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After invoking longjmp(), non-volatile-qualified local objects should not be accessed if their values could have changed since the invocation of setjmp(). Their value in this case is considered indeterminate, and accessing them is undefined behavior. (See Undefined Behavior 121, 10.)
The longjmp() function should never be used to return control to a function that has terminated execution. (See Undefined Behavior 120.)

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Placing the call to setjmp() in the if statement and (, optionally) , comparing it with a constant integer removes the undefined behavior, as shown in this compliant solution.

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jmp_buf buf;
unsigned char b[] = {0xe5, 0x06, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00};

int main(void) {
  if (setjmp(buf) == 0) {
    printf("setjmp() invoked\n");
  } else {
    printf("longjmp() invoked\n");
  }
  do_stuff();
  return 0;
}

void do_stuff(void) {
  char a[8];
  memcpy(a, b, 8);
  /* ... stuff ... */
  longjmp(buf, 1);
}

void bad(void) {
  printf("Should not be called!\n");
  exit(1);
}

In this compliant solution, there There is no risk of overwriting a return address because the stack frame of main() (the function that invoked setjmp()) is still on the stack; so when do_stuff() is invoked, the two stack frames will not overlap.

Noncompliant Code Example

NonIn this noncompliant example, non-volatile-qualified objects local to the function that invoked the corresponding setjmp() have indeterminate values after longjmp() has been executed if their value has been changed since the invocation of setjmp().

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