Why does C allow me to call an undeclared function? [duplicate]

问题

I have two files: test1.c, and test2.c, which contains the main() function.

test1.c:

#include <stdio.h>  // printf() function declaration/prototype

// function definition
void say_hello() {
  printf("\tHello, world!\n");
}

test2.c:

#include <stdio.h>  // printf() function declaration/prototype
int main() {
  printf("Inside main()\n");
  say_hello();

  return 0;
}

And this is my makefile:

a.out: test1.o test2.o
    $(CXX) -o a.out test1.o test2.o

test1.o: test1.c
    $(CXX) -c test1.c

test2.o: test2.c
    $(CXX) -c test2.c

Now it should be clear where the problem lies: The main() function in test2.c calls say_hello() without declaring it! I run the following command, to use the gcc compiler: make CXX=gcc I get this warning to the screen:

gcc -c test1.c
gcc -c test2.c
test2.c: In function ‘main’:
test2.c:16:3: warning: implicit declaration of function ‘say_hello’ [-Wimplicit-function-declaration]
   say_hello();
   ^
gcc -o a.out test1.o test2.o

Although the *.o files got compiled and linked into the executable. That's weird. Imagine my surprise when I run the a.out file, and I see that main() successfully called say_hello(), a function which is not declared inside of main's translation unit, as if there were no issue at all! I reason that since say_hello() was not previously declared in test2.c, it should not allowed to be called by main() at all. Notice that I've added comments to the #include <stdio.h>. These preprocessor directives include the function declarations/prototypes, which extend their scope into the corresponding *.c files. That is why we are able to use them. No function declaration/prototype == no scope in that translation unit, or so I thought until now.

Then I compiled the above code as C++ code: make CXX=g++

I get this error to the screen:

test2.c: In function ‘int main()’:
test2.c:16:13: error: ‘say_hello’ was not declared in this scope
   say_hello();
             ^
makefile:18: recipe for target 'test2.o' failed
make: *** [test2.o] Error 1

g++ does what it's supposed to do, and stops the compilation process. But gcc did not do this! What's going on? Is it a perk of the C programming language? Is it an issue with the compiler?

回答1:

Simple, because C allows undeclared functions to be called and C++ does not. Either way, gcc warns you and you may want to take warnings seriously.

来源：https://stackoverflow.com/questions/50496264/why-does-c-allow-me-to-call-an-undeclared-function

标签

c++

scope

function-declaration