Safe Signal Handlers

• Signal handlers should be reentrant and async-signal-safe
• A signal may interrupt the thread of execution at any point.
• A process is reentrant if it can safely be simultaneously executed by multiple threads in the same process
  • It can not update global structures
  • It can not use static data.
    • Internally (rand/srand)
    • Or for a return values (getpwnam)
    • Man pages usually flag these
    • Look at both rand_r and getpwuid_r
• Example : malloc and free
  • Maintain a "free" list

  •  
    
     tmp = head
     head = tmp->next;
    
          OR
    
     node->next = head;
     head = node;
    
             

  • If the main, while performing either of these is interpreted between 1. and 2. there will be MAJOR problems.
• Async-signal-safe functions
  • These functions are either reentrant
  • Or they are not interruptible by a signal handler.
  • There are a list of these on page 426.