. So actually the correct program would also have a prototype for printSum ( which I forgot, sorry ) above main. void printSum(int, int, int (*)(int,int)); C would give out a bunch of warnings without the prototypes. But in C++ this wouldn't compile.
25th Sep 2018, 2:32 PM
Dennis
Dennis - avatar
0
Dennis Thank you, that worked for me. But why we need a function prototype?
25th Sep 2018, 2:01 PM
BraveHornet
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