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C99, Section 6.8.6.1 "The goto statement", says:

A goto statement shall not jump from outside the scope of an identifier having a variably modified type to inside the scope of that identifier.

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The examples here fall under the exception MSC17:-EX2 in MSC17-C. Finish every set of statements associated with a case label with a break statement.

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This example shows the use of the switch statment statement to jump into a for loop.

int f(int i) {
  int j=0;
  switch (i) {
    case 1:
      for(j=0;j<10;j++) {
      // no break, process case 2 as well
    case 2: // switch jumps inside the for block
        j++;
      // no break, process case 3 as well
    case 3:
        j++;
      }
      break;
  default:
    // default action
    break;
  }
  return j;
}

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In this code, the first iteration of the loop is subject to the switch statement, so it performs count % 8 assignments. Each subsequent iteration of the loop performs 8 assignments. (being Being outside the loop, the switch statement is ignored.) . Consequently, this code performs count assignments, but only n comparisons, so it is usually faster.

The code is widely considered to be legal C and C++ and supported by all compliant compilers. When describing Duff's Device, the creator noted:

Many people ... have said that the worst feature of C is that switches don't break automatically before each case label. This code forms some sort of argument in that debate, but I'm not sure whether it's for or against.

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Other Languages

Related Guidelines

This rule appears in the C++ Secure Coding Standard as : MSC20-CPP. Do not use a switch statement to transfer control into a complex block.

Bibliography

\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.8.6.1 "The goto statement"
[MISRA 042004|AA. Bibliography#ISO/MISRA 04] Rule 15.1
[Tom Duff on Duff's Device|http://www.lysator.liu.se/c/duffs-device.html]

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