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这样的不带参数的函数声明，在c中是合法的，表示任意参数；当然我们自己写代码最好不要这样写了，但是读老代码还是会遇到；
1 #include <stdio.h>
2
3 void fun();
4
5 int main()
6 {
7 fun(1);
8 return 0;
9 }
10
11 void fun(int a)
12 {
13 printf("%d\n", a);
14 }
下面贴一下函数声明的说明：
int func(); is an obsolescent function declaration from the days when there was no C standard, i.e. the days of K&R C (before 1989, the year the first "ANSI C" standard was published).
Remember that there were no prototypes in K&R C and the keyword void was not yet invented. All you could do was to tell the compiler about the return type of a function. The empty parameter list in K&R C means "an unspecified but fixed" number of arguments. Fixed means that you must call the function with the same number of args each time (as opposed to a variadic function like printf, where the number and type can vary for each call).
Many compilers will diagnose this construct; in particular gcc -Wstrict-prototypes will tell you "function declaration isn't a prototype", which is spot on, because it looks like a prototype (especially if you are poisoned by C++!), but isn't. It's an old style K&R C return type declaration.
Rule of thumb: Never leave an empty parameter list declaration empty, use int func(void) to be specific. This turns the K&R return type declaration into a proper C89 prototype. Compilers are happy, developers are happy, static checkers are happy. Those mislead by^W^Wfond of C++ may cringe, though, because they need to type extra characters when they try to exercise their foreign language skills :-)
All the other answers are correct, but just for completion
A function is declared in the following manner:
return-type function-name(parameter-list,...) { body... }
return-type is the variable type that the function returns. This can not be an array type or a function type. If not given, then int is assumed.
function-name is the name of the function.
parameter-list is the list of parameters that the function takes separated by commas. If no parameters are given, then the function does not take any and should be defined with an empty set of parenthesis or with the keyword void. If no variable type is in front of a variable in the paramater list, then int is assumed. Arrays and functions are not passed to functions, but are automatically converted to pointers. If the list is terminated with an ellipsis (,...), then there is no set number of parameters. Note: the header stdarg.h can be used to access arguments when using an ellipsis.
And again for the sake of completeness. From C11 specification 6:11:6 (page: 179)
The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.
stackoverflow:
http://stackoverflow.com/questions/13950642/why-does-a-function-with-no-parameters-compared-to-the-actual-function-definiti
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