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A switch statement can be mixed with a block of code by starting the block in one case label, then having another case label within the block. This  The block can also be pictured as the block spanning more than one case statement.

C99, Section Subclause 6.8.6.1 "The goto statement", says4.2, paragraph 2, of the C Standard [ISO/IEC 9899:2011] says,

If a switch statement has an associated case or default label within A goto statement shall not jump from outside the scope of an identifier having with a variably modified type to inside , the entire switch statement shall be within the scope of that identifier.¹⁵⁴

Footnote 154 says:

That is, the declaration either precedes the switch statement, or it follows the last case or default label associated with the switch that is in the block containing the declaration.

Note that the However, the standard does not disallow jumping via goto or switch into loops that do not involve variably modified type identifiers. Consequently, loops and other blocks can be freely intermixed with switch statements. Unfortunately, this creates  such intermixing creates code that is, at best, confusing and unclear in what it does, which can cause undesirable behavior.

The examples here fall under the exception MSC17-C-EX2 in recommendation 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 statement to jump into a for loop.:

int f(int i) {
  int j=0;
  switch (i) {
    case 1:
      for(j=0;j<10;j++) {
      //* noNo break,; process case 2 as well */
    case 2: //* switch jumps inside the for block */
        j++;
      //* noNo break,; process case 3 as well */
    case 3:
        j++;
      }
      break;
  default:
    //* defaultDefault action */
    break;
  }
  return j;
}

Implementation Details

When i = 1, this executes the entire for loop is executed. When i = 2, two increments to j are made before the loop starts. When i = 3, one increment to j is made before the loop starts. The default case is no loop. Consequently, the function has the following behavior:

Compliant

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Solution

The compliant example solution separates the switch and for blocks.:

int f(int i) {
  int j=0;
  switch (i) {
    case 1:
      //* noNo break,; process case 2 as well */
    case 2:
      j++;
      //* noNo break,; process case 3 as well */
    case 3:
      j++;
      break;
    default:
      //* defaultDefault action */
      return j;
  }
  for(j++;j<10;j++) {
    j+=2;
  }
  return j;
}

Noncompliant Code Example (Duff's Device)

Duff's Device device is a curious optimization applied to code intended to perform a serial copy. That is, it copies a a series of bytes into one memory output in turn. A simple code to do this would be as follows:

size_t count; /* mustMust be nonzero */
char *to;     /* outputOutput destination */
char *from;   /* Points to count bytes to copy */

do {
  *to = *from++;     /*
     /                 * Note that the ''"to''" pointer 
                      * is NOT incremented.
                      */
} while (--count > 0);

However, this code might be unacceptably slow because the while condition is performed count times. The classic code for Duff's Device device unrolls this loop to minimize the number of comparisons performed:

int n = (count + 7) / 8;
switch (count % 8) {
  case 0: do { *to = *from++;
  case 7:      *to = *from++;
  case 6:      *to = *from++;
  case 5:      *to = *from++;
  case 4:      *to = *from++;
  case 3:      *to = *from++;
  case 2:      *to = *from++;
  case 1:      *to = *from++;
	      } while (--n > 0);
}

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 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 valid C and C++ and is supported by all compliant compilers. When describing Duff's Devicedevice, the creator [Duff 1988] 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.

Compliant

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Solution (Duff's Device)

This is an alternate implemention alternative implementation of Duff's Device device, which separates the switch statement and loop.:

int n = (count + 7) / 8;
switch (count % 8) {
  case 0: *to = *from++; /* fallFall through */
  case 7: *to = *from++; /* fallFall through */
  case 6: *to = *from++; /* fallFall through */
  case 5: *to = *from++; /* fallFall through */
  case 4: *to = *from++; /* fallFall through */
  case 3: *to = *from++; /* fallFall through */
  case 2: *to = *from++; /* fallFall through */
  case 1: *to = *from++; /* fallFall through */
}
while (--n > 0) {
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
}

Risk Assessment

Automated Detection

Related Guidelines

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Bibliography

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