Why does gcc allow arguments to be passed to a function defined to be with no arguments?

In C, a function declared with an empty parameter list accepts an arbitrary number of arguments when being called, which are subject to the usual arithmetic promotions. It is the responsibility of the caller to ensure that the arguments supplied are appropriate for the definition of the function.

To declare a function taking zero arguments, you need to write void foo(void);.

This is for historic reasons; originally, C functions didn't have prototypes, as C evolved from B, a typeless language. When prototypes were added, the original typeless declarations were left in the language for backwards compatibility.

To get gcc to warn about empty parameter lists, use -Wstrict-prototypes:

Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.)

For legacy reasons, declaring a function with () for a parameter list essentially means “figure out the parameters when the function is called”. To specify that a function has no parameters, use (void).

Edit: I feel like I am racking up reputation in this problem for being old. Just so you kids know what programming used to be like, here is my first program. (Not C; it shows you what we had to work with before that.)

void foo() {    printf("Hello\n");}foo(str);

in C, this code does not violates a constraint (it would if it was defined in its prototype-form with void foo(void) {/*...*/}) and as there is no constraint violation, the compiler is not required to issue a diagnostic.

But this program has undefined behavior according to the following C rules:

From:

(C99, 6.9.1p7) "If the declarator includes a parameter type list, the list also specifies the types of all the parameters; such a declarator also serves as a function prototype for later calls to the same function in the same translation unit. If the declarator includes an identifier list,142) the types of the parameters shall be declared in a following declaration list."

the foo function does not provide a prototype.

From:

(C99, 6.5.2.2p6) "If the expression that denotes the called function has a type that does not include a prototype [...] If the number of arguments does not equal the number of parameters, the behavior is undefined."

the foo(str) function call is undefined behavior.

C does not mandate the implementation to issue a diagnostic for a program that invokes undefined behavior but your program is still an erroneous program.