Using Goto function across different functions

Question

How can I use goto across different functions? For example:

int main() {
    // ....
    REACH:
    // ....
}
    
void function() {
    goto REACH;
}

Hawser

You should not be using goto xkcd.com/292 – Deformation

Try setjmp instead. – Pardo

Question: Why on earth do you want to do this? Surely there are better ways. – Zabrine

Not answering your question of course, but instead you could make the goto part it's own functions which you call from both main and function. No reason to clutter you code with gotos. – Fatalism

You should probably have a look at this answer: <https://mcmap.net/q/17998/-how-to-quot-goto-quot-into-different-function-in-c> – Husain

@michaelb958 He's probably implementing a continuation. – Syrinx

@Ben don't generalize. goto is not always considered as a bad practice. goto is very useful in C when you need to gracefully cleanup and exit from function. You can see many goto instructions in Linux kernel and many other huge projects. – Solicitous

Also it's good for breaking out of multiple nested loops. – Loafer

You probably should limit this question to either C or C++. If you are using C++, standard exception handling might provide a solution. It is not available in C and you don't show enough of main to say for sure. – Durning

This is exactly the problem with functions. To all the people who say, "Why would you ever need this? You're doing something wrong. Do it differently." Screw off. There are very legitimate cases where you need this. Also sometimes functions are just a replacement for redundant or iterative code and that code ends up needing to goto. – Thoughtful

Answer 1

You can't in Standard C++. From $6.6.4/1 of the C++ Language Standard

The goto statement unconditionally transfers control to the statement labeled by the identifier. The identifier shall be a label (6.1) located in the current function.

...or in Standard C. From $6.8.6.1/1 of the C Language Standard

The identifier in a goto statement shall name a label located somewhere in the enclosing function. A goto statement shall not jump from outside the scope of an identifier having a variably modified type to inside the scope of that identifier.

Musical

Answer 2

You can't in Standard C; labels are local to a single function.

The nearest standard equivalent is the setjmp() and longjmp() pair of functions.

GCC has extensions to support labels more generally.

Wards

Answer 3

You can't. Think of this. There is a function A which is recursively calling another function B which in turn is calling A. Now, suppose that you put a goto statement from A to B. The question now becomes which instance of A do you want to go to which is undefined. Also, if no previous instance of A is defined, you have a bigger problem of no initialized variables in the function that are present before the label.

#include "bits/stdc++.h"
int i=0;
A(){
run:
    B();
}
B(){
if(i==10)
    goto run;
i++;
A();
}

Hardunn

Good point. This is exactly the problem with functions. We have no control over the stack in this sense. We should be able to revert a part of it and/or store certain places in the stack which we want to revert to. There is setjmp() and longjmp(), but these are slow and bring trouble when it comes to "stack" variables. The problem is for sake of convenience we've given up very useful functionality. The whole paradigm is flawed. – Thoughtful

Answer 4

For gcc:

#include <iostream>

void func(void* target){
    std::cout << "func" <<std::endl;
    goto *target;
}


int main() {
    void* target;
    auto flag = true;
l:
    std::cout << "label" <<std::endl;
    target = &&l;
    if (flag) {
        flag = false;
        func(target);
  }
}

Note that this can be an undefined behavior

Opsonize

gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Labels-as-Values.html: "You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. " – Northumbria

this answer should not have any upvotes, it is UB – Crowley