1. Setjmp, Longjmp Useful functions for dealing with errors and interrupts setjmp saves its environment (i.e. registers) in env for later use by longjmp After longjmp completes, program starts after call to setjmp, as if setjmp had returned val. int setjmp(jmp_buf env); void longjmp(jmp_buf env, int val);

2. Setjmp, Longjmp: Example 1 Consider writing a project that will fail if it ever segfaults. Instead just gracefully restart it. How would you go about doing this? Catch the SIGSEGV, and use setjmp/longjmp to jump back to the beginning.

3. Setjmp, Longjmp: Example 1 con’t #include int main() { int opt; while(1) { printf("Directory Main Menu\n"); printf("-------------------------------\n"); printf("1. Find Person\n"); printf("2. Add Person\n"); printf("3. Delete Person\n"); printf("4. Exit\n"); printf("Enter your menu option: "); scanf("%d", &opt); switch(opt) { case 1: /* FindPerson(); */ printf("\n Finding a person\n"); break; case 2: /* AddPerson(); */ printf("\n Adding a person\n"); break; case 3: /* DeletePerson(); */ printf("\n Deleting a person\n"); break; case 4: exit(0); default: printf("\n Invalid Menu option\n"); break; } } /* end while loop */ return; }

4. Setjmp, Longjmp: Example 1 con’t #include jmp_buf Env; void handler(int sig) { /* Can do clean up here */ longjmp(Env, 1); } int main() { int opt; signal(SIGSEGV, handler); if(setjmp(Env) != 0) { printf("\n\n Returning to Main Menu \n"); } while(1) { printf("Directory Main Menu\n"); printf("-------------------------------\n"); printf("1. Find Person\n"); printf("2. Add Person\n"); printf("3. Delete Person\n"); printf("4. Exit\n"); printf("Enter your menu option: "); scanf("%d", &opt); switch(opt) { case 1: /* FindPerson(); */ printf("\n Finding a person\n"); break; case 2: /* AddPerson(); */ printf("\n Adding a person\n"); break; case 3: /* DeletePerson(); */ printf("\n Deleting a person\n"); break; case 4: exit(0); default: printf("\n Invalid Menu option\n"); break; } } /* end while loop */ return; }

5. Setjmp, Longjmp: Example 2 What is the output of the following program -- sj.c? int main() { jmp_buf env; if(a("Bob", env) != 0) exit(0); b(3,env); return 0; } #include int a(char *s, jmp_buf env) { int i; i = setjmp(env); printf("Setjmp returned -- %d\n", i); printf("s = %s\n", s); return i; } int b(int i, jmp_buf env) { printf("In b: i = %d, Calling longjmp\n", i); longjmp(env, i); }

6. Setjmp, Longjmp: Example 2 con’t UNIX> sj Setjmp returned -- 0 s = Bob In b: I = 3, Calling longjmp Setjmp returned -- 3 s = Bob UNIX> sj Setjmp returned -- 0 s = Bob In b: I = 3, Calling longjmp Setjmp returned -- 3 Segmentation Fault Output: ???

7. Setjmp, Longjmp: Example 2 con’t Let’s take a look at the stack to see why we’re segfaulting. old %ebp env[8] …. env[0] %ebp %esp First, main( ) looks like this. %eip --> in main

8. Setjmp, Longjmp: Example 2 con’t old %ebp env[8] …. env[0] %ebp %esp Then main( ) calls a( ). %eip --> in a i s = “Bob” Rtn Addr old %ebp

9. Setjmp, Longjmp: Example 2 con’t old %ebp env[8] …. env[0] %ebp %esp Then main( ) calls a( ). %eip --> in a Then a( ) calls setjmp( ). This saves the current state of the registers. i s = “Bob” Rtn Addr old %ebp

10. Setjmp, Longjmp: Example 2 con’t old %ebp env[8] …. env[0] %ebp %esp Returned to main( ), and main( ) calls b( ). %eip --> in b i i = 3 Rtn Addr old %ebp

11. Setjmp, Longjmp: Example 2 con’t old %ebp env[8] …. env[0] %ebp %esp longjmp( ) is called, and the regs are restored to their values of when a( ) was called. %eip --> in a i s =?? 3 Rtn Addr old %ebp Why the segfault?? --> the stack is in a bad state. a( ) expects a (char *) instead of the int value 3. This a common bug with setjmp/longjmp ---> You CANNOT return from a function that calls setjmp!