MSC01-J. Do not use an empty infinite loop

An empty infinite loop that does not do anything within the loop body is a suboptimal solution, and no code should use it. The solution is suboptimal because it consumes CPU cycles but does nothing. An optimizing compiler can remove such a loop, and it can lead to unexpected results. According to C1X Committee Draft [[ISO/IEC 9899:201x]], Section 6.8.5.6

An iteration statement that performs no input/output operations does not access volatile objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a for statement) its expression-3, may be assumed by the implementation to terminate.¹⁵⁵

155) This is intended to allow compiler transformations, such as removal of empty loops, even when termination cannot be proven.

Noncompliant Code Example

This noncompliant code example implements an idle task that continuously executes a loop without executing any instructions within the loop. An optimizing compiler could remove the while loop in the example.

int main(void) {
  while (1) { }
}

Compliant Solution (POSIX)

To avoid optimizing out of the loop, this compliant solution uses instructions that give up the CPU within the while loop. On POSIX platform, sleep(3) can be used. Other platforms might have equivalent functions.

int main(void) {
  while(1) {
    sleep(DURATION);
  }
}

Noncompliant Code Example

This noncompliant code example sets up interrupt handlers to process packets coming in from the network. It then waits for interrupts within a loop. Any interrupt causes the rcv_intr() function to be invoked.

However, an optimizing compiler could remove the for loop in the example. This example also violates the recommendation SIG02-C. Avoid using signals to implement normal functionality.

int main(void) {
  /* set up buffers, signal handlers for interrupts, etc. */
  /* ... */
  for ( ; ; ) {
    ;	/* let interrupt handler do all the work */
  }
  /* not reached */
}

void rcv_intr(int interrupt) { /* signal handler entered upon data_available interrupt */
  /* ... */
  get_packet();  /* read the packet */
  if (packet.hdr.service == ICMP_ECHO) {
    send_packet();  /* send the packet */
  }
  /* ... */
}

Compliant Solution (POSIX)

This compliant solution avoids optimizing out the loop by using instructions that give up the CPU within the for loop. On POSIX platform, sleep(3) can be used. Other platforms might have equivalent functions.

int main(void) {
  /* set up buffers, signal handlers for interrupts, etc. */
  /* ... */
  for ( ; ; ) {
    sleep(DURATION);	/* let interrupt handler do all the work */
  }
  /* not reached */
}

void rcv_intr(int interrupt) { /* signal handler entered upon data_available interrupt */
  /* ... */
  get_packet();  /* read the packet */
  if (packet.hdr.service == ICMP_ECHO) {
    send_packet();  /* send the packet */
  }
  /* ... */
}

Exceptions

MSC40-EX1: In rare cases, use of an empty infinite loop may be unavoidable. For instance, an empty loop may be necessary on a platform that does not support sleep(3) or an equivalent function. Another example occurs in operating system kernels. A task started before normal scheduler functionality is available may not have access to sleep(3) or an equivalent function. In such a case, it is necessary to adopt alternative solutions that prevent an optimizing compiler from removing the empty infinite loop.

The following solution uses a volatile variable.

int main(void) {
  volatile int true_value = 1;
  while (true_value) { }
}

The following solution is specific to the GCC 4.4.0 (or newer) compiler. It uses the optimize function attribute to turn off optimization at a function level, without disabling optimization for the rest of the code.

int main(void) __attribute__ ((optimize("O0")));

int main(void) {
  while (1) { }
}

Risk Assessment

Bibliography

ISO/IEC 9899:201x Committee Draft October 4, 2010 N1516 Section 6.8.5, Iteration statements.

MSC39-C. Do not call va_arg() on a va_list that has indeterminate value      49. Miscellaneous (MSC)      50. POSIX (POS)