Syntax

#include <setjmp.h>
int setjmp(jmp_buf env);

setjmp saves a stack environment that can subsequently be restored by longjmp. setjmp and longjmp provide a way to perform a nonlocal goto. They are often used in signal handlers.

A call to setjmp causes it to save the current stack environment in env. A subsequent call to longjmp restores the saved environment and returns control to a point corresponding to the setjmp call. The values of all variables (except register variables) accessible to the function receiving control contain the values they had when longjmp was called. The values of variables that are allocated to registers by the compiler are unpredictable. Because any auto variable could be allocated to a register in optimized code, you should consider the values of all auto variables to be unpredictable after a longjmp call.

C++ Considerations When you call setjmp in a C++ program, ensure that that part of the program does not also create C++ objects that need to be destroyed. When you call longjmp, objects existing at the time of the setjmp call will still exist, but any destructors called after setjmp are not called. See longjmp for an example.

setjmp returns the value 0 after saving the stack environment. If setjmp returns as a result of a longjmp call, it returns the value argument of longjmp, or 1 if the value argument of longjmp is 0. There is no error return value.

This example stores the stack environment at the statement

   if(setjmp(mark) != 0) ...

When the system first performs the if statement, it saves the environment in mark and sets the condition to FALSE because setjmp returns a 0 when it saves the environment. The program prints the message

   setjmp has been called

The subsequent call to function p tests for a local error condition, which can cause it to perform the longjmp function. Then, control returns to the original setjmp function using the environment saved in mark. This time, the condition is TRUE because -1 is the return value from the longjmp function. The program then performs the statements in the block and prints

   longjmp has been called

Then the program calls the recover function and exits.

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

jmp_buf mark;

void p(void)
{
   int error = 0;

   error = 9;

   if (error != 0)
      longjmp(mark, -1);
}

void recover(void)
{
}

int main(void)
{
   if (setjmp(mark) != 0) {
      printf("longjmp has been called\n");
      recover();
      return 0;
   }
   printf("setjmp has been called\n");

   p();

   return 0;

   /****************************************************************************
      The output should be:

      setjmp has been called
      longjmp has been called
   ****************************************************************************/
}

• longjmp
  
• goto in the Language Reference

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