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12 Non Local Jump

C mechanism to transfer control to any program point higher in the current stack, ie, to go to a function that called the current function.

It is a form of ‘non-local goto’

For eg, if we have 3 functions f1(), f2(), f3(), such that f1() invokes f2() and f3(). The calling diagram will be represented as follows

flowchart TB
f1 --> f2 & f3
Direction of jump Long jump possible?
Higher \(\to\) lower ❌
Lower \(\to\) Higher ✅
Adjacent ❌

Requirements

Description Returns
setjmp.h Header file N/A
jmp_buf jb Store ‘environment’ of return point.
It is a pointer to a structure.
Stores
- current register content
- stack pointer
- Program counter		 N/A
int setjmp(jb) Set return point, by saving current state of program execution in jb 0
void longjmp(jb, val) Restores register context from jump buffer env
Sets function’s return value register to val
Jumps to the old PC value stored in jump buffer jb		 It itself does not return anything, but
causes setjmp() to return
\[ \text{longjmp}(jb, \textcolor{hotpink}{k}) \text{ causes setjmp to return} \\ \text{return val} = \begin{cases} 1, \textcolor{hotpink}{k} = 0 \\ k, \textcolor{hotpink}{k} \ne 0 \end{cases} \]

Example

#include <stdio.h>
#include <longjmp.h>

jmp_buf jb;

void f1()
{
  printf("Entering f1()");

  f2();

  printf("Exiting f1()");
}

void f2()
{
  printf("Entering f2()");

  f3();

  printf("Exiting f2()");
}

void f3()
{
  printf("Entering f3()");

  longjmp(buf);

  printf("Exiting f3()");
}

void main()
{
  printf("Entering main() function");

  setjmp(jb);
  f1();

  printf("Exiting main() function");
}

Output

Entering main() Function
Entering f1()
Entering f2()
Entering f3()
Exiting main() Function

Notice how the exiting and return statements of f1(), f2(), f3() are skipped.

Last Updated: 2023-01-25 ; Contributors: AhmedThahir

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