Question

I want to use longjmp to simulate goto instruction.I have an array DS containing elements of struct types (int , float, bool ,char). I want to jump to the place labled “lablex” where x is DS[TOP].int_val. how can I handle this?

sample code :

...
jmp_buf *bfj;
...
stringstream s;s<<"label"<<DS[TOP].int_val;
bfj = (jmp_buf *) s.str();
longjmp(*bfj,1);

but as I thought it’s having problem what should I do?

error:

output.cpp: In function ‘int main()’:

output.cpp:101: error: invalid cast from type ‘std::basic_string, std::allocator >’ to type ‘__jmp_buf_tag (*)[1]’

Answer 1

You probably don’t want to use longjmp at all but I hate it when people answer a question with “Why would you want to do that?” As has been pointed out your longjmp() usage is wrong. Here is a simple example of how to use it correctly:

#include <setjmp.h>

#include <iostream>

using namespace std;

jmp_buf jumpBuffer;  // Declared globally but could also be in a class.

void a(int count) {
  // . . .
  cout << "In a(" << count << ") before jump" << endl;
  // Calling longjmp() here is OK because it is above setjmp() on the call
  //   stack.
  longjmp(jumpBuffer, count);  // setjump() will return count
  // . . .
}


void b() {
  int count = 0;

  cout << "Setting jump point" << endl;
  if (setjmp(jumpBuffer) == 9) return;
  cout << "After jump point" << endl;

  a(count++);  // This will loop 10 times.
}


int main(int argc, char *argv[]) {
  b();

  // Note: You cannot call longjmp() here because it is below the setjmp() call
  //  on the call stack.

  return 0;
}

The problems with your usage of longjmp() are as follows:

1. You don’t call setjmp()
2. You haven’t allocated the jmp_buf either on the stack or dynamically. jmp_buf *bfj is just a pointer.
3. You cannot cast a char * to jmp_buf * and expect it to work. C++ not a dynamic language it is statically compiled.

But really, it is very unlikely that you should be using longjmp() at all.