How to jump the program execution to a specific address in C?

Question

I want the program to jump to a specific address in memory and continue execution from that address. I thought about using goto but I don't have a label rather just an address in memory.

There is no need to worry about return back from the jump address.

edit: using GCC compiler

Answer 1

Inline assembly might be the easiest and most "elegant" solution, although doing this is highly unusual, unless you are writing a debugger or some specialized introspective system.

Another option might be to declare a pointer to a void function (void (*foo)(void)), then set the pointer to contain your address, and then invoke it:

void (*foo)(void) = (void (*)())0x12345678;
foo();

There will be things pushed on the stack since the compiler thinks you are doing a subroutine call, but since you don't care about returning, this might work.

Answer 2

gcc has an extension that allows jumping to an arbitrary address:

void *ptr = (void *)0x1234567;  // a random memory address
goto *ptr;                      // jump there -- probably crash

This is pretty much the same as using a function pointer that you set to a fixed value, but it will actually use a jump instruction rather than a call instruction (so the stack won't be modified)

Answer 3

#include <stdio.h>
#include <stdlib.h>

void go(unsigned int addr) {
  (&addr)[-1] = addr;
}

int sub() {
  static int i;
  if(i++ < 10) printf("Hello %d\n", i);
  else exit(0);
  go((unsigned int)sub);
}

int main() {
  sub();
}

Of course, this invokes undefined behavior, is platform-dependent, assumes that code addresses are the same size as int, etc, etc.

Answer 4

It should look something like this:

unsigned long address=0x80; 

void (*func_ptr)(void) = (void (*)(void))address;
func_ptr();

However, it is not a very safe operation, jumping to some unknown address will probably result in a crash!

Answer 5

Since the question has a C++ tag, here's an example of a C++ call to a function with a signature like main()--int main(int argc, char* argv[]):

int main(int argc, char* argv[])
{
    auto funcAddr = 0x12345678; //or use &main...
    auto result = reinterpret_cast<int (*)(int, char**)>(funcAddr)(argc, argv);
}

Answer 6

This is what I am using for my bootstrap loader(MSP430AFE253,Compiler = gcc,CodeCompeserStudio);

#define API_RESET_VECT 0xFBFE
#define JUMP_TO_APP()  {((void (*)()) (*(uint16_t*)API_RESET_VECT)) ();}

Answer 7

Do you have control of the code at the address that you intend to jump to? Is this C or C++?

I hesitantly suggest setjmp() / longjmp() if you're using C and can run setjmp() where you need to jump back to. That being said, you've got to be VERY careful with these.

As for C++, see the following discussion about longjmp() shortcutting exception handling and destructors destructors. This would make me even more hesitant to suggest it's use in C++.

C++: Safe to use longjmp and setjmp?

Answer 8

I Propos this code:

asm(
"LDR R0,=0x0a0000\n\t" /* Or 0x0a0000 for the base Addr. */
"LDR R0, [R0, #4]\n\t" /* Vector+4 for PC */
"BX R0"
);